Compose a position in the shader the way the CPU composes it

The shader used mix() where UiScalar::within writes from + (to - from) *
t, so the two associate the arithmetic differently and can put an edge on
either side of an integer. Writing it out matches them, and is a
multiplication cheaper. The five reference renders and the resize render
are byte-identical either way.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Cargo.toml

@@ -3,6 +3,9 @@ name = "iris"
 version.workspace = true
 edition.workspace = true
 
+[features]
+layout-diagnostics = ["iris-core/layout-diagnostics"]
+
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]

core/Cargo.toml

@@ -3,6 +3,9 @@ name = "iris-core"
 version.workspace = true
 edition.workspace = true
 
+[features]
+layout-diagnostics = []
+
 [dependencies]
 wgpu = { workspace = true }
 bytemuck ={ workspace = true }

core/src/layout_diagnostics.rs

@@ -0,0 +1,488 @@
+//! Opt-in counters and coarse timers for explaining CPU layout cost.
+//!
+//! Enable the `layout-diagnostics` feature. With it disabled, none of the
+//! instrumentation is compiled into Iris. The retained rig in
+//! `tests/layout_diagnostics.rs` is the ordinary entry point.
+//!
+//! Timers are inclusive: `update total` contains `full layout` or
+//! `incremental layout`, and `text render` contains shaping and glyph
+//! placement. They locate cost within one instrumented run and must not be
+//! added together. Use an uninstrumented build under `perf` for final CPU
+//! totals; counting every primitive and distinct widget deliberately perturbs
+//! the instrumented run.
+//!
+//! Call [`trace_widget`] before a frame to retain the ordered constraint,
+//! reuse, size, placement, and text events for one suspicious widget. The
+//! selection is a set and survives [`take`] until cleared.
+
+use crate::{Axis, Len, Size, UiRegion, WidgetId, util::Vec2};
+use std::{
+    cell::RefCell,
+    collections::{HashMap, HashSet},
+    fmt::Write,
+    time::Instant,
+};
+
+#[derive(Clone, Copy)]
+pub(crate) enum Counter {
+    Updates,
+    ResizeDependents,
+    DrawRequests,
+    WidgetDraws,
+    PlaceCalls,
+    SizeReads,
+    HintHits,
+    HintMisses,
+    RetainedSizeHits,
+    ReuseAttempts,
+    ReuseExact,
+    ReuseMoved,
+    ReuseDirty,
+    ReuseWrongParent,
+    ReuseUnslotted,
+    ReuseOwnResize,
+    ReuseDescendantResize,
+    ResizeChecks,
+    ResizeCheckChildren,
+    QueuePops,
+    DepthReads,
+    EagerReaderRedraws,
+    LocalRedraws,
+    SizeChanges,
+    ReaderEdges,
+    PrimitiveWrites,
+    TextRenders,
+    TextShapeHits,
+    TextShapes,
+    TextBreaks,
+    GlyphPlacements,
+}
+
+impl Counter {
+    const COUNT: usize = Self::GlyphPlacements as usize + 1;
+
+    const NAMES: [&'static str; Self::COUNT] = [
+        "updates",
+        "resize dependents",
+        "draw requests",
+        "widget draws",
+        "place calls",
+        "draw-result size reads",
+        "hint hits",
+        "hint misses",
+        "retained size hits",
+        "reuse attempts",
+        "reuse exact",
+        "reuse moved",
+        "reuse: dirty",
+        "reuse: wrong parent",
+        "reuse: unslotted",
+        "reuse: own resize",
+        "reuse: descendant resize",
+        "resize checks",
+        "resize children checked",
+        "redraw queue pops",
+        "depth reads",
+        "eager reader redraws",
+        "local redraws",
+        "size changes",
+        "reader edges",
+        "primitive writes",
+        "text renders",
+        "text shape hits",
+        "text shapes",
+        "text line breaks",
+        "glyph placements",
+    ];
+}
+
+#[derive(Clone, Copy)]
+pub(crate) enum TimerKind {
+    Update,
+    FullLayout,
+    ResizeMarking,
+    IncrementalLayout,
+    TextRender,
+    TextShape,
+    TextBreak,
+    GlyphPlacement,
+}
+
+impl TimerKind {
+    const COUNT: usize = Self::GlyphPlacement as usize + 1;
+
+    const NAMES: [&'static str; Self::COUNT] = [
+        "update total",
+        "full layout",
+        "resize marking",
+        "incremental layout",
+        "text render",
+        "text shape",
+        "text line break",
+        "glyph placement",
+    ];
+}
+
+#[derive(Clone)]
+pub struct Report {
+    counters: [u64; Counter::COUNT],
+    nanos: [u64; TimerKind::COUNT],
+    distinct_widgets: usize,
+    distinct_text_widgets: usize,
+    hot_widgets: Vec<Callsite>,
+    hot_text: Vec<Callsite>,
+    traces: Vec<TraceEvent>,
+}
+
+impl Default for Report {
+    fn default() -> Self {
+        Self {
+            counters: [0; Counter::COUNT],
+            nanos: [0; TimerKind::COUNT],
+            distinct_widgets: 0,
+            distinct_text_widgets: 0,
+            hot_widgets: Vec::new(),
+            hot_text: Vec::new(),
+            traces: Vec::new(),
+        }
+    }
+}
+
+impl Report {
+    pub fn counters(&self) -> impl Iterator<Item = (&'static str, u64)> + '_ {
+        Counter::NAMES.into_iter().zip(self.counters)
+    }
+
+    /// Inclusive elapsed time accumulated for each targeted operation.
+    pub fn timings_ns(&self) -> impl Iterator<Item = (&'static str, u64)> + '_ {
+        TimerKind::NAMES.into_iter().zip(self.nanos)
+    }
+
+    pub fn distinct_widgets(&self) -> usize {
+        self.distinct_widgets
+    }
+
+    pub fn distinct_text_widgets(&self) -> usize {
+        self.distinct_text_widgets
+    }
+
+    pub fn hot_widgets(&self) -> &[Callsite] {
+        &self.hot_widgets
+    }
+
+    pub fn hot_text(&self) -> &[Callsite] {
+        &self.hot_text
+    }
+
+    /// Ordered layout events for widgets selected with [`trace_widget`].
+    pub fn traces(&self) -> &[TraceEvent] {
+        &self.traces
+    }
+
+    /// Formats nonzero totals divided by `frames`.
+    pub fn per_frame(&self, frames: usize) -> String {
+        let divisor = frames.max(1) as f64;
+        let mut out = String::new();
+        for (name, value) in self.counters() {
+            if value != 0 {
+                let _ = writeln!(out, "  {name:<27} {:>12.2}", value as f64 / divisor);
+            }
+        }
+        if self.distinct_widgets != 0 {
+            let _ = writeln!(
+                out,
+                "  {:<27} {:>12}",
+                "distinct widgets", self.distinct_widgets
+            );
+        }
+        if self.distinct_text_widgets != 0 {
+            let _ = writeln!(
+                out,
+                "  {:<27} {:>12}",
+                "distinct text widgets", self.distinct_text_widgets
+            );
+        }
+        for (name, nanos) in self.timings_ns() {
+            if nanos != 0 {
+                let ms = nanos as f64 / divisor / 1_000_000.0;
+                let _ = writeln!(out, "  {name:<27} {ms:>12.3} ms");
+            }
+        }
+        if !self.hot_widgets.is_empty() {
+            let _ = writeln!(out, "  hottest widget draws:");
+            for callsite in &self.hot_widgets {
+                let calls = callsite.calls as f64 / divisor;
+                let _ = writeln!(
+                    out,
+                    "    {calls:>9.2}  {:?} {}",
+                    callsite.id, callsite.label
+                );
+            }
+        }
+        if !self.hot_text.is_empty() {
+            let _ = writeln!(out, "  hottest text renders:");
+            for callsite in &self.hot_text {
+                let calls = callsite.calls as f64 / divisor;
+                let _ = writeln!(
+                    out,
+                    "    {calls:>9.2}  {:>3} widths  {:?} {}",
+                    callsite.distinct_widths, callsite.id, callsite.label
+                );
+            }
+        }
+        if !self.traces.is_empty() {
+            let _ = writeln!(out, "  targeted layout trace:");
+            for event in &self.traces {
+                let _ = writeln!(out, "    {event:?}");
+            }
+        }
+        out
+    }
+}
+
+#[derive(Clone)]
+pub struct Callsite {
+    pub id: WidgetId,
+    pub label: String,
+    pub calls: u64,
+    pub distinct_widths: usize,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum ReuseOutcome {
+    Exact,
+    Moved,
+    Dirty,
+    WrongParent,
+    Unslotted,
+    OwnResize,
+    DescendantResize,
+}
+
+/// One targeted layout event. Events are retained in execution order, making
+/// repeated constraint paths visible without logging every widget globally.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum TraceEvent {
+    DrawRequest {
+        id: WidgetId,
+        parent: Option<WidgetId>,
+        region: UiRegion,
+        pixel_size: Vec2,
+        slotted: bool,
+    },
+    Reuse {
+        id: WidgetId,
+        outcome: ReuseOutcome,
+    },
+    SizeReported {
+        id: WidgetId,
+        size: Size,
+    },
+    Placed {
+        id: WidgetId,
+        parent: WidgetId,
+        region: UiRegion,
+    },
+    SizeRead {
+        id: WidgetId,
+        reader: WidgetId,
+        size: Size,
+    },
+    HintRead {
+        id: WidgetId,
+        reader: WidgetId,
+        axis: Axis,
+        hint: Option<Len>,
+    },
+    TextRendered {
+        id: WidgetId,
+        width: Option<f32>,
+    },
+}
+
+#[derive(Default)]
+struct Calls {
+    label: String,
+    count: u64,
+    widths: HashSet<Option<u32>>,
+}
+
+#[derive(Default)]
+struct Current {
+    report: Report,
+    widgets: HashMap<WidgetId, Calls>,
+    text_widgets: HashMap<WidgetId, Calls>,
+    traced: HashSet<WidgetId>,
+}
+
+thread_local! {
+    static CURRENT: RefCell<Current> = RefCell::new(Current::default());
+}
+
+pub(crate) fn bump(counter: Counter) {
+    CURRENT.with_borrow_mut(|current| current.report.counters[counter as usize] += 1);
+}
+
+pub(crate) fn draw_widget(id: WidgetId, label: &str) {
+    CURRENT.with_borrow_mut(|current| {
+        let calls = current.widgets.entry(id).or_default();
+        if calls.label.is_empty() {
+            calls.label = label.to_owned();
+        }
+        calls.count += 1;
+    });
+}
+
+/// Adds a widget to the targeted trace set. Selection survives [`take`]
+/// until explicitly removed or cleared.
+pub fn trace_widget(id: impl Into<WidgetId>) {
+    CURRENT.with_borrow_mut(|current| {
+        current.traced.insert(id.into());
+    });
+}
+
+pub fn untrace_widget(id: impl Into<WidgetId>) {
+    CURRENT.with_borrow_mut(|current| {
+        current.traced.remove(&id.into());
+    });
+}
+
+pub fn clear_traced_widgets() {
+    CURRENT.with_borrow_mut(|current| current.traced.clear());
+}
+
+fn trace(id: WidgetId, event: TraceEvent) {
+    CURRENT.with_borrow_mut(|current| {
+        if current.traced.contains(&id) {
+            current.report.traces.push(event);
+        }
+    });
+}
+
+pub(crate) fn draw_request(
+    id: WidgetId,
+    parent: Option<WidgetId>,
+    region: UiRegion,
+    pixel_size: Vec2,
+    slotted: bool,
+) {
+    trace(
+        id,
+        TraceEvent::DrawRequest {
+            id,
+            parent,
+            region,
+            pixel_size,
+            slotted,
+        },
+    );
+}
+
+pub(crate) fn reuse(id: WidgetId, outcome: ReuseOutcome) {
+    trace(id, TraceEvent::Reuse { id, outcome });
+}
+
+pub(crate) fn size_reported(id: WidgetId, size: Size) {
+    trace(id, TraceEvent::SizeReported { id, size });
+}
+
+pub(crate) fn placed(id: WidgetId, parent: WidgetId, region: UiRegion) {
+    trace(id, TraceEvent::Placed { id, parent, region });
+}
+
+pub(crate) fn size_read(id: WidgetId, reader: WidgetId, size: Size) {
+    trace(id, TraceEvent::SizeRead { id, reader, size });
+}
+
+pub(crate) fn hint_read(id: WidgetId, reader: WidgetId, axis: Axis, hint: Option<Len>) {
+    trace(
+        id,
+        TraceEvent::HintRead {
+            id,
+            reader,
+            axis,
+            hint,
+        },
+    );
+}
+
+pub(crate) fn render_text(id: WidgetId, label: &str, width: Option<f32>) {
+    CURRENT.with_borrow_mut(|current| {
+        let calls = current.text_widgets.entry(id).or_default();
+        if calls.label.is_empty() {
+            calls.label = label.to_owned();
+        }
+        calls.count += 1;
+        calls.widths.insert(width.map(f32::to_bits));
+        if current.traced.contains(&id) {
+            current
+                .report
+                .traces
+                .push(TraceEvent::TextRendered { id, width });
+        }
+    });
+}
+
+pub(crate) struct Timer {
+    kind: TimerKind,
+    start: Instant,
+}
+
+pub(crate) fn timer(kind: TimerKind) -> Timer {
+    Timer {
+        kind,
+        start: Instant::now(),
+    }
+}
+
+impl Drop for Timer {
+    fn drop(&mut self) {
+        let nanos = self.start.elapsed().as_nanos().min(u64::MAX as u128) as u64;
+        CURRENT.with_borrow_mut(|current| current.report.nanos[self.kind as usize] += nanos);
+    }
+}
+
+/// Takes all diagnostics accumulated on this thread and resets them.
+pub fn take() -> Report {
+    CURRENT.with_borrow_mut(|current| {
+        current.report.distinct_widgets = current.widgets.len();
+        current.report.distinct_text_widgets = current.text_widgets.len();
+        current.report.hot_widgets = hottest(&current.widgets);
+        current.report.hot_text = hottest(&current.text_widgets);
+        let report = std::mem::take(&mut current.report);
+        current.widgets.clear();
+        current.text_widgets.clear();
+        report
+    })
+}
+
+fn hottest(calls: &HashMap<WidgetId, Calls>) -> Vec<Callsite> {
+    let mut calls: Vec<_> = calls
+        .iter()
+        .map(|(&id, calls)| Callsite {
+            id,
+            label: calls.label.clone(),
+            calls: calls.count,
+            distinct_widths: calls.widths.len(),
+        })
+        .collect();
+    calls.sort_by(|a, b| b.calls.cmp(&a.calls).then_with(|| a.label.cmp(&b.label)));
+    calls.truncate(8);
+    calls
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn taking_a_report_resets_its_counters() {
+        let _ = take();
+        bump(Counter::Updates);
+        bump(Counter::Updates);
+
+        let report = take();
+        assert_eq!(report.counters().next(), Some(("updates", 2)));
+        assert!(take().counters().all(|(_, count)| count == 0));
+    }
+}

core/src/lib.rs

@@ -10,6 +10,9 @@
 #![feature(coerce_unsized)]
 #![feature(option_into_flat_iter)]
 
+#[cfg(feature = "layout-diagnostics")]
+pub mod layout_diagnostics;
+
 mod attr;
 mod event;
 mod num;

core/src/orientation/align.rs

@@ -144,10 +144,10 @@ impl UiScalar {
     pub const fn align(&self, align: AxisAlign) -> UiSpan {
         let rel = align.rel();
         let mut start = UiScalar::rel(rel);
-        start.abs -= self.abs * rel;
+        start.px -= self.px * rel;
         start.rel -= self.rel * rel;
         let mut end = UiScalar::rel(rel);
-        end.abs += self.abs * (1.0 - rel);
+        end.px += self.px * (1.0 - rel);
         end.rel += self.rel * (1.0 - rel);
         UiSpan { start, end }
     }

core/src/orientation/axis.rs

@@ -1,6 +1,6 @@
 use super::*;
 
-#[derive(Copy, Clone, Eq, PartialEq)]
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
 pub enum Axis {
     X,
     Y,

core/src/orientation/len.rs

@@ -9,14 +9,14 @@ pub struct Size {
 
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub struct Len {
-    pub abs: f32,
+    pub px: f32,
     pub rel: f32,
     pub rest: f32,
 }
 
 impl<N: UiNum> From<N> for Len {
     fn from(value: N) -> Self {
-        Len::abs(value.to_f32())
+        Len::px(value.to_f32())
     }
 }
 
@@ -46,10 +46,10 @@ impl Size {
         y: Len::REST,
     };
 
-    pub fn abs(v: Vec2) -> Self {
+    pub fn px(v: Vec2) -> Self {
         Self {
-            x: Len::abs(v.x),
-            y: Len::abs(v.y),
+            x: Len::px(v.x),
+            y: Len::px(v.y),
         }
     }
 
@@ -97,13 +97,13 @@ impl Size {
 
 impl Len {
     pub const ZERO: Self = Self {
-        abs: 0.0,
+        px: 0.0,
         rel: 0.0,
         rest: 0.0,
     };
 
     pub const REST: Self = Self {
-        abs: 0.0,
+        px: 0.0,
         rel: 0.0,
         rest: 1.0,
     };
@@ -111,27 +111,27 @@ impl Len {
     pub fn apply_rest(&self) -> UiScalar {
         UiScalar {
             rel: self.rel + if self.rest > 0.0 { 1.0 } else { 0.0 },
-            abs: self.abs,
+            px: self.px,
         }
     }
 
-    pub fn abs(abs: impl UiNum) -> Self {
+    pub fn px(px: impl UiNum) -> Self {
         Self {
-            abs: abs.to_f32(),
+            px: px.to_f32(),
             rel: 0.0,
             rest: 0.0,
         }
     }
     pub fn rel(rel: impl UiNum) -> Self {
         Self {
-            abs: 0.0,
+            px: 0.0,
             rel: rel.to_f32(),
             rest: 0.0,
         }
     }
     pub fn rest(ratio: impl UiNum) -> Self {
         Self {
-            abs: 0.0,
+            px: 0.0,
             rel: 0.0,
             rest: ratio.to_f32(),
         }
@@ -141,31 +141,31 @@ impl Len {
 pub mod len_fns {
     use super::*;
 
-    pub fn abs(abs: impl UiNum) -> Len {
+    pub fn px(px: impl UiNum) -> Len {
         Len {
-            abs: abs.to_f32(),
+            px: px.to_f32(),
             rel: 0.0,
             rest: 0.0,
         }
     }
     pub fn rel(rel: impl UiNum) -> Len {
         Len {
-            abs: 0.0,
+            px: 0.0,
             rel: rel.to_f32(),
             rest: 0.0,
         }
     }
     pub fn rest(ratio: impl UiNum) -> Len {
         Len {
-            abs: 0.0,
+            px: 0.0,
             rel: 0.0,
             rest: ratio.to_f32(),
         }
     }
 }
 
-impl_op!(Len Add add; abs rel rest);
-impl_op!(Len Sub sub; abs rel rest);
+impl_op!(Len Add add; px rel rest);
+impl_op!(Len Sub sub; px rel rest);
 
 impl_op!(Size Add add; x y);
 impl_op!(Size Sub sub; x y);
@@ -184,8 +184,8 @@ impl std::fmt::Display for Size {
 
 impl std::fmt::Display for Len {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        if self.abs != 0.0 {
-            write!(f, "{} abs;", self.abs)?;
+        if self.px != 0.0 {
+            write!(f, "{} px;", self.px)?;
         }
         if self.rel != 0.0 {
             write!(f, "{} rel;", self.rel)?;

core/src/orientation/pos.rs

@@ -23,11 +23,11 @@ impl UiVec2 {
         Self { x, y }
     }
 
-    pub const fn abs(abs: impl const Into<Vec2>) -> Self {
-        let abs = abs.into();
+    pub const fn px(px: impl const Into<Vec2>) -> Self {
+        let px = px.into();
         Self {
-            x: UiScalar::abs(abs.x),
-            y: UiScalar::abs(abs.y),
+            x: UiScalar::px(px.x),
+            y: UiScalar::px(px.y),
         }
     }
 
@@ -70,10 +70,10 @@ impl UiVec2 {
         }
     }
 
-    pub fn to_abs(&self, rel: Vec2) -> Vec2 {
+    pub fn to_px(&self, rel: Vec2) -> Vec2 {
         Vec2 {
-            x: self.x.to_abs(rel.x),
-            y: self.y.to_abs(rel.y),
+            x: self.x.to_px(rel.x),
+            y: self.y.to_px(rel.y),
         }
     }
 
@@ -92,8 +92,8 @@ impl UiVec2 {
         }
     }
 
-    pub fn get_abs(&self) -> Vec2 {
-        (self.x.abs, self.y.abs).into()
+    pub fn get_px(&self) -> Vec2 {
+        (self.x.px, self.y.px).into()
     }
 
     pub fn get_rel(&self) -> Vec2 {
@@ -102,15 +102,15 @@ impl UiVec2 {
 
     pub fn abs_mut(&mut self) -> Vec2View<'_> {
         Vec2View {
-            x: &mut self.x.abs,
-            y: &mut self.y.abs,
+            x: &mut self.x.px,
+            y: &mut self.y.px,
         }
     }
 }
 
 impl Display for UiVec2 {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        write!(f, "rel{};abs{}", self.get_rel(), self.get_abs())
+        write!(f, "rel{};px{}", self.get_rel(), self.get_px())
     }
 }
 
@@ -118,8 +118,8 @@ impl_op!(UiVec2 Add add; x y);
 impl_op!(UiVec2 Sub sub; x y);
 
 const impl From<Vec2> for UiVec2 {
-    fn from(abs: Vec2) -> Self {
-        Self::abs(abs)
+    fn from(px: Vec2) -> Self {
+        Self::px(px)
     }
 }
 
@@ -127,8 +127,8 @@ const impl<T: const UiNum, U: const UiNum> From<(T, U)> for UiVec2
 where
     (T, U): const Destruct,
 {
-    fn from(abs: (T, U)) -> Self {
-        Self::abs(abs)
+    fn from(px: (T, U)) -> Self {
+        Self::px(px)
     }
 }
 
@@ -136,34 +136,34 @@ where
 #[derive(Debug, Copy, Clone, PartialEq, bytemuck::Pod, Default, bytemuck::Zeroable)]
 pub struct UiScalar {
     pub rel: f32,
-    pub abs: f32,
+    pub px: f32,
 }
 
 impl Eq for UiScalar {}
 impl Hash for UiScalar {
     fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
         state.write_u32(self.rel.to_bits());
-        state.write_u32(self.abs.to_bits());
+        state.write_u32(self.px.to_bits());
     }
 }
 
-impl_op!(UiScalar Add add; rel abs);
-impl_op!(UiScalar Sub sub; rel abs);
+impl_op!(UiScalar Add add; rel px);
+impl_op!(UiScalar Sub sub; rel px);
 
 impl UiScalar {
-    pub const ZERO: Self = Self { rel: 0.0, abs: 0.0 };
-    pub const FULL: Self = Self { rel: 1.0, abs: 0.0 };
+    pub const ZERO: Self = Self { rel: 0.0, px: 0.0 };
+    pub const FULL: Self = Self { rel: 1.0, px: 0.0 };
 
-    pub const fn new(rel: f32, abs: f32) -> Self {
-        Self { rel, abs }
+    pub const fn new(rel: f32, px: f32) -> Self {
+        Self { rel, px }
     }
 
     pub const fn rel(rel: f32) -> Self {
-        Self { rel, abs: 0.0 }
+        Self { rel, px: 0.0 }
     }
 
-    pub const fn abs(abs: f32) -> Self {
-        Self { rel: 0.0, abs }
+    pub const fn px(px: f32) -> Self {
+        Self { rel: 0.0, px }
     }
 
     pub const fn rel_min() -> Self {
@@ -177,39 +177,31 @@ impl UiScalar {
     pub const fn max(&self, other: Self) -> Self {
         Self {
             rel: self.rel.max(other.rel),
-            abs: self.abs.max(other.abs),
+            px: self.px.max(other.px),
         }
     }
 
     pub const fn min(&self, other: Self) -> Self {
         Self {
             rel: self.rel.min(other.rel),
-            abs: self.abs.min(other.abs),
+            px: self.px.min(other.px),
         }
     }
 
     pub const fn offset(mut self, amt: f32) -> Self {
-        self.abs += amt;
+        self.px += amt;
         self
     }
 
     pub const fn within(&self, span: &UiSpan) -> Self {
         let anchor = self.rel.lerp(span.start.rel, span.end.rel);
-        let offset = self.abs + self.rel.lerp(span.start.abs, span.end.abs);
+        let offset = self.px + self.rel.lerp(span.start.px, span.end.px);
         Self {
             rel: anchor,
-            abs: offset,
+            px: offset,
         }
     }
 
-    /// Undoes `within`, and `None` where the span has a fixed length: every
-    /// fraction of it lands on the same `rel`, so none can be told apart.
-    pub fn outside(&self, span: &UiSpan) -> Option<Self> {
-        let rel = self.rel.lerp_inv(span.start.rel, span.end.rel)?;
-        let abs = self.abs - rel.lerp(span.start.abs, span.end.abs);
-        Some(Self { rel, abs })
-    }
-
     pub fn within_len(&self, len: UiScalar) -> Self {
         self.within(&UiSpan {
             start: UiScalar::ZERO,
@@ -223,15 +215,15 @@ impl UiScalar {
 
     pub const fn flip(&mut self) {
         self.rel = 1.0 - self.rel;
-        self.abs = -self.abs;
+        self.px = -self.px;
     }
 
     pub const fn to(&self, end: Self) -> UiSpan {
         UiSpan { start: *self, end }
     }
 
-    pub const fn to_abs(&self, rel: f32) -> f32 {
-        self.rel * rel + self.abs
+    pub const fn to_px(&self, rel: f32) -> f32 {
+        self.rel * rel + self.px
     }
 }
 
@@ -263,7 +255,7 @@ impl UiSpan {
         self.start.flip();
         self.end.flip();
         std::mem::swap(&mut self.start.rel, &mut self.end.rel);
-        std::mem::swap(&mut self.start.abs, &mut self.end.abs);
+        std::mem::swap(&mut self.start.px, &mut self.end.px);
     }
 
     pub const fn shift(&mut self, offset: UiScalar) {
@@ -278,13 +270,6 @@ impl UiSpan {
         }
     }
 
-    pub fn outside(&self, parent: &Self) -> Option<Self> {
-        Some(Self {
-            start: self.start.outside(parent)?,
-            end: self.end.outside(parent)?,
-        })
-    }
-
     pub const fn len(&self) -> UiScalar {
         self.end - self.start
     }
@@ -353,8 +338,8 @@ impl UiRegion {
 
     pub fn to_px(&self, size: Vec2) -> PixelRegion {
         PixelRegion {
-            top_left: self.top_left().get_rel() * size + self.top_left().get_abs(),
-            bot_right: self.bot_right().get_rel() * size + self.bot_right().get_abs(),
+            top_left: self.top_left().get_rel() * size + self.top_left().get_px(),
+            bot_right: self.bot_right().get_rel() * size + self.bot_right().get_px(),
         }
     }
 
@@ -397,50 +382,6 @@ impl UiRegion {
     }
 }
 
-/// Taking a drawing out of one box and putting it in another, checked once
-/// for a whole subtree so that applying it cannot fail.
-///
-/// A box of a fixed length holds each part as an offset from its start rather
-/// than as a fraction of it, so those parts can be carried to a box of the
-/// same length but never stretched to a different one.
-#[derive(Debug, Copy, Clone, PartialEq)]
-pub struct Remap {
-    from: UiRegion,
-    to: UiRegion,
-}
-
-impl Remap {
-    pub fn new(from: UiRegion, to: UiRegion) -> Option<Self> {
-        [Axis::X, Axis::Y]
-            .into_iter()
-            .all(|axis| {
-                let (from, to) = (from.axis(axis), to.axis(axis));
-                from.start.rel != from.end.rel || from.len() == to.len()
-            })
-            .then_some(Self { from, to })
-    }
-
-    pub fn apply(&self, region: UiRegion) -> UiRegion {
-        UiRegion {
-            x: Self::span(region.x, self.from.x, self.to.x),
-            y: Self::span(region.y, self.from.y, self.to.y),
-        }
-    }
-
-    fn span(span: UiSpan, from: UiSpan, to: UiSpan) -> UiSpan {
-        match span.outside(&from) {
-            Some(out) => out.within(&to),
-            // `new` admits this only where the two are the same length, so
-            // the difference between their starts is the whole move.
-            None => {
-                let mut span = span;
-                span.shift(to.start - from.start);
-                span
-            }
-        }
-    }
-}
-
 impl Display for UiRegion {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(

core/src/primitive/text.rs

@@ -1,3 +1,5 @@
+#[cfg(feature = "layout-diagnostics")]
+use crate::layout_diagnostics::{self as diag, Counter, TimerKind};
 use crate::{
     Align, GlyphAtlas, GlyphEntry, GlyphKey, PlacedGlyph, RegionAlign, UiColor, util::Vec2,
 };
@@ -5,7 +7,10 @@ use parley::{
     Alignment, AlignmentOptions, FontContext, FontFamily, FontFamilyName, GenericFamily, Layout,
     LayoutContext, LineHeight, PositionedLayoutItem, StyleProperty,
 };
-use std::hash::{DefaultHasher, Hash, Hasher};
+use std::{
+    collections::VecDeque,
+    hash::{DefaultHasher, Hash, Hasher},
+};
 use swash::{
     FontRef,
     scale::{Render, ScaleContext, Source, StrikeWith},
@@ -17,8 +22,32 @@ pub struct TextData {
     pub layout_ctx: LayoutContext<UiColor>,
     scale_ctx: ScaleContext,
     pub atlas: GlyphAtlas,
+    spare: VecDeque<Placed>,
 }
 
+/// The glyphs of one text at one width. A buffer holds the ones it is drawn
+/// as; these are the ones it had before, kept because a container measures a
+/// child by drawing it in a box it may not keep, and so comes back to widths
+/// it has already asked for.
+struct Placed {
+    /// Where the glyphs land is a function of these three and nothing else,
+    /// so no widget or buffer identity is involved and two texts of the same
+    /// words share an answer.
+    text: String,
+    key: LayoutKey,
+    glyphs: RenderedText,
+}
+
+/// How many to keep. Bounding the whole store rather than each buffer is what
+/// makes this a fixed cost instead of one a tree of ten thousand texts pays
+/// ten thousand times; the re-asks come from laying out one subtree, so they
+/// are close together and few are needed. Instructions over 500 resize frames
+/// of `tests/revision_cost.rs`, both the repeating widths and the sweep that
+/// cannot hit across frames: 13.7B at 32, 12.1B at 64, 10.4B and 12.1B at 128,
+/// and nothing past that -- so 128, which is no worse in the case that never
+/// repeats and better in the one that does.
+const SPARE_PLACED: usize = 128;
+
 impl Default for TextData {
     fn default() -> Self {
         Self {
@@ -26,6 +55,7 @@ impl Default for TextData {
             layout_ctx: LayoutContext::new(),
             scale_ctx: ScaleContext::new(),
             atlas: GlyphAtlas::default(),
+            spare: VecDeque::new(),
         }
     }
 }
@@ -76,11 +106,21 @@ impl Default for TextAttrs {
     }
 }
 
+/// How far below the longest line a width may fall and still be answered by
+/// the break in hand. A parent that offers a child the length it reported
+/// composes that length back through the box chain, so the two differ in the
+/// last bits -- and at exactly the longest line, that decides whether a line
+/// fits. Sub-pixel, so no break it admits is one a reader could see.
+const BREAK_EPSILON_PX: f32 = 0.05;
+
 /// Keeps text and its corresponding layout from getting out of sync.
 pub struct TextBuffer {
     text: String,
     layout: Layout<UiColor>,
     layout_key: Option<LayoutKey>,
+    /// The glyphs placed from `layout`, so drawing this text again at the
+    /// width it already has places them once.
+    placed: Option<RenderedText>,
 }
 
 #[derive(PartialEq)]
@@ -95,6 +135,7 @@ impl TextBuffer {
             text: text.into(),
             layout: Layout::new(),
             layout_key: None,
+            placed: None,
         }
     }
 
@@ -119,15 +160,28 @@ impl TextBuffer {
         if text != self.text {
             self.text = text;
             self.layout_key = None;
+            self.placed = None;
         }
     }
 
     /// Invalidates the layout and returns the underlying string for editing.
     pub fn edit(&mut self) -> &mut String {
         self.layout_key = None;
+        self.placed = None;
         &mut self.text
     }
 
+    /// The glyphs of the shaping it is drawn as, once they are placed.
+    pub fn rendered(&self) -> Option<&RenderedText> {
+        self.placed.as_ref()
+    }
+
+    /// The width its shaping wraps at, and `None` where it does not wrap or
+    /// has not been shaped.
+    pub fn wrap_width(&self) -> Option<f32> {
+        self.layout_key.as_ref()?.max_width
+    }
+
     pub fn size(&self) -> Vec2 {
         Vec2::new(self.layout.width(), self.layout.height())
     }
@@ -138,8 +192,58 @@ impl TextBuffer {
             max_width: width,
         };
         if self.layout_key.as_ref() == Some(&layout_key) {
+            #[cfg(feature = "layout-diagnostics")]
+            diag::bump(Counter::TextShapeHits);
             return;
         }
+        // A greedy break at one width is the same break at every width down
+        // to the longest line it produced: each line still fits, and none can
+        // take a word that would not fit in the wider box. So the layout in
+        // hand already answers, and re-breaking would only be a chance to
+        // disagree with itself -- which is what happens when a parent offers
+        // a child the length that child just reported, and the two land
+        // either side of a float.
+        if let Some(key) = &self.layout_key
+            && key.attrs == *attrs
+            && let (Some(broke_at), Some(want)) = (key.max_width, width)
+            && want <= broke_at
+            && want + BREAK_EPSILON_PX >= self.layout.width()
+        {
+            #[cfg(feature = "layout-diagnostics")]
+            diag::bump(Counter::TextShapeHits);
+            return;
+        }
+        let same_shaping = self
+            .layout_key
+            .as_ref()
+            .is_some_and(|key| key.attrs == *attrs);
+        let old_key = self.layout_key.replace(layout_key);
+        // The glyphs it holds are of the width it held, which the layout may
+        // well come back to.
+        if let Some(key) = old_key
+            && let Some(glyphs) = self.placed.take()
+        {
+            data.keep_placed(Placed {
+                text: self.text.clone(),
+                key,
+                glyphs,
+            });
+        }
+        // Only the line breaking depends on the width: the shaped runs under
+        // it are a function of the text and the attrs, and parley re-breaks
+        // them in place. So a new width is a break, not a shaping.
+        if same_shaping {
+            #[cfg(feature = "layout-diagnostics")]
+            diag::bump(Counter::TextBreaks);
+            #[cfg(feature = "layout-diagnostics")]
+            let _break = diag::timer(TimerKind::TextBreak);
+            self.break_lines(width);
+            return;
+        }
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::TextShapes);
+        #[cfg(feature = "layout-diagnostics")]
+        let _shape = diag::timer(TimerKind::TextShape);
         let mut builder = data
             .layout_ctx
             .ranged_builder(&mut data.font_ctx, &self.text, 1.0, true);
@@ -150,10 +254,13 @@ impl TextBuffer {
         )));
         builder.push_default(StyleProperty::Brush(attrs.color));
         builder.build_into(&mut self.layout, &self.text);
+        self.break_lines(width);
+    }
+
+    fn break_lines(&mut self, width: Option<f32>) {
         self.layout.break_all_lines(width);
         self.layout
             .align(Alignment::Start, AlignmentOptions::default());
-        self.layout_key = Some(layout_key);
     }
 }
 
@@ -265,18 +372,54 @@ pub struct RenderedText {
 }
 
 impl TextData {
-    pub fn render(
+    /// The glyphs of this text at this width, taken out of what is kept.
+    fn take_placed(&mut self, text: &str, key: &LayoutKey) -> Option<RenderedText> {
+        // From the newest, since a re-ask is usually of something recent.
+        let at = self
+            .spare
+            .iter()
+            .rposition(|spare| spare.key == *key && spare.text == text)?;
+        self.spare.remove(at).map(|spare| spare.glyphs)
+    }
+
+    fn keep_placed(&mut self, placed: Placed) {
+        if self.spare.len() >= SPARE_PLACED {
+            self.spare.pop_front();
+        }
+        self.spare.push_back(placed);
+    }
+
+    pub fn render<'b>(
         &mut self,
-        buffer: &mut TextBuffer,
+        buffer: &'b mut TextBuffer,
         attrs: &TextAttrs,
         width: Option<f32>,
-    ) -> RenderedText {
+    ) -> &'b RenderedText {
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::TextRenders);
+        #[cfg(feature = "layout-diagnostics")]
+        let _render = diag::timer(TimerKind::TextRender);
         buffer.shape(self, attrs, width);
-        let glyphs = self.place(buffer);
-        RenderedText {
-            glyphs,
-            size: buffer.size(),
-            color: attrs.color,
-        }
+        // Only asked for when the buffer no longer holds them: taking one out
+        // of the store to then drop it would throw an answer away.
+        let placed = buffer.placed.take().or_else(|| {
+            let key = buffer.layout_key.as_ref()?;
+            self.take_placed(&buffer.text, key)
+        });
+        let placed = match placed {
+            Some(placed) => placed,
+            None => {
+                #[cfg(feature = "layout-diagnostics")]
+                diag::bump(Counter::GlyphPlacements);
+                #[cfg(feature = "layout-diagnostics")]
+                let _place = diag::timer(TimerKind::GlyphPlacement);
+                RenderedText {
+                    glyphs: self.place(buffer),
+                    size: buffer.size(),
+                    color: attrs.color,
+                }
+            }
+        };
+        buffer.placed.insert(placed)
     }
 }

core/src/render/data.rs

@@ -1,11 +1,10 @@
-use crate::{UiRegion, util::Id};
+use crate::{UiRegion, util::Id, util::Vec2};
 use wgpu::*;
 
 #[repr(C)]
 #[derive(Copy, Clone, bytemuck::Pod, bytemuck::Zeroable, Default)]
 pub struct WindowUniform {
-    pub width: f32,
-    pub height: f32,
+    pub dim: Vec2,
 }
 
 #[repr(C)]
@@ -13,15 +12,17 @@ pub struct WindowUniform {
 pub struct PrimitiveInstance {
     pub region: UiRegion,
     pub mask_idx: MaskIdx,
+    pub move_idx: MoveIdx,
 }
 
 impl PrimitiveInstance {
-    const ATTRIBS: [VertexAttribute; 5] = vertex_attr_array![
+    const ATTRIBS: [VertexAttribute; 6] = vertex_attr_array![
         0 => Float32x2,
         1 => Float32x2,
         2 => Float32x2,
         3 => Float32x2,
         4 => Uint32,
+        5 => Uint32,
     ];
 
     pub fn desc() -> VertexBufferLayout<'static> {
@@ -43,4 +44,45 @@ impl MaskIdx {
 #[derive(Debug, Copy, Clone, bytemuck::Pod, bytemuck::Zeroable)]
 pub struct Mask {
     pub region: UiRegion,
+    pub move_idx: MoveIdx,
+}
+
+/// Its own type rather than another `Id<u32>`, because it sits beside
+/// `MaskIdx` in an instance and the two must not be swappable.
+#[repr(transparent)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, bytemuck::Pod, bytemuck::Zeroable)]
+pub struct MoveIdx(u32);
+
+impl MoveIdx {
+    pub const NONE: Self = Self(u32::MAX);
+
+    pub(crate) fn slot(idx: usize) -> Self {
+        Self(idx as u32)
+    }
+
+    pub(crate) fn idx(self) -> usize {
+        self.0 as usize
+    }
+}
+
+/// One link of the chain a primitive's position is resolved through: the box
+/// its contents are placed within, given in the coordinates of the slot it
+/// names. Moving or resizing a subtree writes its own slot and nothing else.
+///
+/// The identity is `UiRegion::FULL`, not zero: a zeroed entry is a box of no
+/// extent, which collapses everything under it to a point.
+#[repr(C)]
+#[derive(Debug, Copy, Clone)]
+pub struct MoveOffset {
+    pub region: UiRegion,
+    pub parent: MoveIdx,
+}
+
+unsafe impl bytemuck::Pod for MoveOffset {}
+unsafe impl bytemuck::Zeroable for MoveOffset {}
+
+impl MoveOffset {
+    pub fn new(parent: MoveIdx, region: UiRegion) -> Self {
+        Self { region, parent }
+    }
 }

core/src/render/mod.rs

@@ -17,11 +17,15 @@ mod texture;
 mod util;
 
 pub use atlas::*;
-pub use data::{Mask, MaskIdx};
+pub use data::{Mask, MaskIdx, MoveIdx, MoveOffset};
 pub use primitive::*;
 
 const PRELUDE: &str = include_str!("./shader/prelude.wgsl");
 
+fn module_source(wgsl: &str) -> String {
+    format!("{PRELUDE}\n{wgsl}")
+}
+
 pub struct UiRenderNode {
     shared_layout: BindGroupLayout,
     shared_group: BindGroup,
@@ -34,6 +38,7 @@ pub struct UiRenderNode {
     active: Vec<usize>,
     window_buffer: Buffer,
     masks: ArrBuf<Mask>,
+    moves: ArrBuf<MoveOffset>,
 }
 
 struct RenderLayer {
@@ -127,32 +132,35 @@ impl UiRenderNode {
         for primitive in &mut self.primitives {
             primitive.render.update(ui);
         }
+        let mut regroup = false;
         if ui.masks.changed {
             ui.masks.changed = false;
-            if self.masks.update(device, queue, &ui.masks[..]) {
-                self.shared_group = Self::shared_group(
-                    device,
-                    &self.shared_layout,
-                    &self.window_buffer,
-                    &self.masks,
-                );
-            }
+            regroup |= self.masks.update(device, queue, &ui.masks[..]);
+        }
+        if ui_render.moves.changed {
+            ui_render.moves.changed = false;
+            regroup |= self.moves.update(device, queue, ui_render.moves.entries());
+        }
+        if regroup {
+            self.shared_group = Self::shared_group(
+                device,
+                &self.shared_layout,
+                &self.window_buffer,
+                &self.masks,
+                &self.moves,
+            );
         }
     }
 
     pub fn resize(&mut self, size: impl Into<Vec2>, queue: &Queue) {
         let size = size.into();
-        let slice = &[WindowUniform {
-            width: size.x,
-            height: size.y,
-        }];
+        let slice = &[WindowUniform { dim: size }];
         queue.write_buffer(&self.window_buffer, 0, bytemuck::cast_slice(slice));
     }
 
     pub fn new(device: &Device, config: &SurfaceConfiguration) -> Self {
         let window_uniform = WindowUniform {
-            width: config.width as f32,
-            height: config.height as f32,
+            dim: Vec2::new(config.width as f32, config.height as f32),
         };
         let window_buffer = device.create_buffer_init(&BufferInitDescriptor {
             label: Some("window"),
@@ -166,7 +174,13 @@ impl UiRenderNode {
             BufferUsages::STORAGE | BufferUsages::COPY_DST,
             "ui masks",
         );
-        let shared_group = Self::shared_group(device, &shared_layout, &window_buffer, &masks);
+        let moves = ArrBuf::new(
+            device,
+            BufferUsages::STORAGE | BufferUsages::COPY_DST,
+            "ui move offsets",
+        );
+        let shared_group =
+            Self::shared_group(device, &shared_layout, &window_buffer, &masks, &moves);
 
         Self {
             shared_layout,
@@ -177,6 +191,7 @@ impl UiRenderNode {
             layers: HashMap::default(),
             active: Vec::new(),
             masks,
+            moves,
         }
     }
 
@@ -211,7 +226,7 @@ impl UiRenderNode {
     ) -> RenderPipeline {
         let module = device.create_shader_module(ShaderModuleDescriptor {
             label: Some(label),
-            source: ShaderSource::Wgsl(format!("{PRELUDE}\n{wgsl}").into()),
+            source: ShaderSource::Wgsl(module_source(wgsl).into()),
         });
         device.create_render_pipeline(&RenderPipelineDescriptor {
             label: Some(label),
@@ -252,7 +267,8 @@ impl UiRenderNode {
         })
     }
 
-    /// What every draw in the ui is given: the window and the masks.
+    /// What every draw in the ui is given: the window, the masks and the
+    /// move chain every position is resolved through.
     fn shared_layout(device: &Device) -> BindGroupLayout {
         device.create_bind_group_layout(&BindGroupLayoutDescriptor {
             entries: &[
@@ -276,6 +292,16 @@ impl UiRenderNode {
                     },
                     count: None,
                 },
+                BindGroupLayoutEntry {
+                    binding: 2,
+                    visibility: ShaderStages::VERTEX | ShaderStages::FRAGMENT,
+                    ty: BindingType::Buffer {
+                        ty: BufferBindingType::Storage { read_only: true },
+                        has_dynamic_offset: false,
+                        min_binding_size: BufferSize::new(size_of::<MoveOffset>() as u64),
+                    },
+                    count: None,
+                },
             ],
             label: Some("ui shared"),
         })
@@ -286,6 +312,7 @@ impl UiRenderNode {
         layout: &BindGroupLayout,
         window: &Buffer,
         masks: &ArrBuf<Mask>,
+        moves: &ArrBuf<MoveOffset>,
     ) -> BindGroup {
         device.create_bind_group(&BindGroupDescriptor {
             layout,
@@ -298,6 +325,10 @@ impl UiRenderNode {
                     binding: 1,
                     resource: masks.buffer.as_entire_binding(),
                 },
+                BindGroupEntry {
+                    binding: 2,
+                    resource: moves.buffer.as_entire_binding(),
+                },
             ],
             label: Some("ui shared"),
         })
@@ -374,3 +405,35 @@ impl ListBuffers {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::module_source;
+    use wgpu::naga::{
+        front::wgsl,
+        valid::{Capabilities, ValidationFlags, Validator},
+    };
+
+    /// Every shader file, composed as the renderer composes it, parses and
+    /// validates with no device -- so an edit that breaks one fails here and
+    /// not in the first window opened.
+    #[test]
+    fn every_shader_validates() {
+        let dir = concat!(env!("CARGO_MANIFEST_DIR"), "/src/render/shader");
+        let mut checked = 0;
+        for entry in std::fs::read_dir(dir).unwrap() {
+            let path = entry.unwrap().path();
+            if path.extension().is_none_or(|e| e != "wgsl") || path.ends_with("prelude.wgsl") {
+                continue;
+            }
+            let source = module_source(&std::fs::read_to_string(&path).unwrap());
+            let module = wgsl::parse_str(&source)
+                .unwrap_or_else(|e| panic!("{}: {}", path.display(), e.emit_to_string(&source)));
+            Validator::new(ValidationFlags::all(), Capabilities::all())
+                .validate(&module)
+                .unwrap_or_else(|e| panic!("{}: {e:?}", path.display()));
+            checked += 1;
+        }
+        assert!(checked > 0, "no shaders found in {dir}");
+    }
+}

core/src/render/primitive.rs

@@ -3,7 +3,7 @@ use std::{any::TypeId, marker::PhantomData};
 use crate::{
     Color, TextureHandle, UiData, UiRegion, WidgetId,
     render::{
-        data::{MaskIdx, PrimitiveInstance},
+        data::{MaskIdx, MoveIdx, PrimitiveInstance},
         page::GlyphRender,
         texture::ImageRender,
     },
@@ -246,6 +246,7 @@ impl LayerDraws {
             primitive,
             region,
             mask_idx,
+            move_idx,
         }: PrimitiveInst<P>,
     ) -> PrimitiveHandle {
         self.updated = true;
@@ -258,7 +259,11 @@ impl LayerDraws {
             .get_or_insert_with(InstanceList::new::<P>)
             .push(
                 id,
-                PrimitiveInstance { region, mask_idx },
+                PrimitiveInstance {
+                    region,
+                    mask_idx,
+                    move_idx,
+                },
                 bytemuck::bytes_of(&primitive),
             );
         PrimitiveHandle {
@@ -304,6 +309,7 @@ pub struct PrimitiveInst<P> {
     pub primitive: P,
     pub region: UiRegion,
     pub mask_idx: MaskIdx,
+    pub move_idx: MoveIdx,
 }
 
 pub struct PrimitiveChange {
@@ -347,7 +353,7 @@ impl RectPrimitive {
 
 /// `color` is multiplied by the atlas alpha for a mask glyph; a colour glyph
 /// takes the texel unchanged, which `GlyphEntry::IS_COLORED` selects.
-#[repr(C, align(8))]
+#[repr(C)]
 #[derive(Debug, Copy, Clone)]
 pub struct GlyphPrimitive {
     pub uv_min: Vec2,
@@ -358,8 +364,8 @@ pub struct GlyphPrimitive {
     pub flags: u32,
 }
 
-// Manual rather than derived: the align(8) leaves four bytes of padding, which
-// is how WGSL lays the struct out.
+// Manual rather than derived: `Vec2`'s alignment leaves four bytes of padding
+// here, which is how WGSL lays the struct out.
 unsafe impl bytemuck::Pod for GlyphPrimitive {}
 unsafe impl bytemuck::Zeroable for GlyphPrimitive {}
 impl Primitive for GlyphPrimitive {

core/src/render/shader/prelude.wgsl

@@ -7,6 +7,8 @@
 var<uniform> window: WindowUniform;
 @group(0) @binding(1)
 var<storage> masks: array<Mask>;
+@group(0) @binding(2)
+var<storage> move_offsets: array<MoveOffset>;
 
 struct WindowUniform {
     dim: vec2<f32>,
@@ -15,6 +17,52 @@ struct WindowUniform {
 struct Mask {
     x: UiSpan,
     y: UiSpan,
+    move_idx: u32,
+}
+
+struct MoveOffset {
+    x: UiSpan,
+    y: UiSpan,
+    parent: u32,
+}
+
+struct Region {
+    x: UiSpan,
+    y: UiSpan,
+}
+
+const MOVE_NONE: u32 = 4294967295u;
+// Keep in step with `iris_core::CHAIN_LIMIT`. It bounds a malformed cycle
+// rather than any real tree, and the CPU walk uses the same number so both
+// resolve a deep one the same way.
+const CHAIN_LIMIT: u32 = 64u;
+
+// Written the way `UiScalar::within` writes it rather than as `mix`, so the
+// CPU and the shader compose a position with the same arithmetic and answer
+// the same thing about where a widget is.
+fn scalar_within(s: UiScalar, p: UiSpan) -> UiScalar {
+    return UiScalar(
+        p.start.rel + (p.end.rel - p.start.rel) * s.rel,
+        s.px + (p.start.px + (p.end.px - p.start.px) * s.rel),
+    );
+}
+
+fn span_within(s: UiSpan, p: UiSpan) -> UiSpan {
+    return UiSpan(scalar_within(s.start, p), scalar_within(s.end, p));
+}
+
+fn resolve_move(idx: u32, local: Region) -> Region {
+    var r = local;
+    var at = idx;
+    for (var step = 0u; step < CHAIN_LIMIT; step++) {
+        if at == MOVE_NONE {
+            break;
+        }
+        let entry = move_offsets[at];
+        r = Region(span_within(r.x, entry.x), span_within(r.y, entry.y));
+        at = entry.parent;
+    }
+    return r;
 }
 
 struct UiSpan {
@@ -24,7 +72,7 @@ struct UiSpan {
 
 struct UiScalar {
     rel: f32,
-    abs: f32,
+    px: f32,
 }
 
 struct InstanceInput {
@@ -33,6 +81,7 @@ struct InstanceInput {
     @location(2) y_start: vec2<f32>,
     @location(3) y_end: vec2<f32>,
     @location(4) mask_idx: u32,
+    @location(5) move_idx: u32,
 }
 
 struct VertexOutput {
@@ -52,13 +101,18 @@ fn vs_main(
 ) -> VertexOutput {
     var out: VertexOutput;
 
-    let top_left_rel = vec2(in.x_start.x, in.y_start.x);
-    let top_left_abs = vec2(in.x_start.y, in.y_start.y);
-    let bot_right_rel = vec2(in.x_end.x, in.y_end.x);
-    let bot_right_abs = vec2(in.x_end.y, in.y_end.y);
+    let local = Region(
+        UiSpan(UiScalar(in.x_start.x, in.x_start.y), UiScalar(in.x_end.x, in.x_end.y)),
+        UiSpan(UiScalar(in.y_start.x, in.y_start.y), UiScalar(in.y_end.x, in.y_end.y)),
+    );
+    let r = resolve_move(in.move_idx, local);
+    let top_left_rel = vec2(r.x.start.rel, r.y.start.rel);
+    let top_left_px = vec2(r.x.start.px, r.y.start.px);
+    let bot_right_rel = vec2(r.x.end.rel, r.y.end.rel);
+    let bot_right_px = vec2(r.x.end.px, r.y.end.px);
 
-    let top_left = floor(top_left_rel * window.dim) + floor(top_left_abs);
-    let bot_right = floor(bot_right_rel * window.dim) + floor(bot_right_abs);
+    let top_left = floor(top_left_rel * window.dim) + floor(top_left_px);
+    let bot_right = floor(bot_right_rel * window.dim) + floor(bot_right_px);
     let size = bot_right - top_left;
 
     let uv = vec2<f32>(
@@ -81,13 +135,16 @@ fn masked(in: VertexOutput, color: vec4<f32>) -> vec4<f32> {
         return color;
     }
     let mask = masks[in.mask_idx];
-    let tl = vec2(mask.x.start.rel, mask.y.start.rel);
-    let tl_abs = vec2(mask.x.start.abs, mask.y.start.abs);
-    let br = vec2(mask.x.end.rel, mask.y.end.rel);
-    let br_abs = vec2(mask.x.end.abs, mask.y.end.abs);
+    // Its own chain, not the drawn primitive's, so a stationary viewport
+    // clips content that moves inside it.
+    let m = resolve_move(mask.move_idx, Region(mask.x, mask.y));
+    let tl = vec2(m.x.start.rel, m.y.start.rel);
+    let tl_px = vec2(m.x.start.px, m.y.start.px);
+    let br = vec2(m.x.end.rel, m.y.end.rel);
+    let br_px = vec2(m.x.end.px, m.y.end.px);
 
-    let top_left = floor(tl * window.dim) + floor(tl_abs);
-    let bot_right = floor(br * window.dim) + floor(br_abs);
+    let top_left = floor(tl * window.dim) + floor(tl_px);
+    let bot_right = floor(br * window.dim) + floor(br_px);
     let pos = in.clip_position.xy;
     if pos.x < top_left.x || pos.x > bot_right.x || pos.y < top_left.y || pos.y > bot_right.y {
         return color * 0.0;

core/src/ui/active.rs

@@ -1,4 +1,6 @@
-use crate::{LayerId, MaskIdx, PrimitiveHandle, Size, TextureHandle, UiRegion, WidgetId};
+use crate::{
+    LayerId, MaskIdx, MoveIdx, PrimitiveHandle, Size, TextureHandle, UiRegion, WidgetId, util::Vec2,
+};
 
 /// important non rendering data for retained drawing
 #[derive(Debug)]
@@ -7,14 +9,37 @@ pub struct ActiveData {
     pub region: UiRegion,
     /// What the widget said it used of `region`, the last time it drew.
     pub size: Size,
+    /// The pixel size of the box it drew against. `region` alone cannot say:
+    /// it is a fraction of a slot's box, and the same fraction of a box that
+    /// has since changed is a different number of pixels.
+    pub px: Vec2,
+    /// The pixel size of the box its parent first asked about it in, before
+    /// knowing what it came to. `px` may be a box derived from that answer,
+    /// and a size measured there is only the same answer asked again.
+    pub offered_px: Vec2,
     pub parent: Option<WidgetId>,
+    /// How far down the tree it was drawn, the root being 1. Carried down a
+    /// draw rather than worked out by walking up, so it is right for every
+    /// widget a frame visits and cannot drift while one is being drawn.
+    pub depth: usize,
     pub textures: Vec<TextureHandle>,
     pub primitives: Vec<PrimitiveHandle>,
     pub children: Vec<WidgetId>,
     /// The children whose size this widget read while drawing.
     pub size_deps: Vec<WidgetId>,
-    /// Whether it read the output's size, and so is wrong when that changes.
-    pub reads_output: bool,
+    /// Offered pixel axes which flowed into this widget's reported size,
+    /// directly or through a child size it read.
+    pub size_box_inputs: [bool; 2],
+    /// Output axes read while producing `size`, distinct from the widget's
+    /// own box when that box has a fixed pixel length.
+    pub size_output_inputs: [bool; 2],
+    /// The output dimensions against which those dependencies were observed.
+    pub output_px: Vec2,
+    /// The slot its primitives are positioned through: its own if its parent
+    /// placed it, otherwise the nearest ancestor that has one.
+    pub move_idx: MoveIdx,
+    /// The slot `region` is given in, which is whatever its parent drew in.
+    pub parent_move: MoveIdx,
     pub mask: MaskIdx,
     pub layer: LayerId,
 }

core/src/ui/mod.rs

@@ -1,7 +1,14 @@
 use crate::{
-    Mask, PrimitiveRegistry, TextData, Textures, WeakWidget, WidgetId, Widgets, util::TrackedArena,
+    Mask, MoveIdx, MoveOffset, PrimitiveRegistry, TextData, Textures, UiRegion, WeakWidget,
+    WidgetId, Widgets,
+    util::{Arena, Id, TrackedArena},
 };
 
+/// How far the shader will walk a move chain. It bounds a malformed cycle
+/// rather than any real tree; `Moves::resolve` uses the same number so the
+/// two agree on what a deep tree resolves to.
+pub const CHAIN_LIMIT: u32 = 64;
+
 mod active;
 mod painter;
 mod render_state;
@@ -20,6 +27,88 @@ pub struct UiData {
     pub masks: TrackedArena<Mask, u32>,
 }
 
+/// Where each widget's drawing sits relative to its parent's slot, so moving
+/// a subtree writes one entry rather than every descendant's primitives.
+#[derive(Default)]
+pub struct Moves {
+    arena: Arena<MoveOffset, u32>,
+    pub changed: bool,
+}
+
+impl Moves {
+    pub fn push(&mut self, parent: MoveIdx, region: UiRegion) -> MoveIdx {
+        self.changed = true;
+        MoveIdx::slot(self.arena.push(MoveOffset::new(parent, region)).idx())
+    }
+
+    /// Re-points a slot at a different parent, for a widget drawn somewhere
+    /// else in the tree than it was.
+    pub fn set_parent(&mut self, idx: MoveIdx, parent: MoveIdx) {
+        let entry = self.arena.get_mut(Id::preset(idx.idx() as u32));
+        if entry.parent != parent {
+            entry.parent = parent;
+            self.changed = true;
+        }
+    }
+
+    pub fn remove(&mut self, idx: MoveIdx) {
+        self.changed = true;
+        self.arena.remove(Id::preset(idx.idx() as u32));
+    }
+
+    /// Sets the box a slot's contents are placed within, itself given in the
+    /// coordinates of its parent slot.
+    pub fn set(&mut self, idx: MoveIdx, region: UiRegion) {
+        let entry = self.arena.get_mut(Id::preset(idx.idx() as u32));
+        if entry.region != region {
+            entry.region = region;
+            self.changed = true;
+        }
+    }
+
+    /// Composes a region held in `idx`'s coordinates down the chain, which is
+    /// the same walk the vertex shader does.
+    pub fn resolve(&self, idx: MoveIdx, local: UiRegion) -> UiRegion {
+        let mut region = local;
+        let mut at = idx;
+        for _ in 0..CHAIN_LIMIT {
+            if at == MoveIdx::NONE {
+                return region;
+            }
+            let entry = self.arena[at.idx()];
+            region = region.within(&entry.region);
+            at = entry.parent;
+        }
+        debug_assert!(
+            at == MoveIdx::NONE,
+            "a move chain longer than {CHAIN_LIMIT} resolves to the wrong place, \
+             and the shader stops at the same depth"
+        );
+        region
+    }
+
+    /// How many slots a region in `idx` is composed through, which is what
+    /// the shader's walk costs per primitive.
+    pub fn depth(&self, idx: MoveIdx) -> usize {
+        let mut depth = 0;
+        let mut at = idx;
+        while at != MoveIdx::NONE && depth < CHAIN_LIMIT as usize {
+            at = self.arena[at.idx()].parent;
+            depth += 1;
+        }
+        depth
+    }
+
+    pub fn entries(&self) -> &[MoveOffset] {
+        &self.arena
+    }
+
+    pub fn clear(&mut self) {
+        self.changed = true;
+        self.arena = Arena::default();
+    }
+}
+
 pub trait UiRsc {
     fn ui(&self) -> &UiData;
     fn ui_mut(&mut self) -> &mut UiData;

core/src/ui/painter.rs

@@ -1,9 +1,11 @@
+#[cfg(feature = "layout-diagnostics")]
+use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
     Axis, Len, RenderedText, Size, StrongWidget, TextAttrs, TextBuffer, TextData, TextureHandle,
     UiRegion, UiRenderState, UiRsc, UiScalar, UiVec2, WidgetId,
     render::{
-        GlyphPrimitive, Mask, MaskIdx, Primitive, PrimitiveHandle, PrimitiveInst, PrimitiveKind,
-        TexturePrimitive,
+        GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveHandle, PrimitiveInst,
+        PrimitiveKind, TexturePrimitive,
     },
     util::Vec2,
 };
@@ -13,15 +15,25 @@ pub struct Painter<'a> {
     pub(super) state: &'a mut UiRenderState,
     pub(super) rsc: &'a mut dyn UiRsc,
 
+    /// This widget's box, in the coordinates of `move_idx`.
     pub(super) region: UiRegion,
     pub(super) mask: MaskIdx,
     pub(super) textures: Vec<TextureHandle>,
     pub(super) primitives: Vec<PrimitiveHandle>,
     pub(super) children: Vec<WidgetId>,
+    /// The children asked about so far, so the first box each was asked
+    /// about is the one recorded as its offer.
+    pub(super) offered: Vec<WidgetId>,
     /// The children whose size this widget read while drawing.
     pub(super) size_deps: Vec<WidgetId>,
-    pub(super) reads_output: bool,
+    /// Offered pixel axes which can affect the size this draw reports.
+    pub(super) size_box_inputs: [bool; 2],
+    pub(super) size_output_inputs: [bool; 2],
+    /// The slot this widget's primitives are positioned through: its own if
+    /// its parent placed it, otherwise the nearest ancestor that has one.
+    pub(super) move_idx: MoveIdx,
     pub layer: usize,
+    pub(super) depth: usize,
     pub(super) id: WidgetId,
 }
 
@@ -33,6 +45,8 @@ impl<'a> Painter<'a> {
 
     /// Takes the kind, for a caller writing many of one primitive.
     fn write<P: Primitive>(&mut self, kind: PrimitiveKind<P>, primitive: P, region: UiRegion) {
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::PrimitiveWrites);
         let h = self.state.layers.write(
             self.layer,
             PrimitiveInst {
@@ -41,6 +55,7 @@ impl<'a> Painter<'a> {
                 primitive,
                 region,
                 mask_idx: self.mask,
+                move_idx: self.move_idx,
             },
         );
         self.push_primitive(h);
@@ -67,29 +82,51 @@ impl<'a> Painter<'a> {
 
     pub fn set_mask(&mut self, region: UiRegion) {
         assert!(self.mask == MaskIdx::NONE);
-        self.mask = self.rsc.ui_mut().masks.push(Mask { region });
+        self.mask = self.rsc.ui_mut().masks.push(Mask {
+            region,
+            move_idx: self.move_idx,
+        });
     }
 
     /// Draws a widget within this widget's region.
     pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
-        self.widget_at(id, self.region)
+        self.widget_at(id, self.region, false)
     }
 
-    /// Draws a widget somewhere within this one. Drawing one a second time
-    /// gives it a new box, keeping the drawing it already has where it can.
+    /// Draws a widget somewhere within this one.
     pub fn widget_within<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
         region: UiRegion,
     ) -> DrawResult<'s, 'a, W> {
         let region = region.within(&self.region);
-        self.widget_at(id, region)
+        self.widget_at(id, region, false)
+    }
+
+    /// Draws a child this widget decides the box of, and may decide again
+    /// once it knows what the child came to. The child gets a slot of its
+    /// own, so placing it a second time writes one entry however much it
+    /// drew -- moved or resized alike, since everything under the slot is
+    /// held as a fraction of its box. A child drawn any other way has no slot
+    /// and can only be given a different box by drawing again.
+    pub fn place<'s, W: ?Sized>(
+        &'s mut self,
+        id: &'s StrongWidget<W>,
+        region: UiRegion,
+    ) -> DrawResult<'s, 'a, W> {
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::PlaceCalls);
+        let region = region.within(&self.region);
+        #[cfg(feature = "layout-diagnostics")]
+        diag::placed(id.id(), self.id, region);
+        self.widget_at(id, region, true)
     }
 
     fn widget_at<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
         region: UiRegion,
+        slotted: bool,
     ) -> DrawResult<'s, 'a, W> {
         // A child listed twice would be moved twice.
         if !self.children.contains(&id.id()) {
@@ -100,10 +137,14 @@ impl<'a> Painter<'a> {
             id.id(),
             region,
             Some(self.id),
+            self.depth + 1,
+            self.move_idx,
+            slotted,
             self.mask,
             None,
             self.rsc,
         );
+        self.offer(id.id(), region);
         DrawResult {
             child: id,
             painter: self,
@@ -114,23 +155,119 @@ impl<'a> Painter<'a> {
     /// What a child says its length is without being drawn, if it can say.
     /// Asking counts as reading its size.
     pub fn size_hint<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<Len> {
-        let hint = self.rsc.widgets().get_dyn(id.id())?.size_hint(axis)?;
-        self.depend_on_size(id);
-        Some(hint)
-    }
-
-    fn depend_on_size<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
-        if !self.size_deps.contains(&child.id()) {
-            self.size_deps.push(child.id());
+        let hint = self
+            .rsc
+            .widgets()
+            .get_dyn(id.id())
+            .and_then(|widget| widget.size_hint(axis));
+        #[cfg(feature = "layout-diagnostics")]
+        diag::hint_read(id.id(), self.id, axis, hint);
+        match hint {
+            Some(hint) => {
+                #[cfg(feature = "layout-diagnostics")]
+                diag::bump(Counter::HintHits);
+                self.depend_on_hint(id);
+                Some(hint)
+            }
+            None => {
+                #[cfg(feature = "layout-diagnostics")]
+                diag::bump(Counter::HintMisses);
+                None
+            }
         }
     }
 
-    pub fn render_text(
+    /// A retained child length valid under the region it is about to be
+    /// offered. Unlike a hint, this is contextual: it is kept only when none
+    /// of the offered pixel axes which produced it changed.
+    pub fn known_len<W: ?Sized>(
         &mut self,
-        buffer: &mut TextBuffer,
+        child: &StrongWidget<W>,
+        axis: Axis,
+        region: UiRegion,
+    ) -> Option<Len> {
+        let region = region.within(&self.region);
+        self.offer(child.id(), region);
+        if let Some(hint) = self.size_hint(child, axis) {
+            return Some(hint);
+        }
+        self.retained_size(child, region)
+            .map(|size| size.axis(axis))
+    }
+
+    /// `region` in this widget's own coordinates.
+    fn retained_size<W: ?Sized>(
+        &mut self,
+        child: &StrongWidget<W>,
+        region: UiRegion,
+    ) -> Option<Size> {
+        let (size, box_inputs, output_inputs) =
+            self.state
+                .retained_size(child.id(), region, self.move_idx, self.rsc.widgets())?;
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::RetainedSizeHits);
+        self.depend_on_size_inputs(child, box_inputs, output_inputs);
+        Some(size)
+    }
+
+    /// Records the box a child was first asked about in this draw. Any later
+    /// box this draw gives it was decided knowing its answer, so a size the
+    /// child measures there is not an answer to this widget's question.
+    fn offer(&mut self, child: WidgetId, region: UiRegion) {
+        if self.offered.contains(&child) {
+            return;
+        }
+        self.offered.push(child);
+        let px = self.state.px_of(self.move_idx, region);
+        if let Some(active) = self.state.active.get_mut(&child) {
+            active.offered_px = px;
+        }
+    }
+
+    /// Depends on a length the child gave without being drawn. A hint is
+    /// context-free, so this depends on the child but on no pixel axis.
+    fn depend_on_hint<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
+        self.depend_on_size_inputs(child, [false; 2], [false; 2]);
+    }
+
+    /// Depends on a size the child produced by drawing, which carries
+    /// whatever the child read to produce it.
+    fn depend_on_drawn_size<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
+        let (box_inputs, output_inputs) = self
+            .state
+            .active
+            .get(&child.id())
+            .map_or(([false; 2], [false; 2]), |active| {
+                (active.size_box_inputs, active.size_output_inputs)
+            });
+        self.depend_on_size_inputs(child, box_inputs, output_inputs);
+    }
+
+    fn depend_on_size_inputs<W: ?Sized>(
+        &mut self,
+        child: &StrongWidget<W>,
+        box_inputs: [bool; 2],
+        output_inputs: [bool; 2],
+    ) {
+        if !self.size_deps.contains(&child.id()) {
+            self.size_deps.push(child.id());
+        }
+        for (own, child) in self.size_box_inputs.iter_mut().zip(box_inputs) {
+            *own |= child;
+        }
+        for (own, child) in self.size_output_inputs.iter_mut().zip(output_inputs) {
+            *own |= child;
+        }
+    }
+
+    pub fn render_text<'b>(
+        &mut self,
+        buffer: &'b mut TextBuffer,
         attrs: &TextAttrs,
         width: Option<f32>,
-    ) -> RenderedText {
+    ) -> &'b RenderedText {
+        #[cfg(feature = "layout-diagnostics")]
+        diag::render_text(self.id, self.rsc.widgets().label(self.id), width);
         let ui = self.rsc.ui_mut();
         ui.text.render(buffer, attrs, width)
     }
@@ -142,9 +279,9 @@ impl<'a> Painter<'a> {
             let mut region = origin;
             region.x.end = region.x.start;
             region.y.end = region.y.start;
-            let mut region = region.offset(UiVec2::abs(glyph.offset));
-            region.x.end = region.x.start + UiScalar::abs(glyph.entry.width as f32);
-            region.y.end = region.y.start + UiScalar::abs(glyph.entry.height as f32);
+            let mut region = region.offset(UiVec2::px(glyph.offset));
+            region.x.end = region.x.start + UiScalar::px(glyph.entry.width as f32);
+            region.y.end = region.y.start + UiScalar::px(glyph.entry.height as f32);
             self.write(
                 kind,
                 GlyphPrimitive {
@@ -159,6 +296,8 @@ impl<'a> Painter<'a> {
         }
     }
 
+    /// This widget's box, in the coordinates its own primitives are written
+    /// in -- so a region composed `within` it may be drawn directly.
     pub fn region(&self) -> UiRegion {
         self.region
     }
@@ -166,15 +305,42 @@ impl<'a> Painter<'a> {
     /// The output's size in pixels. A widget that reads it draws again when
     /// the output changes, since nothing else can put that right.
     pub fn output_size(&mut self) -> Vec2 {
-        self.reads_output = true;
+        self.size_output_inputs = [true; 2];
         self.state.output_size
     }
 
-    /// This widget's box in pixels. Resolved against the output's size, so a
-    /// widget that reads it draws again when the output changes.
+    /// One axis of the output in pixels. Prefer this to [`Self::output_size`]
+    /// when the other axis cannot affect the size this widget reports.
+    pub fn output_len(&mut self, axis: Axis) -> f32 {
+        self.size_output_inputs[axis as usize] = true;
+        self.state.output_size.axis(axis)
+    }
+
+    /// This widget's box in pixels. Resolved against the output's size and
+    /// the boxes it sits within, so a widget that reads it draws again when
+    /// the output changes.
     pub fn px_size(&mut self) -> Vec2 {
-        self.reads_output = true;
-        self.region.size().to_abs(self.state.output_size)
+        self.size_box_inputs = [true; 2];
+        let region = self.state.moves.resolve(self.move_idx, self.region);
+        region.size().to_px(self.state.output_size)
+    }
+
+    /// One axis of this widget's box in pixels. Prefer this to
+    /// [`Self::px_size`] when the other axis cannot affect the reported size.
+    pub fn px_len(&mut self, axis: Axis) -> f32 {
+        self.size_box_inputs[axis as usize] = true;
+        self.px_len_for_draw(axis)
+    }
+
+    /// One axis of this widget's box in pixels, for a draw whose reported
+    /// size does not follow from it -- a clamp or a position. Nothing records
+    /// the read, so a size that does depend on it would go stale.
+    pub fn px_len_for_draw(&self, axis: Axis) -> f32 {
+        let region = self.state.moves.resolve(self.move_idx, self.region);
+        region
+            .size()
+            .axis(axis)
+            .to_px(self.state.output_size.axis(axis))
     }
 
     pub fn text_data(&mut self) -> &mut TextData {
@@ -209,7 +375,12 @@ pub struct DrawResult<'p, 'a, W: ?Sized> {
 
 impl<W: ?Sized> DrawResult<'_, '_, W> {
     pub fn size(self) -> Size {
-        self.painter.depend_on_size(self.child);
+        #[cfg(feature = "layout-diagnostics")]
+        {
+            diag::bump(Counter::SizeReads);
+            diag::size_read(self.child.id(), self.painter.id, self.size);
+        }
+        self.painter.depend_on_drawn_size(self.child);
         self.size
     }
 

core/src/ui/render_state.rs

@@ -1,17 +1,40 @@
+#[cfg(feature = "layout-diagnostics")]
+use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
 use crate::{
-    ActiveData, Axis, DrawLayers, IdLike, MaskIdx, OnResize, Painter, PixelRegion, Remap, Size,
-    StrongWidget, UiRegion, UiRsc, WidgetId, Widgets,
-    util::{HashMap, HashSet, Vec2, forget_ref},
+    ActiveData, Axis, DrawLayers, IdLike, MaskIdx, MoveIdx, Moves, OnResize, Painter, PixelRegion,
+    Size, StrongWidget, UiRegion, UiRsc, UiScalar, UiSpan, WidgetId, Widgets,
+    util::{HashMap, HashSet, Vec2},
 };
 
+const AXES: [Axis; 2] = [Axis::X, Axis::Y];
+const LAYOUT_EPSILON_PX: f32 = 0.05;
+
+fn pixel_len_changed(old: f32, new: f32) -> bool {
+    (old - new).abs() > LAYOUT_EPSILON_PX
+}
+
 pub struct UiRenderState {
     pub active: HashMap<WidgetId, ActiveData>,
     pub layers: DrawLayers,
     pub(super) output_size: Vec2,
 
     old_root: Option<WidgetId>,
-    resized: bool,
+    /// The slot every chain bottoms out in, holding the output as a box.
+    root_move: MoveIdx,
+    /// Widgets whose reported size depends on the root box rather than on
+    /// their own, so nothing below them changing length can reach them.
+    root_readers: HashSet<WidgetId>,
+    /// Content/state dirtiness whose retained size cannot answer a layout
+    /// question until that widget has drawn again.
+    invalid_sizes: HashSet<WidgetId>,
+    /// What has already been drawn during the pass under way, so a widget
+    /// reached by redrawing an ancestor is not drawn again on its own
+    /// account. Emptied when the pass ends.
     draw_started: HashSet<WidgetId>,
+    /// A widget's move slot, which outlives any one `ActiveData`: a redraw
+    /// replaces that while its children go on pointing at the slot.
+    slots: HashMap<WidgetId, MoveIdx>,
+    pub moves: Moves,
 }
 
 impl UiRenderState {
@@ -21,14 +44,48 @@ impl UiRenderState {
             layers: Default::default(),
             output_size: Vec2::ZERO,
             old_root: None,
-            resized: false,
+            invalid_sizes: Default::default(),
             draw_started: Default::default(),
+            slots: Default::default(),
+            moves: Default::default(),
+            root_move: MoveIdx::NONE,
+            root_readers: Default::default(),
+        }
+    }
+
+    /// The window as a box, so a chain bottoms out in one rather than in a
+    /// multiplication applied after it. Composing through a box held in
+    /// pixels leaves everything below it in pixels, which is why nothing
+    /// downstream has to know the output's size to resolve a position.
+    fn write_root(&mut self) {
+        let region = UiRegion::new(
+            UiSpan::new(UiScalar::ZERO, UiScalar::px(self.output_size.x)),
+            UiSpan::new(UiScalar::ZERO, UiScalar::px(self.output_size.y)),
+        );
+        match self.root_move == MoveIdx::NONE {
+            true => self.root_move = self.moves.push(MoveIdx::NONE, region),
+            false => self.moves.set(self.root_move, region),
         }
     }
 
     pub fn resize(&mut self, size: impl Into<Vec2>) {
         self.output_size = size.into();
-        self.resized = true;
+        self.write_root();
+    }
+
+    /// Which axes of the root widget's box are no longer the ones the root
+    /// slot holds, which is all a resize now is: one slot written, found by
+    /// the same comparison every other box change is found by.
+    fn root_axes_changed(&self) -> [bool; 2] {
+        let Some(active) = self.old_root.and_then(|root| self.active.get(&root)) else {
+            return [false; 2];
+        };
+        let px = self.px_of(active.parent_move, active.region);
+        let mut changed = [false; 2];
+        for (axis, c) in AXES.into_iter().zip(changed.iter_mut()) {
+            *c = pixel_len_changed(active.px.axis(axis), px.axis(axis));
+        }
+        changed
     }
 
     pub fn output_size(&self) -> Vec2 {
@@ -36,6 +93,13 @@ impl UiRenderState {
     }
 
     pub fn update<'a>(&mut self, root: impl Into<Option<&'a StrongWidget>>, rsc: &mut dyn UiRsc) {
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::Updates);
+        #[cfg(feature = "layout-diagnostics")]
+        let _update = diag::timer(TimerKind::Update);
+        self.invalid_sizes.clear();
+        self.invalid_sizes
+            .extend(rsc.widgets().needs_redraw.iter().copied());
         // safety mechanism for memory leaks; might wanna return a result instead so user can
         // decide whether to panic or not
         if !rsc.widgets().waiting.is_empty() {
@@ -56,27 +120,74 @@ impl UiRenderState {
         if self.root_changed(root) {
             self.redraw_all(root, rsc);
             self.old_root = root.map(|r| r.id());
-        } else if self.resized {
-            // A region is a fraction of the output plus an offset, resolved
-            // against the window in the shader, so a resize moves the whole
-            // drawing on its own. Only a widget that read pixels can be wrong.
-            for (&id, active) in &self.active {
-                if active.reads_output {
-                    rsc.widgets_mut().needs_redraw.insert(id);
+        } else if self.root_axes_changed().iter().any(|&c| c) {
+            // Every box is a part of the root box, so writing it is a box
+            // that changed length like any other. Offering the root widget
+            // its box again puts that through `try_reuse`, which answers per
+            // axis and lets `redraws_under` price the subtree -- rather than
+            // marking it, which would redraw it whichever axis moved. What
+            // that cannot reach is a widget whose size came from the root box
+            // instead of its own, since its own box need not have changed.
+            #[cfg(feature = "layout-diagnostics")]
+            let _marking = diag::timer(TimerKind::ResizeMarking);
+            let changed = self.root_axes_changed();
+            for id in self.root_readers.clone() {
+                let reads = self
+                    .active
+                    .get(&id)
+                    .map_or([false; 2], |active| active.size_output_inputs);
+                if !AXES
+                    .into_iter()
+                    .zip(changed)
+                    .any(|(axis, c)| c && reads[axis as usize])
+                {
+                    continue;
                 }
+                #[cfg(feature = "layout-diagnostics")]
+                diag::bump(Counter::ResizeDependents);
+                rsc.widgets_mut().needs_redraw.insert(id);
+            }
+            if let Some(root) = root {
+                self.draw_inner(
+                    0,
+                    root.id(),
+                    UiRegion::FULL,
+                    None,
+                    1,
+                    self.root_move,
+                    false,
+                    MaskIdx::NONE,
+                    None,
+                    rsc,
+                );
             }
         }
-        self.resized = false;
         if rsc.widgets().has_updates() {
             self.redraw_updates(rsc);
         }
+        self.invalid_sizes.clear();
+        self.draw_started.clear();
     }
 
     fn redraw_all(&mut self, root: Option<&StrongWidget>, rsc: &mut dyn UiRsc) {
+        #[cfg(feature = "layout-diagnostics")]
+        let _layout = diag::timer(TimerKind::FullLayout);
         self.clear(rsc);
         // free all resources & cache
+        self.write_root();
         if let Some(id) = root {
-            self.draw_inner(0, id.id(), UiRegion::FULL, None, MaskIdx::NONE, None, rsc);
+            self.draw_inner(
+                0,
+                id.id(),
+                UiRegion::FULL,
+                None,
+                1,
+                self.root_move,
+                false,
+                MaskIdx::NONE,
+                None,
+                rsc,
+            );
         }
     }
 
@@ -88,53 +199,90 @@ impl UiRenderState {
         id: WidgetId,
         region: UiRegion,
         parent: Option<WidgetId>,
+        depth: usize,
+        parent_move: MoveIdx,
+        slotted: bool,
         mask: MaskIdx,
-        old_children: Option<Vec<WidgetId>>,
+        mut old: Option<ActiveData>,
         rsc: &mut dyn UiRsc,
     ) -> Size {
-        let mut old_children = old_children.unwrap_or_default();
+        #[cfg(feature = "layout-diagnostics")]
+        {
+            diag::bump(Counter::DrawRequests);
+            diag::draw_request(id, parent, region, self.px_of(parent_move, region), slotted);
+        }
         if self.active.contains_key(&id) {
-            if let Some(size) = self.try_reuse(id, region, rsc) {
+            if let Some(size) = self.try_reuse(id, region, depth, parent_move, rsc) {
                 return size;
             }
             // if not, then maintain resize and track old children to remove unneeded
-            let active = self.remove(id, false, rsc).unwrap();
-            old_children = active.children;
+            old = self.remove(id, false, rsc);
         }
 
         // draw widget
+        let (move_idx, local) = match slotted {
+            // Its box becomes its slot's, so it draws in the slot's own
+            // coordinates and the box it was given is one entry to rewrite.
+            true => (self.move_slot(id, parent_move, region), UiRegion::FULL),
+            false => {
+                self.drop_slot(id);
+                (parent_move, region)
+            }
+        };
+        let px = self.px_of(move_idx, local);
+        // Drawn again in a box its parent already decided: the offer is the
+        // one recorded when the parent first asked, not this box.
+        let (old_children, offered_px) = match old {
+            Some(old) => (old.children, old.offered_px),
+            None => (Vec::new(), px),
+        };
         rsc.widgets_mut().needs_redraw.remove(&id);
         self.draw_started.insert(id);
 
         let mut painter = Painter {
             state: self,
-            region,
+            region: local,
             mask,
             layer,
             id,
             textures: Vec::new(),
             primitives: Vec::new(),
             children: Vec::new(),
+            offered: Vec::new(),
             size_deps: Vec::new(),
-            reads_output: false,
+            depth,
+            size_box_inputs: [false; 2],
+            size_output_inputs: [false; 2],
+            move_idx,
             rsc,
         };
 
+        #[cfg(feature = "layout-diagnostics")]
+        {
+            diag::bump(Counter::WidgetDraws);
+            diag::draw_widget(id, painter.rsc.widgets().label(id));
+        }
         let mut widget = painter.rsc.widgets().get_dyn_dynamic(id);
         let size = widget.draw(&mut painter);
         drop(widget);
+        #[cfg(feature = "layout-diagnostics")]
+        diag::size_reported(id, size);
 
         let Painter {
             state: _,
             rsc: _,
-            region,
+            region: _,
             mask,
             textures,
             primitives,
             children,
+            offered: _,
             size_deps,
-            reads_output,
+            size_box_inputs,
+            size_output_inputs,
+            move_idx,
             layer,
+            depth: _,
             id,
         } = painter;
 
@@ -149,16 +297,22 @@ impl UiRenderState {
             id,
             region,
             size,
+            px,
+            offered_px,
             parent,
+            depth,
             textures,
             primitives,
             children,
             size_deps,
-            reads_output,
+            size_box_inputs,
+            size_output_inputs,
+            output_px: self.output_size,
+            move_idx,
+            parent_move,
             mask,
             layer,
         };
-
         // remove old children that weren't kept
         for c in &old_children {
             if !active.children.contains(c) {
@@ -166,54 +320,260 @@ impl UiRenderState {
             }
         }
 
+        match active.size_output_inputs.iter().any(|&reads| reads) {
+            true => self.root_readers.insert(id),
+            false => self.root_readers.remove(&id),
+        };
         rsc.on_draw(&active);
         self.active.insert(id, active);
+        self.invalid_sizes.remove(&id);
         size
     }
 
+    /// The slot a widget's box is held in, made on its first placed draw and
+    /// kept until it stops being drawn -- a redraw replaces its `ActiveData`
+    /// while descendants go on naming the slot.
+    fn move_slot(&mut self, id: WidgetId, parent: MoveIdx, region: UiRegion) -> MoveIdx {
+        if let Some(&idx) = self.slots.get(&id) {
+            self.moves.set_parent(idx, parent);
+            self.moves.set(idx, region);
+            return idx;
+        }
+        let idx = self.moves.push(parent, region);
+        self.slots.insert(id, idx);
+        idx
+    }
+
+    /// Gives up a slot a widget no longer needs, because it is drawn somewhere
+    /// that does not place it. Its descendants name it, so this is only
+    /// reached where they are about to be drawn again.
+    fn drop_slot(&mut self, id: WidgetId) {
+        if let Some(idx) = self.slots.remove(&id) {
+            self.moves.remove(idx);
+        }
+    }
+
+    /// The pixel size of a region held in `slot`'s coordinates.
+    pub(super) fn px_of(&self, slot: MoveIdx, region: UiRegion) -> Vec2 {
+        self.moves
+            .resolve(slot, region)
+            .size()
+            .to_px(self.output_size)
+    }
+
+    /// A clean widget's retained size, when the offered pixel axes which
+    /// produced that answer are unchanged. This observes the old answer only;
+    /// it does not move or otherwise reuse the widget's drawing.
+    pub(super) fn retained_size(
+        &self,
+        id: WidgetId,
+        region: UiRegion,
+        parent_move: MoveIdx,
+        widgets: &Widgets,
+    ) -> Option<(Size, [bool; 2], [bool; 2])> {
+        if self.size_is_invalid(id, widgets) || self.dirty_size_under(id, widgets) {
+            return None;
+        }
+        let active = self.active.get(&id)?;
+        if active.parent_move != parent_move {
+            return None;
+        }
+        let px = self.px_of(parent_move, region);
+        let valid_box = AXES
+            .into_iter()
+            .zip(active.size_box_inputs)
+            .all(|(axis, depends)| {
+                !depends || !pixel_len_changed(active.px.axis(axis), px.axis(axis))
+            });
+        let valid_output =
+            AXES.into_iter()
+                .zip(active.size_output_inputs)
+                .all(|(axis, depends)| {
+                    !depends
+                        || !pixel_len_changed(
+                            active.output_px.axis(axis),
+                            self.output_size.axis(axis),
+                        )
+                });
+        (valid_box && valid_output).then_some((
+            active.size,
+            active.size_box_inputs,
+            active.size_output_inputs,
+        ))
+    }
+
+    fn size_is_invalid(&self, id: WidgetId, widgets: &Widgets) -> bool {
+        self.invalid_sizes.contains(&id) || widgets.needs_redraw.contains(&id)
+    }
+
+    fn dirty_size_under(&self, id: WidgetId, widgets: &Widgets) -> bool {
+        self.active.get(&id).is_some_and(|active| {
+            active.size_deps.iter().any(|child| {
+                self.size_is_invalid(*child, widgets) || self.dirty_size_under(*child, widgets)
+            })
+        })
+    }
+
     /// The drawing a widget already has, kept for a new box if the box has not
     /// changed in a way it depends on.
-    fn try_reuse(&mut self, id: WidgetId, region: UiRegion, rsc: &dyn UiRsc) -> Option<Size> {
+    fn try_reuse(
+        &mut self,
+        id: WidgetId,
+        region: UiRegion,
+        depth: usize,
+        parent_move: MoveIdx,
+        rsc: &mut dyn UiRsc,
+    ) -> Option<Size> {
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::ReuseAttempts);
+        // Only its own dirtiness, not anything dirty under it that could
+        // change the size this hands back. What makes that safe is the order
+        // `redraw_updates` settles in, and nothing else: by the time a reader
+        // draws, everything dirty below it has been drawn and has propagated.
+        // Draw in another order and this returns a stale size -- measured, on
+        // seed 2 of `tests/generated.rs`.
         if rsc.widgets().needs_redraw.contains(&id) {
+            #[cfg(feature = "layout-diagnostics")]
+            {
+                diag::bump(Counter::ReuseDirty);
+                diag::reuse(id, ReuseOutcome::Dirty);
+            }
             return None;
         }
         let active = self.active.get(&id)?;
-        let (size, old) = (active.size, active.region);
-        if old == region {
-            return Some(size);
-        }
-        // TODO: epsilon?
-        if old.size() != region.size() && !self.reusable(id, region, rsc) {
+        // Drawn somewhere else in the tree: its box is in coordinates it no
+        // longer sits in, and its slot names the wrong parent.
+        if active.parent_move != parent_move {
+            #[cfg(feature = "layout-diagnostics")]
+            {
+                diag::bump(Counter::ReuseWrongParent);
+                diag::reuse(id, ReuseOutcome::WrongParent);
+            }
             return None;
         }
-        // Its drawing stands, if the new box can be reached from the old one.
-        self.mov(id, &Remap::new(old, region)?);
+        let (size, old_region, slot, old_px) =
+            (active.size, active.region, active.move_idx, active.px);
+        // In pixels, because `region` is a fraction of a slot's box and that
+        // box may be what changed -- an unchanged fraction of a box half the
+        // size is half the widget.
+        let px = self.px_of(parent_move, region);
+        let mut changed = [false; 2];
+        for (axis, c) in AXES.into_iter().zip(changed.iter_mut()) {
+            *c = pixel_len_changed(old_px.axis(axis), px.axis(axis));
+        }
+        if !changed.iter().any(|&c| c) && old_region == region {
+            #[cfg(feature = "layout-diagnostics")]
+            {
+                diag::bump(Counter::ReuseExact);
+                diag::reuse(id, ReuseOutcome::Exact);
+            }
+            self.keep_depth(id, depth);
+            return Some(size);
+        }
+        // Only a placed widget can be given a different *region* without
+        // drawing again: it has an entry of its own to say where it went,
+        // where an unslotted one shares its parent's and has nothing to
+        // write. Its parent's box changing length is not that -- everything
+        // it drew is a fraction of that box, so the slot already above it
+        // carries the change and `on_resize` below decides whether the
+        // drawing survives it.
+        if slot == parent_move && old_region != region {
+            #[cfg(feature = "layout-diagnostics")]
+            {
+                diag::bump(Counter::ReuseUnslotted);
+                diag::reuse(id, ReuseOutcome::Unslotted);
+            }
+            return None;
+        }
+        if changed.iter().any(|&c| c) {
+            let widget = rsc.widgets().get_dyn(id)?;
+            let redraws = AXES
+                .into_iter()
+                .zip(changed)
+                .any(|(axis, c)| c && widget.on_resize(axis) != OnResize::Scale);
+            // Anything under it that has to be drawn again is drawn by drawing
+            // this, because whatever reads that widget's size sits in between
+            // and has to lay out around what it comes to.
+            if redraws {
+                #[cfg(feature = "layout-diagnostics")]
+                {
+                    diag::bump(Counter::ReuseOwnResize);
+                    diag::reuse(id, ReuseOutcome::OwnResize);
+                }
+                return None;
+            }
+            if self.redraws_under(id, changed, rsc) {
+                #[cfg(feature = "layout-diagnostics")]
+                {
+                    diag::bump(Counter::ReuseDescendantResize);
+                    diag::reuse(id, ReuseOutcome::DescendantResize);
+                }
+                return None;
+            }
+        }
+        if slot != parent_move {
+            self.moves.set(slot, region);
+        }
+        self.keep_depth(id, depth);
+        let active = self.active.get_mut(&id).unwrap();
+        active.region = region;
+        #[cfg(feature = "layout-diagnostics")]
+        {
+            diag::bump(Counter::ReuseMoved);
+            diag::reuse(id, ReuseOutcome::Moved);
+        }
         Some(size)
     }
 
-    /// Whether the widget can keep the drawing it has and be given `region`
-    /// instead, asked one axis at a time: a change on an axis it does not
-    /// depend on costs nothing, whatever it depends on elsewhere.
-    fn reusable(&self, id: WidgetId, region: UiRegion, rsc: &dyn UiRsc) -> bool {
+    /// Whether anything under `id` would have to be drawn again for the box
+    /// it is a fraction of changing length, `changed` saying which axes of
+    /// that box did.
+    ///
+    /// A part of a box with no relative extent on an axis is a fixed length,
+    /// held as offsets from that box's start, and composing anything into it
+    /// leaves no relative extent either. So a widget whose own box did not
+    /// change length has no descendant whose box did, and the walk stops
+    /// there -- an 80-wide child of a widened row is not asked at all.
+    fn redraws_under(&self, id: WidgetId, changed: [bool; 2], rsc: &dyn UiRsc) -> bool {
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::ResizeChecks);
         let Some(active) = self.active.get(&id) else {
             return false;
         };
-        let Some(widget) = rsc.widgets().get_dyn(id) else {
-            return false;
-        };
-        [Axis::X, Axis::Y].into_iter().all(|axis| {
-            let offered = region.axis(axis).len();
-            let had = active.region.axis(axis).len();
-            match widget.on_resize(axis) {
-                OnResize::Scale => true,
-                // `Translate` is not acted on yet, and cannot be until a
-                // drawing can sit somewhere other than its box. `region` is
-                // both the box a widget was given and the box its primitives
-                // are in, and `mov` remaps from it -- so carrying a drawing at
-                // its old size while the box grows makes the next move stretch
-                // it. The offset chain is what separates the two.
-                OnResize::Translate | OnResize::Redraw => offered == had,
+        let size_deps = &active.size_deps;
+        active.children.iter().any(|&child| {
+            #[cfg(feature = "layout-diagnostics")]
+            diag::bump(Counter::ResizeCheckChildren);
+            let Some(data) = self.active.get(&child) else {
+                return false;
+            };
+            let Some(widget) = rsc.widgets().get_dyn(child) else {
+                return true;
+            };
+            // What it drew to learn this child's size was the child in *this*
+            // box, so a different box is a different answer -- unless the
+            // child gave an exact one without being drawn at all.
+            if size_deps.contains(&child) {
+                let measured = AXES
+                    .into_iter()
+                    .zip(changed)
+                    .any(|(axis, c)| c && widget.size_hint(axis).is_none());
+                if measured {
+                    return true;
+                }
             }
+            let mut own = changed;
+            for (axis, c) in AXES.into_iter().zip(own.iter_mut()) {
+                *c &= data.region.axis(axis).len().rel != 0.0;
+            }
+            if !own.iter().any(|&c| c) {
+                return false;
+            }
+            let redraws = AXES
+                .into_iter()
+                .zip(own)
+                .any(|(axis, c)| c && widget.on_resize(axis) != OnResize::Scale);
+            redraws || self.redraws_under(child, own, rsc)
         })
     }
 
@@ -221,27 +581,13 @@ impl UiRenderState {
         let Some(widget) = rsc.widgets().get_dyn(id) else {
             return true;
         };
-        [Axis::X, Axis::Y].into_iter().all(|axis| {
+        AXES.into_iter().all(|axis| {
             widget
                 .size_hint(axis)
                 .is_none_or(|hint| hint == size.axis(axis))
         })
     }
 
-    fn mov(&mut self, id: WidgetId, remap: &Remap) {
-        let active = self.active.get_mut(&id).unwrap();
-        for h in &active.primitives {
-            let region = self.layers[h.layer].region_mut(h);
-            *region = remap.apply(*region);
-        }
-        active.region = remap.apply(active.region);
-        // SAFETY: children cannot be recursive
-        let children = unsafe { forget_ref(&active.children) };
-        for child in children {
-            self.mov(*child, remap);
-        }
-    }
-
     /// NOTE: instance textures are cleared and self.textures freed
     fn remove(&mut self, id: WidgetId, undraw: bool, rsc: &mut dyn UiRsc) -> Option<ActiveData> {
         let mut active = self.active.remove(&id);
@@ -268,6 +614,10 @@ impl UiRenderState {
                 self.remove_rec(*c, rsc);
             }
         }
+        // After the descendants, whose slots name this one as their parent.
+        if let Some(idx) = self.slots.remove(&id) {
+            self.moves.remove(idx);
+        }
         inst
     }
 
@@ -275,18 +625,70 @@ impl UiRenderState {
         for (_, active) in self.active.drain() {
             rsc.on_undraw(&active);
         }
+        self.slots.clear();
+        self.moves.clear();
+        self.root_move = MoveIdx::NONE;
         self.layers.clear();
+        self.invalid_sizes.clear();
+        self.draw_started.clear();
         rsc.widgets_mut().needs_redraw.clear();
         rsc.free();
     }
 
     pub fn redraw_updates(&mut self, rsc: &mut dyn UiRsc) {
-        while let Some(&id) = rsc.widgets().needs_redraw.iter().next() {
+        #[cfg(feature = "layout-diagnostics")]
+        let _layout = diag::timer(TimerKind::IncrementalLayout);
+        // A reader's answer is only valid after every dirty size it reads has
+        // settled, and taking the deepest first is what arranges that --
+        // `try_reuse` hands back a retained size without asking whether
+        // anything dirty sits under it, so this order is load-bearing for the
+        // answer and not only for the cost. Equal-depth widgets are
+        // independent, so their order does not matter. Resize dirtiness already marks whole reader chains, so
+        // choosing their shallowest roots coalesces descendants that share a
+        // reader and gives each changing box its final constraints first.
+        while let Some(id) = {
+            let dirty = rsc.widgets().needs_redraw.iter().copied();
+            dirty.max_by_key(|&id| self.depth(id))
+        } {
+            #[cfg(feature = "layout-diagnostics")]
+            diag::bump(Counter::QueuePops);
             self.redraw(id, rsc);
         }
         rsc.free();
     }
 
+    /// Keeps a reused widget's depth current, since being reused is being
+    /// visited: only a subtree nobody looked at can hold a stale one.
+    fn keep_depth(&mut self, id: WidgetId, depth: usize) {
+        if let Some(active) = self.active.get_mut(&id) {
+            active.depth = depth;
+        }
+    }
+
+    fn depth(&self, id: WidgetId) -> usize {
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::DepthReads);
+        let depth = self.active.get(&id).map_or(1, |active| active.depth);
+        debug_assert_eq!(
+            depth,
+            self.walked_depth(id),
+            "a widget's kept depth is not the one its ancestry says"
+        );
+        depth
+    }
+
+    /// What the kept depth is checked against, and the only thing that reads
+    /// the ancestry to find one.
+    fn walked_depth(&self, id: WidgetId) -> usize {
+        let mut depth = 0;
+        let mut at = Some(id);
+        while let Some(id) = at {
+            at = self.active.get(&id).and_then(|active| active.parent);
+            depth += 1;
+        }
+        depth
+    }
+
     pub fn root_changed<'a>(&self, root: impl Into<Option<&'a StrongWidget>>) -> bool {
         root.into().map(|r| r.id()) != self.old_root
     }
@@ -296,7 +698,9 @@ impl UiRenderState {
         root: impl Into<Option<&'a StrongWidget>>,
         widgets: &Widgets,
     ) -> bool {
-        self.root_changed(root) || self.resized || widgets.has_updates()
+        self.root_changed(root)
+            || self.root_axes_changed().iter().any(|&c| c)
+            || widgets.has_updates()
     }
 
     pub fn active_widgets(&self) -> usize {
@@ -322,22 +726,43 @@ impl UiRenderState {
         }
     }
 
+    /// Where a widget is on screen: its box composed through the boxes it
+    /// sits within, which is the walk the vertex shader does.
     pub fn window_region(&self, id: &impl IdLike) -> Option<PixelRegion> {
-        let region = self.active.get(&id.id())?.region;
+        let active = self.active.get(&id.id())?;
+        let region = self.moves.resolve(active.parent_move, active.region);
         Some(region.to_px(self.output_size))
     }
 
     /// redraws a widget that's currently active (drawn)
     pub fn redraw(&mut self, id: WidgetId, rsc: &mut dyn UiRsc) {
         self.draw_started.remove(&id);
-        // Whoever read this widget's size may be a different size now, so the
-        // highest reader is what draws. Everything between the two is marked
-        // as well: their own boxes have not changed, so the mark is the only
-        // thing stopping the draw reusing its way past this widget.
-        if let Some(top) = self.mark_readers(id, rsc) {
+        if rsc.widgets().needs_redraw.contains(&id) {
+            self.invalid_sizes.insert(id);
+        }
+        // A widget can only answer whether its size changed by drawing in the
+        // box its parent chose. If that box changed in pixels, its retained
+        // placement is stale and the highest size reader must choose the new
+        // box first. The same holds when the box was decided from the
+        // widget's own answer: measuring there again can only repeat it,
+        // whatever the content now says. Otherwise the widget can draw
+        // locally, and its readers only matter if the returned size actually
+        // changed.
+        let box_changed = self.active.get(&id).is_some_and(|active| {
+            let px = self.px_of(active.parent_move, active.region);
+            AXES.into_iter()
+                .any(|axis| pixel_len_changed(active.px.axis(axis), px.axis(axis)))
+        });
+        let top = match box_changed {
+            true => self.top_reader(id),
+            false => None,
+        }
+        .or_else(|| self.derived_box_reader(id));
+        if let Some(top) = top {
+            #[cfg(feature = "layout-diagnostics")]
+            diag::bump(Counter::EagerReaderRedraws);
+            self.mark_below(id, top, rsc);
             self.redraw(top, rsc);
-            // Cleared by that draw if it reached here; if it did not, this is
-            // no longer drawn and asking again would not end.
             rsc.widgets_mut().needs_redraw.remove(&id);
             return;
         }
@@ -350,35 +775,91 @@ impl UiRenderState {
         let Some(active) = self.remove(id, false, rsc) else {
             return;
         };
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::LocalRedraws);
 
-        self.draw_inner(
+        let old_size = active.size;
+        let size = self.draw_inner(
             active.layer,
             id,
             active.region,
             active.parent,
+            active.depth,
+            active.parent_move,
+            active.move_idx != active.parent_move,
             active.mask,
-            Some(active.children),
+            Some(active),
             rsc,
         );
+
+        if size != old_size {
+            #[cfg(feature = "layout-diagnostics")]
+            diag::bump(Counter::SizeChanges);
+            if let Some(parent) = self.active.get(&id).and_then(|active| active.parent)
+                && self
+                    .active
+                    .get(&parent)
+                    .is_some_and(|active| active.size_deps.contains(&id))
+            {
+                // Propagate one dependency edge at a time. If drawing the reader
+                // does not change its own size, nothing above it can observe this.
+                rsc.widgets_mut().needs_redraw.insert(parent);
+                self.invalid_sizes.insert(parent);
+                #[cfg(feature = "layout-diagnostics")]
+                diag::bump(Counter::ReaderEdges);
+            }
+        }
+    }
+
+    /// The highest reader up the chain that gave what it read a box other
+    /// than the one it asked in, on an axis this widget's size reads. Above
+    /// it every box is a constraint rather than an answer. It is the highest
+    /// and not the nearest because a pass-through hands a derived box down
+    /// unchanged.
+    fn derived_box_reader(&self, id: WidgetId) -> Option<WidgetId> {
+        let reads = self.active.get(&id)?.size_box_inputs;
+        let mut top = None;
+        for (active, parent) in self.reader_chain(id) {
+            let px = self.px_of(active.parent_move, active.region);
+            if AXES.into_iter().zip(reads).any(|(axis, r)| {
+                r && pixel_len_changed(active.offered_px.axis(axis), px.axis(axis))
+            }) {
+                top = Some(parent);
+            }
+        }
+        top
     }
 
     /// The furthest ancestor that read this widget's size, directly or through
-    /// widgets that did the same, marking everything below it on the way.
-    fn mark_readers(&self, id: WidgetId, rsc: &mut dyn UiRsc) -> Option<WidgetId> {
-        let mut top = None;
+    /// widgets that did the same.
+    fn top_reader(&self, id: WidgetId) -> Option<WidgetId> {
+        self.reader_chain(id).last().map(|(_, parent)| parent)
+    }
+
+    /// Each widget from `id` upward whose parent read its size, with that
+    /// parent.
+    fn reader_chain(&self, id: WidgetId) -> impl Iterator<Item = (&ActiveData, WidgetId)> {
+        let mut at = Some(id);
+        std::iter::from_fn(move || {
+            let active = self.active.get(&at?)?;
+            let parent = active.parent?;
+            let read = self.active.get(&parent)?.size_deps.contains(&active.id);
+            at = read.then_some(parent);
+            read.then_some((active, parent))
+        })
+    }
+
+    /// Marks everything from `id` up to, and not including, `top`, so that
+    /// drawing `top` draws each of them rather than reusing it.
+    fn mark_below(&self, id: WidgetId, top: WidgetId, rsc: &mut dyn UiRsc) {
         let mut at = id;
-        while let Some(active) = self.active.get(&at)
-            && let Some(parent) = active.parent
-            && self
-                .active
-                .get(&parent)
-                .is_some_and(|p| p.size_deps.contains(&at))
-        {
+        while at != top {
             rsc.widgets_mut().needs_redraw.insert(at);
-            top = Some(parent);
+            let Some(parent) = self.active.get(&at).and_then(|active| active.parent) else {
+                return;
+            };
             at = parent;
         }
-        top
     }
 }
 

core/src/util/arena.rs

@@ -34,6 +34,10 @@ impl<T, I: IdNum> Arena<T, I> {
         self.tracker.free(id);
         self.data[i]
     }
+
+    pub fn get_mut(&mut self, id: Id<I>) -> &mut T {
+        &mut self.data[id.idx()]
+    }
 }
 
 impl<T, I: IdNum> Default for Arena<T, I> {

core/src/util/math.rs

@@ -1,6 +1,5 @@
-pub const trait LerpUtil: Sized {
+pub const trait LerpUtil {
     fn lerp(self, from: Self, to: Self) -> Self;
-    fn lerp_inv(self, from: Self, to: Self) -> Option<Self>;
 }
 
 const impl LerpUtil for f32 {
@@ -9,14 +8,6 @@ const impl LerpUtil for f32 {
     fn lerp(self, from: Self, to: Self) -> Self {
         from + (to - from) * self
     }
-    /// inverse of lerp, and `None` where `from` and `to` are the same point:
-    /// every input lerps to it, so there is no one answer to come back to.
-    fn lerp_inv(self, from: Self, to: Self) -> Option<Self> {
-        match to == from {
-            true => None,
-            false => Some((self - from) / (to - from)),
-        }
-    }
 }
 
 macro_rules! impl_op {

core/src/util/trust.rs

@@ -1,8 +1,3 @@
-#[allow(clippy::missing_safety_doc)]
-pub(crate) unsafe fn forget_ref<'a, T>(x: &T) -> &'a T {
-    unsafe { std::mem::transmute::<&T, &T>(x) }
-}
-
 #[allow(clippy::missing_safety_doc)]
 pub(crate) unsafe fn forget_mut<'a, T>(x: &mut T) -> &'a mut T {
     unsafe { std::mem::transmute::<&mut T, &mut T>(x) }

core/src/util/vec2.rs

@@ -1,7 +1,11 @@
 use crate::util::impl_op;
 use std::{hash::Hash, ops::*};
 
-#[repr(C)]
+/// `align(8)` because that is WGSL's alignment for a `vec2<f32>`, so any GPU
+/// struct holding one is laid out the way its shader reads it without having
+/// to say so itself. Those structs still need a manual `unsafe impl Pod`,
+/// since the trailing padding this introduces is what `derive(Pod)` refuses.
+#[repr(C, align(8))]
 #[derive(Clone, Copy, PartialEq, Default, bytemuck::Pod, bytemuck::Zeroable)]
 pub struct Vec2 {
     pub x: f32,

core/src/widget/mod.rs

@@ -21,6 +21,9 @@ pub use widgets::*;
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
 pub enum OnResize {
     Scale,
+    /// Reserved: nothing reads this yet, so a widget saying it is redrawn.
+    /// Keeping an unchanged drawing in a bigger box needs the widget to say
+    /// *where* in that box it should sit, which is the alignment work.
     Translate,
     #[default]
     Redraw,

examples/random.rs

@@ -0,0 +1,31 @@
+//! The seeded random tree `tests/generated.rs` checks, drawn so it can be
+//! looked at. `IRIS_SEED` and `IRIS_DEPTH` choose which one.
+
+use iris::prelude::*;
+use iris::random::Edits;
+
+fn env(name: &str, fallback: u64) -> u64 {
+    std::env::var(name)
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(fallback)
+}
+
+fn main() {
+    DefaultApp::<State>::run();
+}
+
+#[derive(DefaultUiState)]
+struct State {
+    ui_state: DefaultUiState,
+}
+
+impl DefaultAppState for State {
+    fn new(mut ui_state: DefaultUiState, rsc: &mut DefaultRsc<Self>, _: Proxy<Self>) -> Self {
+        let seed = env("IRIS_SEED", 1);
+        let depth = env("IRIS_DEPTH", 4) as usize;
+        let (root, _) = iris::random::grow(rsc, seed, depth, &Edits::default());
+        ui_state.set_root(root);
+        Self { ui_state }
+    }
+}

scripts/run-headless.sh

@@ -17,6 +17,10 @@
 # custom one would otherwise inherit the other's output and quietly screenshot
 # the wrong size.
 #
+# `--resize WxH@Hz` changes the output under the app once it is up, then
+# screenshots. A resize is its own case: what it has to match is a cold start
+# at that size, byte for byte, and nothing in `cargo test` can see it.
+#
 # `--replay FILE` drives a `.touch` recording into the window through
 # `replay-touch`, which reads it with the same parser `iris::harness` uses. A
 # recording is `<ms> down|move|up <x> <y>` in the output's own pixels. With
@@ -46,6 +50,7 @@ run="${XDG_RUNTIME_DIR:-/tmp}/iris-headless"
 seconds=3
 shot=""
 replay=""
+resize=""
 example=""
 kind=example
 mode=1920x1200@60Hz
@@ -57,13 +62,14 @@ while [ $# -gt 0 ]; do
         --seconds) seconds=$2; shift 2 ;;
         --bin) kind=bin; shift ;;
         --mode) mode=$2; shift 2 ;;
+        --resize) resize=$2; shift 2 ;;
         --replay) replay=$2; shift 2 ;;
         --dir) workdir=$(cd "$2" && pwd); shift 2 ;;
         --) shift; break ;;
         *) example=$1; shift ;;
     esac
 done
-[ -n "$example" ] || { echo "usage: $0 NAME [--bin] [--dir DIR] [--mode WxH@Hz] [--replay TOUCH] [--shot PNG] [--seconds N] [-- cargo args]" >&2; exit 2; }
+[ -n "$example" ] || { echo "usage: $0 NAME [--bin] [--dir DIR] [--mode WxH@Hz] [--resize WxH@Hz] [--replay TOUCH] [--shot PNG] [--seconds N] [-- cargo args]" >&2; exit 2; }
 [ -z "$replay" ] || [ -f "$replay" ] || { echo "run-headless: no touch script at $replay" >&2; exit 2; }
 [ -z "$shot" ] || need grim "the screenshot --shot writes"
 
@@ -142,6 +148,12 @@ while [ $i -lt "$((seconds * 2))" ]; do
     i=$((i + 1)); sleep 0.5
 done
 
+if [ -n "$resize" ] && kill -0 "$pid" 2>/dev/null; then
+    swaymsg output HEADLESS-1 mode "$resize" >/dev/null
+    echo "run-headless: resized to $resize" >&2
+    sleep 2
+fi
+
 if [ -n "$replay" ] && kill -0 "$pid" 2>/dev/null; then
     if [ -n "$shot" ]; then
         grim "${shot%.png}-before.png"

src/lib.rs

@@ -8,6 +8,7 @@
 pub mod default;
 pub mod event;
 pub mod harness;
+pub mod random;
 pub mod widget;
 
 pub use iris_core as core;

src/random.rs

@@ -0,0 +1,317 @@
+//! A seeded random widget tree, for tests and for looking at.
+//!
+//! One seed is one tree, on any machine and after any upgrade, so a test can
+//! grow the same tree twice and a failing seed is reproduced by its number.
+//! `examples/random.rs` draws one; `tests/generated.rs` checks that laying one
+//! out again lands where growing it from scratch would.
+
+use crate::prelude::*;
+use std::collections::HashMap;
+
+/// The declared lengths of one `SetSize`, by axis.
+pub type Lens = [Option<Len>; 2];
+
+/// What a test changes between two trees grown from the same seed, so the
+/// warm one can be mutated and the cold one grown that way to begin with.
+#[derive(Default)]
+pub struct Edits {
+    /// Declared sizes, by the order the `SetSize` wrappers were made.
+    pub sizes: HashMap<usize, Lens>,
+    /// Which children a span has, by the order the spans were made.
+    pub spans: HashMap<usize, SpanEdit>,
+}
+
+#[derive(Default, Clone)]
+pub struct SpanEdit {
+    /// Children to leave out, by index among the ones grown.
+    pub detach: Vec<usize>,
+    /// How many of the span's spares are in it, appended in order.
+    pub attach: usize,
+}
+
+/// xorshift64, written out rather than taken from a crate so that a seed
+/// keeps meaning the same tree.
+pub struct Rng(u64);
+
+impl Rng {
+    pub fn new(seed: u64) -> Self {
+        Self(seed | 1)
+    }
+
+    pub fn bits(&mut self) -> u64 {
+        self.0 ^= self.0 << 13;
+        self.0 ^= self.0 >> 7;
+        self.0 ^= self.0 << 17;
+        self.0
+    }
+
+    pub fn below(&mut self, n: usize) -> usize {
+        (self.bits() % n as u64) as usize
+    }
+
+    pub fn chance(&mut self) -> bool {
+        self.bits() & 1 == 0
+    }
+}
+
+const COLORS: [UiColor; 6] = [
+    UiColor::RED,
+    UiColor::GREEN,
+    UiColor::BLUE,
+    UiColor::YELLOW,
+    UiColor::CYAN,
+    UiColor::MAGENTA,
+];
+
+/// Leaves grown beside every span, for a test to put into it.
+const SPARES: usize = 3;
+
+const WORDS: &str = "Wrapping shapes one source into as many lines as the box \
+    leaves room for, so a paragraph's height is an answer and not a setting.";
+
+/// What growing a tree gives back: every widget in creation order, so two
+/// trees from one seed line up index for index, and the declared sizes, which
+/// are what a test changes to watch the change propagate.
+#[derive(Default)]
+pub struct Tree {
+    pub ids: Vec<WidgetId>,
+    pub sized: Vec<WeakWidget<SetSize>>,
+    pub spans: Vec<Spanned>,
+    pub scrolls: Vec<WeakWidget<Scroll>>,
+    /// Children a `SpanEdit` took out, held so that dropping the last share
+    /// of one does not free its id for the next widget to be given -- which
+    /// would put the two trees' `ids` out of step.
+    pub detached: Vec<StrongWidget>,
+}
+
+/// Branches on a child's measured length. Comparing boxes catches a widget
+/// that moved; this catches one that believed a measurement a cold start
+/// would not have given it, by turning that into a different tree. Its own
+/// configuration never changes, so which side draws is a property of the
+/// layout alone.
+pub struct Branch {
+    pub probe: StrongWidget,
+    pub wide: StrongWidget,
+    pub narrow: StrongWidget,
+    pub threshold: f32,
+}
+
+impl Widget for Branch {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        let mut top = UiRegion::FULL;
+        top.y.end = top.y.start.offset(40.0);
+        let measured = painter.place(&self.probe, top).len(Axis::X);
+        let px = measured.apply_rest().to_px(painter.px_len(Axis::X));
+
+        let mut rest = UiRegion::FULL;
+        rest.y.start = rest.y.start.offset(40.0);
+        match px > self.threshold {
+            true => painter.place(&self.wide, rest),
+            false => painter.place(&self.narrow, rest),
+        };
+        Size::REST
+    }
+}
+
+pub struct Spanned {
+    pub id: WeakWidget<Span>,
+    /// Leaves grown with the span whether or not they end up in it, so both
+    /// trees make the same widgets in the same order either way. Attaching
+    /// one moves it out of here: a widget belongs to one parent, and one that
+    /// belongs to nobody still has to be held or it reads as a leak.
+    pub spares: Vec<StrongWidget>,
+    /// How many children it was grown with, before any edit.
+    pub grown: usize,
+}
+
+/// Grows the tree `seed` describes, `edits` replacing the declared sizes it
+/// would otherwise have given those wrappers.
+pub fn grow<Rsc: UiRsc + 'static>(
+    rsc: &mut Rsc,
+    seed: u64,
+    depth: usize,
+    edits: &Edits,
+) -> (StrongWidget, Tree) {
+    let mut grow = Grow {
+        rsc,
+        rng: Rng::new(seed),
+        tree: Tree::default(),
+        edits,
+    };
+    let root = grow.node(depth);
+    (root, grow.tree)
+}
+
+struct Grow<'a, Rsc> {
+    rsc: &'a mut Rsc,
+    rng: Rng,
+    tree: Tree,
+    edits: &'a Edits,
+}
+
+impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
+    fn leaf(&mut self) -> StrongWidget {
+        let id: StrongWidget = match self.rng.below(4) {
+            // Wrapped and unwrapped, because only one of them reads the width
+            // it is given and so only one has to be drawn again for a new one.
+            0 => wtext(WORDS).size(16).wrap(true).add_strong(self.rsc),
+            1 => wtext("one line, overflowing whatever it is given")
+                .size(16)
+                .wrap(false)
+                .add_strong(self.rsc),
+            _ => {
+                let color = COLORS[self.rng.below(COLORS.len())];
+                let alpha = (self.rng.below(5) * 63) as u8;
+                rect(color.alpha(alpha)).add_strong(self.rsc)
+            }
+        };
+        self.tree.ids.push(id.id());
+        id
+    }
+
+    fn len(&mut self) -> Option<Len> {
+        match self.rng.below(4) {
+            0 => Some(Len::px(20.0 + self.rng.below(180) as f32)),
+            1 => Some(Len::REST),
+            _ => None,
+        }
+    }
+
+    fn align(&mut self) -> Align {
+        let mut axis = || match self.rng.below(4) {
+            0 => None,
+            1 => Some(AxisAlign::Neg),
+            2 => Some(AxisAlign::Center),
+            _ => Some(AxisAlign::Pos),
+        };
+        let (mut x, y) = (axis(), axis());
+        // Aligning on neither axis is just another transparent wrapper and
+        // would leave this branch unexercised.
+        if x.is_none() && y.is_none() {
+            x = Some(AxisAlign::Center);
+        }
+        Align { x, y }
+    }
+
+    /// A declared size over half the tree, kept where a test can change it.
+    fn sized(&mut self, inner: StrongWidget) -> StrongWidget {
+        if !self.rng.chance() {
+            return inner;
+        }
+        let idx = self.tree.sized.len();
+        let lens = [self.len(), self.len()];
+        let lens = self.edits.sizes.get(&idx).copied().unwrap_or(lens);
+        let id = SetSize {
+            inner,
+            x: lens[0],
+            y: lens[1],
+        }
+        .add(self.rsc);
+        self.tree.sized.push(id);
+        self.tree.ids.push(id.id());
+        id.add_strong(self.rsc)
+    }
+
+    fn node(&mut self, depth: usize) -> StrongWidget {
+        if depth == 0 {
+            return self.leaf();
+        }
+        let positioned = self.rng.below(6);
+        if positioned == 0 {
+            // Scrolling reads the pixel length of its box, which nothing
+            // else here does, and gives its child a box longer than its own.
+            let inner = self.node(depth - 1);
+            let inner = self.sized(inner);
+            let axis = if self.rng.chance() { Axis::X } else { Axis::Y };
+            let id = Scroll::new(inner, axis).add(self.rsc);
+            self.tree.scrolls.push(id);
+            self.tree.ids.push(id.id());
+            return id.add_strong(self.rsc);
+        }
+        if positioned == 2 {
+            // Both sides are grown either way, so a tree that draws one has
+            // the same ids as a tree that draws the other.
+            let probe = self.node(depth - 1);
+            let wide = self.node(depth - 1);
+            let narrow = self.node(depth - 1);
+            let threshold = self.rng.below(500) as f32;
+            let id = Branch {
+                probe,
+                wide,
+                narrow,
+                threshold,
+            }
+            .add(self.rsc);
+            self.tree.ids.push(id.id());
+            return id.add_strong(self.rsc);
+        }
+        if positioned == 1 {
+            let inner = self.node(depth - 1);
+            let inner = self.sized(inner);
+            let id = Aligned {
+                inner,
+                align: self.align(),
+            }
+            .add_strong(self.rsc);
+            self.tree.ids.push(id.id());
+            return id;
+        }
+        if self.rng.below(4) == 0 {
+            let inner = self.node(depth - 1);
+            let inner = self.sized(inner);
+            // Each side its own, since a padding that is the same all round
+            // hides anything that treats one edge differently from another.
+            let mut side = || self.rng.below(24) as f32;
+            let padding = Padding {
+                left: side(),
+                right: side(),
+                top: side(),
+                bottom: side(),
+            };
+            let id = Pad { padding, inner }.add_strong(self.rsc);
+            self.tree.ids.push(id.id());
+            return id;
+        }
+        let grown = 2 + self.rng.below(3);
+        let mut children = Vec::with_capacity(grown);
+        for _ in 0..grown {
+            let child = self.node(depth - 1);
+            children.push(self.sized(child));
+        }
+        if self.rng.chance() {
+            let id = Stack {
+                children,
+                size: StackSize::Child(0),
+            }
+            .add_strong(self.rsc);
+            self.tree.ids.push(id.id());
+            return id;
+        }
+        // Grown either way, so the widget after them has the same id in a
+        // tree that leaves them out as in one that puts them in.
+        let mut spares: Vec<StrongWidget> = (0..SPARES).map(|_| self.leaf()).collect();
+        let idx = self.tree.spans.len();
+        let edit = self.edits.spans.get(&idx).cloned().unwrap_or_default();
+        // Highest first, so an index means the same child however many of its
+        // neighbours are going too.
+        let mut detach = edit.detach.clone();
+        detach.sort_unstable();
+        for j in detach.into_iter().rev() {
+            if j < children.len() {
+                self.tree.detached.push(children.remove(j));
+            }
+        }
+        let attach = edit.attach.min(spares.len());
+        children.extend(spares.drain(..attach));
+        let dir = [Dir::RIGHT, Dir::DOWN, Dir::LEFT, Dir::UP][self.rng.below(4)];
+        let id = Span {
+            children,
+            dir,
+            gap: self.rng.below(3) as f32 * 4.0,
+        }
+        .add(self.rsc);
+        self.tree.ids.push(id.id());
+        self.tree.spans.push(Spanned { id, spares, grown });
+        id.add_strong(self.rsc)
+    }
+}

src/widget/image.rs

@@ -8,11 +8,11 @@ pub struct Image {
 impl Widget for Image {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         painter.primitive(&self.handle);
-        Size::abs(self.handle.size())
+        Size::px(self.handle.size())
     }
 
     fn size_hint(&self, axis: Axis) -> Option<Len> {
-        Some(Len::abs(self.handle.size().axis(axis)))
+        Some(Len::px(self.handle.size().axis(axis)))
     }
 
     fn on_resize(&self, _: Axis) -> OnResize {

src/widget/position/align.rs

@@ -7,16 +7,36 @@ pub struct Aligned {
 
 impl Widget for Aligned {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        // Drawn where it may be too big, then given its aligned box once its
-        // size is known.
-        let size = painter.widget(&self.inner).size();
+        let known = match self.align.tuple() {
+            (Some(_), Some(_)) => painter
+                .known_len(&self.inner, Axis::X, UiRegion::FULL)
+                .zip(painter.known_len(&self.inner, Axis::Y, UiRegion::FULL))
+                .map(|(x, y)| Size { x, y }),
+            (Some(_), None) => painter
+                .known_len(&self.inner, Axis::X, UiRegion::FULL)
+                .map(|x| Size { x, y: Len::REST }),
+            (None, Some(_)) => painter
+                .known_len(&self.inner, Axis::Y, UiRegion::FULL)
+                .map(|y| Size { x: Len::REST, y }),
+            (None, None) => Some(Size::REST),
+        };
+        // Drawn where it may be too big only when the aligned axes are not
+        // already known, then given its aligned box once its size is known.
+        let had_size = known.is_some();
+        let size = known.unwrap_or_else(|| painter.place(&self.inner, UiRegion::FULL).size());
         let region = match self.align.tuple() {
             (Some(x), Some(y)) => size.to_uivec2().align(RegionAlign { x, y }),
             (Some(x), None) => UiRegion::new(size.x.apply_rest().align(x), UiSpan::FULL),
             (None, Some(y)) => UiRegion::new(UiSpan::FULL, size.y.apply_rest().align(y)),
             (None, None) => UiRegion::FULL,
         };
-        painter.widget_within(&self.inner, region);
-        size
+        let placed = painter.place(&self.inner, region).size();
+        if had_size { placed } else { size }
+    }
+
+    /// The aligned box is a fraction of its own, so the child keeps its
+    /// length and stays against the edge it was aligned to.
+    fn on_resize(&self, _: Axis) -> OnResize {
+        OnResize::Scale
     }
 }

src/widget/position/layer.rs

@@ -12,4 +12,8 @@ impl Widget for LayerOffset {
         }
         painter.widget(&self.inner).size()
     }
+
+    fn on_resize(&self, _: Axis) -> OnResize {
+        OnResize::Scale
+    }
 }

src/widget/position/max_size.rs

@@ -19,7 +19,7 @@ impl Widget for MaxSize {
 
 fn capped(len: Len, max: Option<Len>, output: f32) -> Len {
     match max {
-        Some(max) if len.apply_rest().to_abs(output) > max.apply_rest().to_abs(output) => max,
+        Some(max) if len.apply_rest().to_px(output) > max.apply_rest().to_px(output) => max,
         _ => len,
     }
 }

src/widget/position/offset.rs

@@ -10,4 +10,8 @@ impl Widget for Offset {
         let region = UiRegion::FULL.offset(self.amt);
         painter.widget_within(&self.inner, region).size()
     }
+
+    fn on_resize(&self, _: Axis) -> OnResize {
+        OnResize::Scale
+    }
 }

src/widget/position/pad.rs

@@ -12,15 +12,21 @@ impl Widget for Pad {
             .size();
         Size {
             x: Len {
-                abs: inner.x.abs + self.padding.left + self.padding.right,
+                px: inner.x.px + self.padding.left + self.padding.right,
                 ..inner.x
             },
             y: Len {
-                abs: inner.y.abs + self.padding.top + self.padding.bottom,
+                px: inner.y.px + self.padding.top + self.padding.bottom,
                 ..inner.y
             },
         }
     }
+
+    /// The padding is an offset from each edge, so a longer box pads the same
+    /// amount and the child takes the rest.
+    fn on_resize(&self, _: Axis) -> OnResize {
+        OnResize::Scale
+    }
 }
 
 pub struct Padding {
@@ -49,10 +55,10 @@ impl Padding {
     }
     pub fn region(&self) -> UiRegion {
         let mut region = UiRegion::FULL;
-        region.x.start.abs += self.left;
-        region.y.start.abs += self.top;
-        region.x.end.abs -= self.right;
-        region.y.end.abs -= self.bottom;
+        region.x.start.px += self.left;
+        region.y.start.px += self.top;
+        region.x.end.px -= self.right;
+        region.y.end.px -= self.bottom;
         region
     }
     pub fn x(amt: impl UiNum) -> Self {

src/widget/position/scroll.rs

@@ -11,17 +11,20 @@ pub struct Scroll {
 
 impl Widget for Scroll {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let output_len = painter.output_size().axis(self.axis);
-        let container_len = painter.region().axis(self.axis).len();
-        // Drawn in the whole container to learn its length, then placed at
-        // the scrolled offset.
-        let child = painter.widget(&self.inner).size();
-        let content_len = child
-            .axis(self.axis)
+        let output_len = painter.output_len(self.axis);
+        // Its size is its content's, whatever box that is scrolled within.
+        let container_len = UiScalar::px(painter.px_len_for_draw(self.axis));
+        // Draw in the whole container only when its scrolling-axis length is
+        // not already known, then place it at the scrolled offset.
+        let known_len = painter.known_len(&self.inner, self.axis, UiRegion::FULL);
+        let measured = known_len.is_none();
+        let child = measured.then(|| painter.place(&self.inner, UiRegion::FULL).size());
+        let content_len = known_len
+            .unwrap_or_else(|| child.unwrap().axis(self.axis))
             .apply_rest()
             .within_len(container_len)
-            .to_abs(output_len);
-        self.container_len = container_len.to_abs(output_len);
+            .to_px(output_len);
+        self.container_len = container_len.to_px(output_len);
         self.content_len = content_len;
 
         if self.snap_end {
@@ -31,8 +34,8 @@ impl Widget for Scroll {
 
         let mut region = UiRegion::FULL.offset(Vec2::from_axis(self.axis, -self.amt, 0.0));
         region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
-        painter.widget_within(&self.inner, region);
-        child
+        let placed = painter.place(&self.inner, region).size();
+        child.unwrap_or(placed)
     }
 }
 

src/widget/position/set_size.rs

@@ -8,7 +8,20 @@ pub struct SetSize {
 
 impl Widget for SetSize {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let child = painter.widget(&self.inner).size();
+        // A declared length is what the child gets, whatever box this widget
+        // was offered before its parent knew that. Measuring it anywhere else
+        // asks about a box it will not have, and the answer on the other axis
+        // is taken under that: a wrapping text measured in the whole width
+        // reports one line, and nothing revisits it once the real width
+        // arrives.
+        let mut region = UiRegion::FULL;
+        for (axis, len) in [(Axis::X, self.x), (Axis::Y, self.y)] {
+            if let Some(len) = len {
+                let span = region.axis_mut(axis);
+                span.end = span.start + len.apply_rest();
+            }
+        }
+        let child = painter.widget_within(&self.inner, region).size();
         Size {
             x: self.x.unwrap_or(child.x),
             y: self.y.unwrap_or(child.y),
@@ -23,4 +36,8 @@ impl Widget for SetSize {
             Axis::Y => self.y,
         }
     }
+
+    fn on_resize(&self, _: Axis) -> OnResize {
+        OnResize::Scale
+    }
 }

src/widget/position/span.rs

@@ -12,17 +12,25 @@ impl Widget for Span {
         let axis = self.dir.axis;
         // A length for every child before any is placed: from its own hint
         // where it has one, and from drawing it where it does not.
-        let lens: Vec<Len> = self
-            .children
-            .iter()
-            .map(|child| match painter.size_hint(child, axis) {
+        let mut cursor = UiScalar::rel_min();
+        let mut lens = Vec::with_capacity(self.children.len());
+        for child in &self.children {
+            let mut span = UiSpan::new(cursor, UiScalar::rel_max());
+            if self.dir.sign == Sign::Neg {
+                span.flip();
+            }
+            let region = UiRegion::from_axis(axis, span, UiSpan::FULL);
+            let len = match painter.known_len(child, axis, region) {
                 Some(len) => len,
-                None => painter.widget(child).len(axis),
-            })
-            .collect();
+                None => painter.place(child, region).len(axis),
+            };
+            cursor.px += len.px + self.gap;
+            cursor.rel += len.rel;
+            lens.push(len);
+        }
 
         let gap = self.gap * self.children.len().saturating_sub(1) as f32;
-        let total = lens.iter().fold(Len::abs(gap), |sum, len| sum + *len);
+        let total = lens.iter().fold(Len::px(gap), |sum, len| sum + *len);
 
         let mut start = UiScalar::rel_min();
         let mut ortho = Len::ZERO;
@@ -30,34 +38,44 @@ impl Widget for Span {
             let mut span = UiSpan::FULL;
             span.start = start;
             if len.rest > 0.0 {
-                let offset = UiScalar::new(total.rel, total.abs);
+                let offset = UiScalar::new(total.rel, total.px);
                 let rel_end = UiScalar::rel(len.rest / total.rest);
                 let end = (UiScalar::rel_max() + start) - offset;
                 start = rel_end.within(&start.to(end));
             }
-            start.abs += len.abs;
+            start.px += len.px;
             start.rel += len.rel;
             span.end = start;
             let mut region = UiRegion::from_axis(axis, span, UiSpan::FULL);
             if self.dir.sign == Sign::Neg {
                 region.flip(axis);
             }
-            let used = painter.widget_within(child, region).size().axis(!axis);
+            let used = painter.place(child, region).size().axis(!axis);
             // TODO: rel shouldn't do this, but no easy way before actually calculating pixels
             if used.rel > 0.0 || used.rest > 0.0 {
                 ortho = Len::REST;
             } else if ortho.rest == 0.0 {
-                ortho.abs = ortho.abs.max(used.abs);
+                ortho.px = ortho.px.max(used.px);
             }
-            start.abs += self.gap;
+            start.px += self.gap;
         }
 
-        let along = match total.rest == 0.0 && total.rel == 0.0 {
-            true => total,
-            false => Len::default(),
-        };
+        // Carried whole rather than collapsed to one share: a span that sizes
+        // from its children does not resolve `rest`, it passes the weight up,
+        // so nesting spans divides the same space rather than re-dividing a
+        // share of it. Four `rest(1)` children under two spans under one span
+        // get a quarter each, which collapsing to `rest(1)` per level does
+        // not give. Resolution happens at the nearest ancestor with a length,
+        // and the root always has one.
+        let along = total;
         Size::from_axis(axis, along, ortho)
     }
+
+    /// Every child is placed in fractions and offsets of the span's own box,
+    /// so a longer box holds the same layout and the children follow it.
+    fn on_resize(&self, _: Axis) -> OnResize {
+        OnResize::Scale
+    }
 }
 
 impl Span {

src/widget/position/stack.rs

@@ -28,6 +28,10 @@ impl Widget for Stack {
         }
         size
     }
+
+    fn on_resize(&self, _: Axis) -> OnResize {
+        OnResize::Scale
+    }
 }
 
 #[derive(Default, Debug)]

src/widget/text/edit.rs

@@ -130,7 +130,6 @@ impl<'a> TextEditCtx<'a> {
     pub fn set(&mut self, text: &str) {
         let text = self.string(text);
         self.text.view.buf.set_text(text);
-        self.text.view.buf.changed = true;
         self.text.selection = None;
     }
 
@@ -177,7 +176,6 @@ impl<'a> TextEditCtx<'a> {
         };
         let at = at.min(self.text.view.buf.text().len());
         self.text.view.buf.edit().insert_str(at, text);
-        self.text.view.buf.changed = true;
         self.set_caret(at + text.len());
     }
 
@@ -190,7 +188,6 @@ impl<'a> TextEditCtx<'a> {
         }
         let range = sel.text_range();
         self.text.view.buf.edit().replace_range(range.clone(), "");
-        self.text.view.buf.changed = true;
         self.set_caret(range.start);
         true
     }
@@ -268,7 +265,6 @@ impl<'a> TextEditCtx<'a> {
 
     fn delete_range(&mut self, start: usize, end: usize) {
         self.text.view.buf.edit().replace_range(start..end, "");
-        self.text.view.buf.changed = true;
         self.set_caret(start);
     }
 
@@ -284,7 +280,7 @@ impl<'a> TextEditCtx<'a> {
     }
 
     pub fn select(&mut self, pos: Vec2, size: Vec2, drag: bool, recent: bool) {
-        let pos = pos - self.text.region().top_left().to_abs(size);
+        let pos = pos - self.text.region().top_left().to_px(size);
         let prev_sel = self.text.selection;
         let prev_hit = self.text.double_hit;
 

src/widget/text/mod.rs

@@ -14,11 +14,8 @@ pub struct Text {
 }
 
 pub struct TextView {
-    pub attrs: MutDetect<TextAttrs>,
-    pub buf: MutDetect<TextBuffer>,
-    // cache
-    tex: Option<RenderedText>,
-    width: Option<f32>,
+    pub attrs: TextAttrs,
+    pub buf: TextBuffer,
     pub hint: Option<StrongWidget>,
 }
 
@@ -28,19 +25,13 @@ impl TextView {
     }
 
     pub fn wrap_width(&self) -> Option<f32> {
-        self.width
+        self.buf.wrap_width()
     }
 }
 
 impl TextView {
     pub fn new(buf: TextBuffer, attrs: TextAttrs, hint: Option<StrongWidget>) -> Self {
-        Self {
-            attrs: attrs.into(),
-            buf: buf.into(),
-            tex: None,
-            width: None,
-            hint,
-        }
+        Self { attrs, buf, hint }
     }
 
     /// region where the text should be draw
@@ -52,22 +43,20 @@ impl TextView {
             .align(self.align)
     }
 
+    /// The text shaped for the width it is drawn in. The buffer keeps its
+    /// answers under the attrs too, so changing those asks a new question
+    /// rather than invalidating anything.
     fn render(&mut self, painter: &mut Painter) -> &RenderedText {
         let width = if self.attrs.wrap {
-            Some(painter.px_size().x)
+            Some(painter.px_len(Axis::X))
         } else {
             None
         };
-        if width != self.width || self.tex.is_none() || self.attrs.changed || self.buf.changed {
-            self.width = width;
-            self.tex = Some(painter.render_text(&mut self.buf, &self.attrs, width));
-            self.attrs.changed = false;
-            self.buf.changed = false;
-        }
-        self.tex.as_ref().unwrap()
+        painter.render_text(&mut self.buf, &self.attrs, width)
     }
+
     pub fn tex(&self) -> Option<&RenderedText> {
-        self.tex.as_ref()
+        self.buf.rendered()
     }
     /// Draws the text, and says where the glyphs went and what they use.
     pub fn draw(&mut self, painter: &mut Painter) -> (UiRegion, Size) {
@@ -83,7 +72,7 @@ impl TextView {
 
         let tex = self.render(painter);
         let region = tex.size.align(align);
-        let size = Size::abs(tex.size);
+        let size = Size::px(tex.size);
         let within = region.within(&painter.region());
         painter.glyphs(tex, within);
         (region, size)

tests/chain_cost.rs

@@ -0,0 +1,224 @@
+//! What the vertex shader's move-chain walk costs, against how deep the chain
+//! is. Every active widget owns a slot, so the depth a primitive resolves
+//! through is its depth in the widget tree.
+//!
+//!     cargo test --release --test chain_cost -- --ignored --nocapture
+//!
+//! Timed on the GPU with timestamp queries rather than by the clock: wall time
+//! here varied by 2x between runs of one unchanged binary. The pass is
+//! submitted and waited on, so this is the GPU's cost and not the recording
+//! loop's -- which is what `draw_cost.rs` measures instead.
+//!
+//! The instances are two pixels wide so that vertex work dominates; a chain
+//! walk that does not show up against small quads will not show up against
+//! anything.
+//!
+//! The instance is leaked deliberately, for the reason `draw_cost.rs` gives.
+
+use iris::prelude::*;
+use iris_core::{
+    MaskIdx, MoveIdx, PrimitiveInst, RectPrimitive, UiData, UiRegion, UiRenderNode, UiRenderState,
+    UiScalar, UiSpan,
+};
+use wgpu::{Color as GpuColor, *};
+
+const SIZE: u32 = 1024;
+const INSTANCES: usize = 200_000;
+const FRAMES: u32 = 20;
+/// Reported as the best of this many batches, since the mean moves by more
+/// than the thing being measured.
+const BATCHES: u32 = 8;
+
+fn gpu() -> Option<(Device, Queue, f32)> {
+    let all = Instance::new(InstanceDescriptor::new_without_display_handle());
+    let instance = match pollster::block_on(all.request_adapter(&RequestAdapterOptions::default()))
+    {
+        Ok(_) => all,
+        Err(_) => Instance::new(InstanceDescriptor {
+            backends: Backends::GL,
+            ..InstanceDescriptor::new_without_display_handle()
+        }),
+    };
+    let instance: &'static Instance = Box::leak(Box::new(instance));
+    let adapter =
+        pollster::block_on(instance.request_adapter(&RequestAdapterOptions::default())).ok()?;
+    if !adapter.features().contains(Features::TIMESTAMP_QUERY) {
+        println!("no timestamp queries on {:?}", adapter.get_info().name);
+        return None;
+    }
+    println!("adapter: {:?}", adapter.get_info().name);
+    let (device, queue) = pollster::block_on(adapter.request_device(&DeviceDescriptor {
+        required_features: Features::TIMESTAMP_QUERY,
+        ..Default::default()
+    }))
+    .ok()?;
+    let period = queue.get_timestamp_period();
+    Some((device, queue, period))
+}
+
+fn config(format: TextureFormat) -> SurfaceConfiguration {
+    SurfaceConfiguration {
+        usage: TextureUsages::RENDER_ATTACHMENT,
+        format,
+        color_space: SurfaceColorSpace::Auto,
+        width: SIZE,
+        height: SIZE,
+        present_mode: PresentMode::Fifo,
+        desired_maximum_frame_latency: 2,
+        alpha_mode: CompositeAlphaMode::Auto,
+        view_formats: vec![],
+    }
+}
+
+/// A chain `depth` slots long, and instances that all resolve through its end.
+fn fill(ui: &mut UiData, render: &mut UiRenderState, depth: usize) {
+    let kind = ui.primitives.kind::<RectPrimitive>();
+    let id = ui.widgets.add_strong(Rect::new(UiColor::WHITE)).id();
+
+    let mut slot = MoveIdx::NONE;
+    for _ in 0..depth {
+        slot = render.moves.push(slot, UiRegion::FULL);
+    }
+
+    let px = |v: f32| UiScalar { rel: 0.0, px: v };
+    for i in 0..INSTANCES {
+        let x = (i % (SIZE as usize / 2)) as f32 * 2.0;
+        let y = (i / (SIZE as usize / 2)) as f32;
+        render.layers.write(
+            0,
+            PrimitiveInst {
+                kind,
+                id,
+                primitive: RectPrimitive::color(UiColor::WHITE),
+                region: UiRegion::new(
+                    UiSpan::new(px(x), px(x + 2.0)),
+                    UiSpan::new(px(y), px(y + 1.0)),
+                ),
+                mask_idx: MaskIdx::NONE,
+                move_idx: slot,
+            },
+        );
+    }
+}
+
+/// Nanoseconds the pass took on the GPU, best of `BATCHES`.
+fn pass_cost(device: &Device, queue: &Queue, period: f32, depth: usize) -> f64 {
+    let format = TextureFormat::Bgra8Unorm;
+    let mut node = UiRenderNode::new(device, &config(format));
+    let mut ui = UiData::default();
+    let mut render = UiRenderState::new();
+    fill(&mut ui, &mut render, depth);
+    node.update(device, queue, &mut ui, &mut render);
+
+    let target = device.create_texture(&TextureDescriptor {
+        label: Some("chain cost"),
+        size: Extent3d {
+            width: SIZE,
+            height: SIZE,
+            depth_or_array_layers: 1,
+        },
+        mip_level_count: 1,
+        sample_count: 1,
+        dimension: TextureDimension::D2,
+        format,
+        usage: TextureUsages::RENDER_ATTACHMENT,
+        view_formats: &[],
+    });
+    let view = target.create_view(&TextureViewDescriptor::default());
+
+    let queries = device.create_query_set(&QuerySetDescriptor {
+        label: Some("chain cost"),
+        ty: QueryType::Timestamp,
+        count: 2,
+    });
+    let resolved = device.create_buffer(&BufferDescriptor {
+        label: Some("resolved"),
+        size: 16,
+        usage: BufferUsages::QUERY_RESOLVE | BufferUsages::COPY_SRC,
+        mapped_at_creation: false,
+    });
+    let readback = device.create_buffer(&BufferDescriptor {
+        label: Some("readback"),
+        size: 16,
+        usage: BufferUsages::MAP_READ | BufferUsages::COPY_DST,
+        mapped_at_creation: false,
+    });
+
+    let frame = || {
+        let mut encoder = device.create_command_encoder(&CommandEncoderDescriptor::default());
+        {
+            let pass = &mut encoder.begin_render_pass(&RenderPassDescriptor {
+                label: None,
+                color_attachments: &[Some(RenderPassColorAttachment {
+                    view: &view,
+                    resolve_target: None,
+                    ops: Operations {
+                        load: LoadOp::Clear(GpuColor::BLACK),
+                        store: StoreOp::Store,
+                    },
+                    depth_slice: None,
+                })],
+                depth_stencil_attachment: None,
+                timestamp_writes: Some(RenderPassTimestampWrites {
+                    query_set: &queries,
+                    beginning_of_pass_write_index: Some(0),
+                    end_of_pass_write_index: Some(1),
+                }),
+                occlusion_query_set: None,
+                multiview_mask: None,
+            });
+            node.draw(pass);
+        }
+        encoder.resolve_query_set(&queries, 0..2, &resolved, 0);
+        encoder.copy_buffer_to_buffer(&resolved, 0, &readback, 0, 16);
+        queue.submit(Some(encoder.finish()));
+
+        let slice = readback.slice(..);
+        slice.map_async(MapMode::Read, |_| {});
+        let _ = device.poll(PollType::Wait {
+            submission_index: None,
+            timeout: None,
+        });
+        let ns = {
+            let view = slice.get_mapped_range().expect("timestamps did not map");
+            let stamps: [u64; 2] = [
+                u64::from_le_bytes(view[..8].try_into().unwrap()),
+                u64::from_le_bytes(view[8..16].try_into().unwrap()),
+            ];
+            (stamps[1].saturating_sub(stamps[0])) as f64 * period as f64
+        };
+        readback.unmap();
+        ns
+    };
+
+    frame();
+    let mut best = f64::MAX;
+    for _ in 0..BATCHES {
+        let mut total = 0.0;
+        for _ in 0..FRAMES {
+            total += frame();
+        }
+        best = best.min(total / FRAMES as f64);
+    }
+    best
+}
+
+#[test]
+#[ignore = "measurement, not a check"]
+fn chain_cost_by_depth() {
+    let Some((device, queue, period)) = gpu() else {
+        println!("no gpu with timestamps; nothing measured");
+        return;
+    };
+    println!("{INSTANCES} instances, {SIZE}x{SIZE}, best of {BATCHES} batches");
+    let mut base = None;
+    for depth in [1, 2, 4, 8, 16, 32, 64] {
+        let ns = pass_cost(&device, &queue, period, depth);
+        let base = *base.get_or_insert(ns);
+        println!(
+            "depth {depth:>3}: {:>9.1} us   {:+6.1}% against depth 1",
+            ns / 1000.0,
+            (ns - base) / base * 100.0
+        );
+    }
+}

tests/determinism.rs

@@ -0,0 +1,101 @@
+//! A measurement that decides control flow.
+//!
+//! Comparing boxes catches a widget that moved. It does not catch a widget
+//! that measured a child, believed a different answer from the one a cold
+//! start would give, and took the other branch -- which is the same defect
+//! arriving somewhere it cannot be ignored. A widget here branches on what it
+//! measured, so a disagreement shows up as a different tree.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+/// Measures `probe` across `axis` and draws one of two children on the
+/// answer. Its own configuration never changes, so which child is drawn is a
+/// property of the layout alone.
+struct BranchesOnMeasurement {
+    probe: StrongWidget,
+    wide: StrongWidget,
+    narrow: StrongWidget,
+    threshold: f32,
+}
+
+impl Widget for BranchesOnMeasurement {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        let mut top = UiRegion::FULL;
+        top.y.end = top.y.start.offset(40.0);
+        let measured = painter.place(&self.probe, top).len(Axis::X);
+        let px = measured.apply_rest().to_px(painter.px_len(Axis::X));
+
+        let mut rest = UiRegion::FULL;
+        rest.y.start = rest.y.start.offset(40.0);
+        match px > self.threshold {
+            true => painter.place(&self.wide, rest),
+            false => painter.place(&self.narrow, rest),
+        };
+        Size::REST
+    }
+}
+
+fn plant(h: &mut Harness, threshold: f32) -> (WidgetId, WidgetId) {
+    let words = "the quick brown fox jumps over the lazy dog and keeps running";
+    let probe = wtext(words).size(16).wrap(true).add(&mut h.rsc);
+    let wide = rect(Color::RED).add(&mut h.rsc);
+    let narrow = rect(Color::BLUE).add(&mut h.rsc);
+    let branch = BranchesOnMeasurement {
+        probe: probe.add_strong(&mut h.rsc),
+        wide: wide.add_strong(&mut h.rsc),
+        narrow: narrow.add_strong(&mut h.rsc),
+        threshold,
+    }
+    .add(&mut h.rsc);
+    let side = rect(Color::GREEN).width(120).add(&mut h.rsc);
+    h.set_root((side, branch).span(Dir::RIGHT));
+    (wide.id(), narrow.id())
+}
+
+/// Which of the two branches drew, as a pair a test can compare.
+fn taken(h: &Harness, wide: WidgetId, narrow: WidgetId) -> (bool, bool) {
+    (h.region(&wide).is_some(), h.region(&narrow).is_some())
+}
+
+#[test]
+fn a_branch_taken_on_a_measurement_holds_across_repaints() {
+    for threshold in [0.0, 200.0, 400.0, 600.0, 779.0, 780.0, 781.0, 2000.0] {
+        let mut h = Harness::new((900, 600));
+        let (wide, narrow) = plant(&mut h, threshold);
+        let first = taken(&h, wide, narrow);
+        assert_ne!(first, (false, false), "threshold {threshold}: neither drew");
+
+        for frame in 0..4 {
+            h.rsc.widgets_mut().get_dyn_mut(wide);
+            h.rsc.widgets_mut().get_dyn_mut(narrow);
+            h.frame();
+            assert_eq!(
+                taken(&h, wide, narrow),
+                first,
+                "threshold {threshold}, repaint {frame}: the branch moved when nothing did"
+            );
+        }
+    }
+}
+
+#[test]
+fn a_branch_taken_on_a_measurement_is_the_one_a_cold_start_takes() {
+    for threshold in [0.0, 200.0, 400.0, 600.0, 779.0, 780.0, 781.0, 2000.0] {
+        let mut warm = Harness::new((900, 600));
+        let (wide, narrow) = plant(&mut warm, threshold);
+        warm.resize((640, 480));
+        warm.frame();
+        warm.rsc.widgets_mut().get_dyn_mut(wide);
+        warm.frame();
+
+        let mut cold = Harness::new((640, 480));
+        let (cwide, cnarrow) = plant(&mut cold, threshold);
+
+        assert_eq!(
+            taken(&warm, wide, narrow),
+            taken(&cold, cwide, cnarrow),
+            "threshold {threshold}: warm and cold took different branches"
+        );
+    }
+}

tests/draw_cost.rs

@@ -22,8 +22,8 @@ use std::time::Instant;
 
 use iris::prelude::*;
 use iris_core::{
-    GlyphPrimitive, MaskIdx, PrimitiveInst, RectPrimitive, TextureHandle, TexturePrimitive, UiData,
-    UiRegion, UiRenderNode, UiRenderState,
+    GlyphPrimitive, MaskIdx, MoveIdx, PrimitiveInst, RectPrimitive, TextureHandle,
+    TexturePrimitive, UiData, UiRegion, UiRenderNode, UiRenderState,
 };
 use wgpu::{Color as GpuColor, *};
 
@@ -95,6 +95,7 @@ fn fill(
                     primitive: RectPrimitive::color(UiColor::WHITE),
                     region: UiRegion::FULL,
                     mask_idx: MaskIdx::NONE,
+                    move_idx: MoveIdx::NONE,
                 },
             );
             render.layers.write(
@@ -111,6 +112,7 @@ fn fill(
                     },
                     region: UiRegion::FULL,
                     mask_idx: MaskIdx::NONE,
+                    move_idx: MoveIdx::NONE,
                 },
             );
         }
@@ -123,6 +125,7 @@ fn fill(
                     primitive: TexturePrimitive::from(h),
                     region: UiRegion::FULL,
                     mask_idx: MaskIdx::NONE,
+                    move_idx: MoveIdx::NONE,
                 },
             );
         }

tests/generated.rs

@@ -0,0 +1,468 @@
+//! Random trees, checked against building the same tree cold.
+//!
+//! A frame reaches its layout by keeping most of the last one: slots
+//! rewritten, some widgets drawn again, the rest untouched. The property here
+//! is that what comes out is the tree a cold start would have produced, so
+//! anything the retained path carried over that it should not have shows up
+//! as a difference in somebody's box.
+//!
+//! `iris::random` grows the tree and `examples/random.rs` draws one. A seed is
+//! the whole reproduction; `a_long_run_of_seeds_agrees` is the ignored sweep
+//! for when it is worth spending the time.
+
+use std::collections::HashMap;
+
+use iris::harness::Harness;
+use iris::prelude::*;
+use iris::random::{Edits, Lens, Rng, SpanEdit, Tree, grow};
+
+/// How deep the generator branches. The generator widens two to four ways per
+/// level, so depth is exponential in width and a deep narrow tree is not
+/// reachable by raising this -- it buys more overlap between dependency
+/// paths, not more ancestry.
+fn depth() -> usize {
+    env("IRIS_GENERATED_DEPTH", 4)
+}
+
+fn env<T: std::str::FromStr>(name: &str, fallback: T) -> T {
+    std::env::var(name)
+        .ok()
+        .and_then(|value| value.parse().ok())
+        .unwrap_or(fallback)
+}
+const SEEDS: [u64; 7] = [1, 2, 3, 5, 8, 13, 98];
+const REGION_EPSILON_PX: f32 = 0.05;
+
+fn same_coordinate(got: f32, want: f32) -> bool {
+    (got - want).abs() <= REGION_EPSILON_PX
+}
+
+fn same_region(got: Option<PixelRegion>, want: Option<PixelRegion>) -> bool {
+    match (got, want) {
+        (Some(got), Some(want)) => {
+            same_coordinate(got.top_left.x, want.top_left.x)
+                && same_coordinate(got.top_left.y, want.top_left.y)
+                && same_coordinate(got.bot_right.x, want.bot_right.x)
+                && same_coordinate(got.bot_right.y, want.bot_right.y)
+        }
+        (None, None) => true,
+        _ => false,
+    }
+}
+
+fn plant(h: &mut Harness, seed: u64, edits: &Edits) -> Tree {
+    let (root, tree) = grow(&mut h.rsc, seed, depth(), edits);
+    h.state.root = Some(root);
+    h.frame();
+    tree
+}
+
+fn resize_one(h: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> Lens {
+    let lens = [
+        Some(Len::px(20.0 + rng.below(180) as f32)),
+        Some(Len::px(20.0 + rng.below(180) as f32)),
+    ];
+    let sized = &mut h.rsc[tree.sized[idx]];
+    sized.x = lens[0];
+    sized.y = lens[1];
+    lens
+}
+
+/// Changes a few of the declared sizes, and says which, so the cold tree can
+/// be grown with the same ones.
+fn edit(h: &mut Harness, tree: &Tree, rng: &mut Rng) -> HashMap<usize, Lens> {
+    let mut edits = HashMap::new();
+    for _ in 0..4 {
+        let idx = rng.below(tree.sized.len());
+        edits.insert(idx, resize_one(h, tree, idx, rng));
+    }
+    edits
+}
+
+/// Every declared size at once, so every reader of a size in the tree has a
+/// changed descendant in the same frame and the whole dirty set has to settle
+/// together.
+fn edit_every(h: &mut Harness, tree: &Tree, rng: &mut Rng) -> HashMap<usize, Lens> {
+    (0..tree.sized.len())
+        .map(|idx| (idx, resize_one(h, tree, idx, rng)))
+        .collect()
+}
+
+/// A way of changing what a span holds. Each is a shape worth its own case:
+/// taking a child out of the middle is not the same as emptying a span, and
+/// adding one is not the same as adding three.
+#[derive(Clone, Copy, Debug)]
+enum Shuffle {
+    /// Every other child, so what is left is interleaved with what went.
+    EveryOther,
+    /// Everything but the first, which is the last step before empty.
+    AllButFirst,
+    /// Three more on the end at once.
+    AddThree,
+    /// The first out and three more on, so the count moves both ways.
+    SwapForThree,
+    /// One out of the middle and one on the end.
+    TradeOne,
+}
+
+const SHUFFLES: [Shuffle; 5] = [
+    Shuffle::EveryOther,
+    Shuffle::AllButFirst,
+    Shuffle::AddThree,
+    Shuffle::SwapForThree,
+    Shuffle::TradeOne,
+];
+
+impl Shuffle {
+    fn of(self, grown: usize) -> SpanEdit {
+        let all = |step: usize, from: usize| (from..grown).step_by(step).collect();
+        match self {
+            Self::EveryOther => SpanEdit {
+                detach: all(2, 0),
+                attach: 0,
+            },
+            Self::AllButFirst => SpanEdit {
+                detach: all(1, 1),
+                attach: 0,
+            },
+            Self::AddThree => SpanEdit {
+                detach: Vec::new(),
+                attach: 3,
+            },
+            Self::SwapForThree => SpanEdit {
+                detach: vec![0],
+                attach: 3,
+            },
+            Self::TradeOne => SpanEdit {
+                detach: vec![grown / 2],
+                attach: 1,
+            },
+        }
+    }
+}
+
+/// Applies `shuffle` to every third span, and says what it did so the cold
+/// tree can be grown that way. The widgets it takes out are given back: the
+/// last share of one must outlive the comparison, or its id is handed to
+/// something else and the two trees stop lining up.
+fn reshuffle(
+    h: &mut Harness,
+    tree: &mut Tree,
+    shuffle: Shuffle,
+) -> (HashMap<usize, SpanEdit>, Vec<StrongWidget>) {
+    let mut edits = HashMap::new();
+    let mut detached = Vec::new();
+    for (idx, span) in tree.spans.iter_mut().enumerate().step_by(3) {
+        let span_edit = shuffle.of(span.grown);
+        let mut take = span_edit.detach.clone();
+        take.sort_unstable();
+        let children = &mut h.rsc[span.id].children;
+        // Highest first, so an index means the same child however many of
+        // its neighbours are going too.
+        for j in take.into_iter().rev() {
+            if j < children.len() {
+                detached.push(children.remove(j));
+            }
+        }
+        let attach = span_edit.attach.min(span.spares.len());
+        children.extend(span.spares.drain(..attach));
+        edits.insert(idx, span_edit);
+    }
+    (edits, detached)
+}
+
+/// What a widget was configured with, so a tree the generator found can be
+/// written out by hand. A fuzz failure is a lead; the fast test that replaces
+/// it has to be buildable from what the failure printed.
+fn describe(id: WidgetId, h: &Harness) -> String {
+    let label = h.rsc.widgets().label(id).to_string();
+    let Some(widget) = h.rsc.widgets().get_dyn(id) else {
+        return label;
+    };
+    let any: &dyn std::any::Any = widget;
+    let len = |l: &Option<Len>| match l {
+        Some(l) => format!("{l}"),
+        None => "-".into(),
+    };
+    if let Some(w) = any.downcast_ref::<SetSize>() {
+        return format!("SetSize{{x:{},y:{}}}", len(&w.x), len(&w.y));
+    }
+    if let Some(w) = any.downcast_ref::<Span>() {
+        let sign = if w.dir.sign == Sign::Neg { "-" } else { "+" };
+        return format!(
+            "Span{{dir:{:?}{sign},gap:{},n:{}}}",
+            w.dir.axis,
+            w.gap,
+            w.children.len()
+        );
+    }
+    if let Some(w) = any.downcast_ref::<Pad>() {
+        let p = &w.padding;
+        return format!(
+            "Pad{{l:{},r:{},t:{},b:{}}}",
+            p.left, p.right, p.top, p.bottom
+        );
+    }
+    if let Some(w) = any.downcast_ref::<Aligned>() {
+        let a = |v: Option<AxisAlign>| match v {
+            None => "-",
+            Some(AxisAlign::Neg) => "neg",
+            Some(AxisAlign::Center) => "mid",
+            Some(AxisAlign::Pos) => "pos",
+        };
+        return format!("Aligned{{x:{},y:{}}}", a(w.align.x), a(w.align.y));
+    }
+    if let Some(w) = any.downcast_ref::<Stack>() {
+        return format!("Stack{{n:{}}}", w.children.len());
+    }
+    label
+}
+
+/// Every widget in one tree against the matching widget in the other. A
+/// mismatch prints the widget's ancestry, marking the ones that own a slot,
+/// since where two trees disagree is rarely where the cause is.
+fn assert_same(seed: u64, what: &str, warm: (&Harness, &Tree), cold: (&Harness, &Tree)) {
+    let ((wh, wt), (ch, ct)) = (warm, cold);
+    assert_eq!(wt.ids.len(), ct.ids.len(), "seed {seed}: different trees");
+    let mut drawn = 0;
+    let mut wrong = 0;
+    for (i, (&w, &c)) in wt.ids.iter().zip(&ct.ids).enumerate() {
+        let (got, want) = (wh.region(&w), ch.region(&c));
+        drawn += usize::from(got.is_some());
+        // This oracle cares where rasterization lands, not whether equivalent
+        // arithmetic produced the same f32. Keep the tolerance to one
+        // twentieth of a physical pixel, while whether a widget drew remains
+        // exact.
+        if same_region(got, want) {
+            continue;
+        }
+        wrong += 1;
+        if wrong <= 3 {
+            let mut chain = Vec::new();
+            let mut at = Some(w);
+            while let Some(id) = at {
+                let active = &wh.render.active[&id];
+                let slot = match active.move_idx == active.parent_move {
+                    true => "",
+                    false => "*",
+                };
+                chain.push(format!("{}{slot}", describe(id, wh)));
+                at = active.parent;
+            }
+            println!(
+                "seed {seed} after {what}: widget {i}\n  warm {got:?}\n  cold {want:?}\n  {}",
+                chain.join(" < ")
+            );
+        }
+    }
+    assert!(drawn > 0, "seed {seed}: nothing was drawn");
+    assert_eq!(wrong, 0, "seed {seed}: {wrong} widgets differ after {what}");
+}
+
+fn changed_size(seed: u64) {
+    let mut warm = Harness::new((900, 1200));
+    let grown = plant(&mut warm, seed, &Edits::default());
+    // Not every tree grows a declared size to change.
+    if grown.sized.is_empty() {
+        return;
+    }
+
+    let mut rng = Rng::new(seed ^ 0x5eed);
+    let sizes = edit(&mut warm, &grown, &mut rng);
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let same = plant(
+        &mut cold,
+        seed,
+        &Edits {
+            sizes,
+            ..Default::default()
+        },
+    );
+
+    assert_same(seed, "a size change", (&warm, &grown), (&cold, &same));
+}
+
+fn reshuffled(seed: u64, shuffle: Shuffle) {
+    let mut warm = Harness::new((900, 1200));
+    let mut grown = plant(&mut warm, seed, &Edits::default());
+    // Some seeds grow nothing but wrappers, and a shuffle with no span to
+    // shuffle is not the same thing as one that had no effect. A span behind
+    // a branch nobody took is the same kind of nothing: it is not drawn, so
+    // shuffling it cannot move anything.
+    let shuffles = grown
+        .spans
+        .iter()
+        .step_by(3)
+        .any(|span| warm.region(&span.id.id()).is_some());
+    if !shuffles {
+        return;
+    }
+    let before: Vec<_> = grown.ids.iter().map(|id| warm.region(id)).collect();
+
+    let (spans, _held) = reshuffle(&mut warm, &mut grown, shuffle);
+    warm.frame();
+
+    // Or the two trees would agree for want of anything having happened.
+    let after = grown.ids.iter().map(|id| warm.region(id));
+    let moved = before.iter().zip(after).filter(|(a, b)| *a != b).count();
+    assert!(moved > 0, "seed {seed}: {shuffle:?} changed nothing");
+
+    let mut cold = Harness::new((900, 1200));
+    let same = plant(
+        &mut cold,
+        seed,
+        &Edits {
+            spans,
+            ..Default::default()
+        },
+    );
+
+    let what = format!("{shuffle:?}");
+    assert_same(seed, &what, (&warm, &grown), (&cold, &same));
+}
+
+fn changed_every_size(seed: u64) {
+    let mut warm = Harness::new((900, 1200));
+    let grown = plant(&mut warm, seed, &Edits::default());
+    if grown.sized.is_empty() {
+        return;
+    }
+
+    let mut rng = Rng::new(seed ^ 0xa11);
+    let sizes = edit_every(&mut warm, &grown, &mut rng);
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let same = plant(
+        &mut cold,
+        seed,
+        &Edits {
+            sizes,
+            ..Default::default()
+        },
+    );
+
+    assert_same(seed, "every size at once", (&warm, &grown), (&cold, &same));
+}
+
+/// Marks a spread of widgets for redraw at once. Nothing changes, so no box
+/// may either; what this exercises is the order a frame settles a dirty set
+/// in, which the other cases reach one dependency path at a time.
+fn repainted_together(seed: u64) {
+    let mut warm = Harness::new((900, 1200));
+    let grown = plant(&mut warm, seed, &Edits::default());
+    for &id in grown.ids.iter().step_by(5) {
+        warm.rsc.widgets_mut().get_dyn_mut(id);
+    }
+    assert!(
+        !warm.rsc.widgets().needs_redraw.is_empty(),
+        "seed {seed}: nothing was marked"
+    );
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let same = plant(&mut cold, seed, &Edits::default());
+
+    let what = "many repaints at once";
+    assert_same(seed, what, (&warm, &grown), (&cold, &same));
+}
+
+fn resized(seed: u64) {
+    let mut warm = Harness::new((1920, 1200));
+    let grown = plant(&mut warm, seed, &Edits::default());
+    warm.resize((640, 900));
+    warm.frame();
+
+    let mut cold = Harness::new((640, 900));
+    let same = plant(&mut cold, seed, &Edits::default());
+
+    assert_same(seed, "a resize", (&warm, &grown), (&cold, &same));
+}
+
+fn resized_then_changed(seed: u64) {
+    let mut warm = Harness::new((1920, 1200));
+    let grown = plant(&mut warm, seed, &Edits::default());
+    if grown.sized.is_empty() {
+        return;
+    }
+    warm.resize((640, 900));
+    warm.frame();
+
+    let mut rng = Rng::new(seed ^ 0xb0a7);
+    let sizes = edit(&mut warm, &grown, &mut rng);
+    warm.frame();
+
+    let mut cold = Harness::new((640, 900));
+    let same = plant(
+        &mut cold,
+        seed,
+        &Edits {
+            sizes,
+            ..Default::default()
+        },
+    );
+
+    let what = "a resize then a size change";
+    assert_same(seed, what, (&warm, &grown), (&cold, &same));
+}
+
+#[test]
+fn a_changed_size_lands_where_growing_it_that_way_would() {
+    SEEDS.into_iter().for_each(changed_size);
+}
+
+#[test]
+fn every_size_changing_at_once_lands_where_growing_it_that_way_would() {
+    SEEDS.into_iter().for_each(changed_every_size);
+}
+
+#[test]
+fn many_widgets_redrawing_at_once_leaves_every_box_where_it_was() {
+    SEEDS.into_iter().for_each(repainted_together);
+}
+
+#[test]
+fn a_resize_lands_where_starting_at_that_size_would() {
+    SEEDS.into_iter().for_each(resized);
+}
+
+#[test]
+fn a_size_change_after_a_resize_lands_the_same_way() {
+    SEEDS.into_iter().for_each(resized_then_changed);
+}
+
+#[test]
+fn adding_and_removing_span_children_lands_where_growing_it_that_way_would() {
+    for shuffle in SHUFFLES {
+        for seed in SEEDS {
+            reshuffled(seed, shuffle);
+        }
+    }
+}
+
+/// The same property over a hundred seeds and every scenario. What it has
+/// found so far was never where the trees disagreed: a text measured in a box
+/// it was not going to get, and a widget re-measured in a box its own answer
+/// had decided. `tests/shrink.rs` is how a seed from here becomes a tree
+/// small enough to read.
+#[test]
+#[ignore = "a hundred seeds, rather than the seven the others check"]
+fn a_long_run_of_seeds_agrees() {
+    let seeds = std::env::var("IRIS_GENERATED_SEED")
+        .ok()
+        .and_then(|seed| seed.parse().ok())
+        .map(|seed| seed..=seed)
+        .unwrap_or_else(|| 1..=env("IRIS_GENERATED_SEEDS", 100));
+    for seed in seeds {
+        changed_size(seed);
+        changed_every_size(seed);
+        repainted_together(seed);
+        resized(seed);
+        resized_then_changed(seed);
+        for shuffle in SHUFFLES {
+            reshuffled(seed, shuffle);
+        }
+    }
+}

tests/idempotence.rs

@@ -0,0 +1,29 @@
+//! Whether measuring a widget and then giving it the length it reported is a
+//! fixed point, which is what a span that sizes to its children needs.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+#[test]
+fn a_wrapping_text_in_a_span_settles_on_one_width() {
+    let mut h = Harness::new((900, 600));
+    let words = "the quick brown fox jumps over the lazy dog and keeps on running \
+                 until it reaches the end of a rather long line of text";
+    let t = wtext(words).size(16).wrap(true).add(&mut h.rsc);
+    let filler = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((t, filler).span(Dir::RIGHT));
+
+    let mut widths = Vec::new();
+    for _ in 0..6 {
+        let r = h.region(&t.id()).unwrap();
+        widths.push(r.bot_right.x - r.top_left.x);
+        // Redrawing it changes nothing about the state, so nothing may move.
+        h.rsc.widgets_mut().get_dyn_mut(t.id());
+        h.frame();
+    }
+    println!("widths over six frames: {widths:?}");
+    assert!(
+        widths.windows(2).all(|w| w[0] == w[1]),
+        "a repaint that changed nothing moved it: {widths:?}"
+    );
+}

tests/layout.rs

@@ -54,7 +54,7 @@ fn a_child_drawn_twice_moves_once() {
     h.set_root((left, centered).span(Dir::RIGHT));
     assert_corners!(h, inner, (100, 0), (300, 200));
 
-    h.rsc[left].x = Some(Len::abs(150));
+    h.rsc[left].x = Some(Len::px(150));
     h.frame();
 
     assert_corners!(h, inner, (150, 0), (350, 200));
@@ -104,8 +104,248 @@ fn a_fixed_box_is_drawn_again_rather_than_stretched() {
     h.set_root(stack.align(Align::TOP));
     assert_corners!(h, panel, (0, 0), (400, 100));
 
-    h.rsc[leaf].y = Some(Len::abs(250));
+    h.rsc[leaf].y = Some(Len::px(250));
     h.frame();
 
     assert_corners!(h, panel, (0, 0), (400, 250));
 }
+
+#[test]
+fn a_moved_subtree_takes_its_children_with_it() {
+    let mut h = Harness::new((400, 400));
+    let first = rect(Color::RED).height(40).add(&mut h.rsc);
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let row = inner.pad(10).height(40).add(&mut h.rsc);
+    h.set_root((first, row).span(Dir::DOWN));
+    assert_corners!(h, inner, (10, 50), (390, 70));
+
+    h.rsc[first].y = Some(Len::px(80));
+    h.frame();
+
+    // The row is the same shape somewhere else, so one slot moved it and
+    // `inner`'s own region was never rewritten.
+    assert_corners!(h, inner, (10, 90), (390, 110));
+}
+
+#[test]
+fn a_fixed_length_child_keeps_it_when_the_box_around_it_grows() {
+    let mut h = Harness::new((400, 200));
+    let fixed = rect(Color::BLUE).width(50).add(&mut h.rsc);
+    let rest = rect(Color::GREEN).add(&mut h.rsc);
+    let panel = (fixed, rest).span(Dir::RIGHT).add(&mut h.rsc);
+    // Changing the bar's width is the only thing that changes the box the
+    // panel and everything under it was drawn for.
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, panel).span(Dir::RIGHT));
+    assert_corners!(h, fixed, (100, 0), (150, 200));
+    assert_corners!(h, rest, (150, 0), (400, 200));
+
+    h.rsc[bar].x = Some(Len::px(200));
+    h.frame();
+
+    // The panel's box is 100 shorter, so the fixed child is the same 50 wide
+    // against its new start and the one taking the rest absorbs the change.
+    assert_corners!(h, fixed, (200, 0), (250, 200));
+    assert_corners!(h, rest, (250, 0), (400, 200));
+}
+
+#[test]
+fn a_box_with_a_fixed_length_can_be_stretched_on_its_other_axis() {
+    let mut h = Harness::new((400, 200));
+    // The row is 40 tall whatever happens, which used to make its drawing
+    // impossible to take out of: recovering a fraction of a box needs a
+    // relative extent, and it has none on that axis.
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let row = inner.pad(10).height(40).add(&mut h.rsc);
+    let filler = rect(Color::GREEN).add(&mut h.rsc);
+    let column = (row, filler).span(Dir::DOWN).add(&mut h.rsc);
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, column).span(Dir::RIGHT));
+    assert_corners!(h, inner, (110, 10), (390, 30));
+
+    h.rsc[bar].x = Some(Len::px(200));
+    h.frame();
+
+    assert_corners!(h, inner, (210, 10), (390, 30));
+}
+
+#[test]
+fn only_a_container_that_places_its_children_lengthens_the_chain() {
+    let mut h = Harness::new((400, 200));
+    let leaf = rect(Color::BLUE).add(&mut h.rsc);
+    // Four widgets between the span and the leaf, none of which places what
+    // it draws, so all of them share the span's slot.
+    let buried = leaf.pad(4).pad(4).pad(4).pad(4).add(&mut h.rsc);
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, buried).span(Dir::RIGHT));
+
+    let slot = h.render.active[&leaf.id()].parent_move;
+    assert_eq!(
+        h.render.moves.depth(slot),
+        2,
+        "the span above the leaf, and the root the window is held in"
+    );
+}
+
+/// A span that sizes from its children passes their `rest` weight up rather
+/// than collapsing it to one share, so nesting divides the same space instead
+/// of re-dividing a share of it.
+#[test]
+fn nested_spans_divide_the_space_once_however_deep_the_nesting_is() {
+    let mut h = Harness::new((400, 200));
+    let (a, b, c, d) = (
+        rect(Color::RED).add(&mut h.rsc),
+        rect(Color::BLUE).add(&mut h.rsc),
+        rect(Color::GREEN).add(&mut h.rsc),
+        rect(Color::WHITE).add(&mut h.rsc),
+    );
+    let left = (a, b).span(Dir::RIGHT).add(&mut h.rsc);
+    let right = (c, d).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((left, right).span(Dir::RIGHT));
+
+    for (i, id) in [a, b, c, d].into_iter().enumerate() {
+        let x = i as f32 * 100.0;
+        assert_corners!(h, id, (x, 0), (x + 100.0, 200));
+    }
+}
+
+/// The same space, unevenly nested: weights carried up mean a share is a
+/// share of the whole, not of whatever branch a widget happens to sit in.
+#[test]
+fn an_uneven_nesting_still_gives_every_share_the_same_length() {
+    let mut h = Harness::new((400, 200));
+    let (a, b, c, d) = (
+        rect(Color::RED).add(&mut h.rsc),
+        rect(Color::BLUE).add(&mut h.rsc),
+        rect(Color::GREEN).add(&mut h.rsc),
+        rect(Color::WHITE).add(&mut h.rsc),
+    );
+    let one = (a,).span(Dir::RIGHT).add(&mut h.rsc);
+    let three = (b, c, d).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((one, three).span(Dir::RIGHT));
+
+    for (i, id) in [a, b, c, d].into_iter().enumerate() {
+        let x = i as f32 * 100.0;
+        assert_corners!(h, id, (x, 0), (x + 100.0, 200));
+    }
+}
+
+/// Where the shader puts an edge: the two parts of a scalar are floored
+/// apart, so a fraction and a pixel offset snap independently.
+fn drawn_edges(h: &Harness, id: WidgetId, axis: Axis) -> (f32, f32) {
+    let active = &h.render.active[&id];
+    let region = h.render.moves.resolve(active.parent_move, active.region);
+    let dim = h.size().axis(axis);
+    let edge = |s: UiScalar| (s.rel * dim).floor() + s.px.floor();
+    let span = region.axis(axis);
+    (edge(span.start), edge(span.end))
+}
+
+fn hairline(h: &mut Harness, marks: &mut Vec<WidgetId>) -> StrongWidget {
+    let inner = rect(Color::RED).add_strong(&mut h.rsc);
+    let mark = SetSize {
+        inner,
+        x: Some(Len::px(1.0)),
+        y: None,
+    }
+    .add_strong(&mut h.rsc);
+    marks.push(mark.id());
+    mark
+}
+
+fn share(h: &mut Harness, inner: StrongWidget, ratio: f32) -> StrongWidget {
+    SetSize {
+        inner,
+        x: Some(Len::rest(ratio)),
+        y: None,
+    }
+    .add_strong(&mut h.rsc)
+}
+
+/// Shares in weights no binary fraction lands on, a padding on one branch
+/// and not the other, so an edge falls near an integer as often as it can.
+fn hairlines(h: &mut Harness, depth: usize, marks: &mut Vec<WidgetId>) -> StrongWidget {
+    let mut span = Span::empty(Dir::RIGHT);
+    if depth == 0 {
+        let left = rect(Color::BLUE).add_strong(&mut h.rsc);
+        let left = share(h, left, 3.0);
+        span.push(left);
+        let mark = hairline(h, marks);
+        span.push(mark);
+        let right = rect(Color::BLUE).add_strong(&mut h.rsc);
+        let right = share(h, right, 7.0);
+        span.push(right);
+        return span.add_strong(&mut h.rsc);
+    }
+    let first = hairlines(h, depth - 1, marks);
+    let first = share(h, first, 3.0);
+    span.push(first);
+    let second = hairlines(h, depth - 1, marks);
+    let second = Pad {
+        padding: Padding {
+            left: 3.0,
+            right: 7.0,
+            top: 0.0,
+            bottom: 0.0,
+        },
+        inner: second,
+    }
+    .add_strong(&mut h.rsc);
+    let second = share(h, second, 5.0);
+    span.push(second);
+    span.add_strong(&mut h.rsc)
+}
+
+/// A one-pixel line is a pixel wherever it is drawn. Both edges of a fixed
+/// length share their box's fraction, so composing the chain moves them
+/// together and the shader's `floor` cannot round the pixel between them
+/// away -- only shift it. A separator that disappeared at one window size
+/// would be a defect no size comparison catches.
+#[test]
+fn a_one_pixel_line_keeps_its_pixel_through_a_chain() {
+    let mut h = Harness::new((1920, 1200));
+    let mut marks = Vec::new();
+    let root = hairlines(&mut h, 4, &mut marks);
+    h.state.set_root(root);
+    h.frame();
+    assert_eq!(marks.len(), 16);
+
+    for size in [(1920, 1200), (1919, 1201), (997, 1003), (1367, 733)] {
+        h.resize(size);
+        h.frame();
+        for mark in &marks {
+            let (start, end) = drawn_edges(&h, *mark, Axis::X);
+            assert_eq!(end - start, 1.0, "at {size:?}, mark {mark:?}");
+        }
+    }
+}
+
+/// A span short of room takes it from its shares, which go to nothing and
+/// then to nothing wider; the fixed lengths between them keep their pixels.
+/// Collapsing those to make room would delete a separator the caller asked
+/// for, which is worse than overflowing.
+#[test]
+fn a_span_out_of_room_shrinks_its_shares_and_not_its_fixed_lengths() {
+    let mut h = Harness::new((400, 20));
+    let mut marks = Vec::new();
+    let mut span = Span::empty(Dir::RIGHT);
+    for _ in 0..3 {
+        let share_of = rect(Color::BLUE).add_strong(&mut h.rsc);
+        let share_of = share(&mut h, share_of, 1.0);
+        span.push(share_of);
+        let mark = hairline(&mut h, &mut marks);
+        span.push(mark);
+    }
+    let root = span.add_strong(&mut h.rsc);
+    h.state.set_root(root);
+    h.frame();
+
+    for width in [400, 10, 3, 1] {
+        h.resize((width, 20));
+        h.frame();
+        for mark in &marks {
+            let (start, end) = drawn_edges(&h, *mark, Axis::X);
+            assert_eq!(end - start, 1.0, "at {width} wide, mark {mark:?}");
+        }
+    }
+}

tests/layout_diagnostics.rs

@@ -0,0 +1,240 @@
+//! Retained CPU-layout diagnostics on one reproducible random tree.
+//!
+//! Counters and phase timers:
+//!
+//!     cargo test --release --features layout-diagnostics \
+//!       --test layout_diagnostics -- --ignored --nocapture
+//!
+//! Uninstrumented hardware totals for one phase:
+//!
+//!     IRIS_PHASE=resize IRIS_FRAMES=1000 perf stat \
+//!       -e cycles:u,instructions:u cargo test --release \
+//!       --test layout_diagnostics -- --ignored --nocapture
+//!
+//! `IRIS_PHASE` is `cold`, `repaint`, `many`, `size`, `scroll`, `resize`, or
+//! `all`. `IRIS_SEED`, `IRIS_DEPTH`, and `IRIS_FRAMES` select the load, and
+//! `IRIS_DIRTY` how many widgets `many` marks at once.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+use iris::random::{Edits, Tree, grow};
+use std::time::Instant;
+
+const OUTPUT: (f32, f32) = (1920.0, 1200.0);
+
+#[cfg(feature = "layout-diagnostics")]
+#[test]
+fn a_selected_widget_retains_its_layout_events() {
+    use iris::core::layout_diagnostics::{self as diagnostics, TraceEvent};
+
+    diagnostics::clear_traced_widgets();
+    let _ = diagnostics::take();
+    let mut harness = Harness::new((400, 200));
+    let leaf = rect(Color::RED).add(&mut harness.rsc);
+    let other = rect(Color::BLUE).add(&mut harness.rsc);
+    let root = (leaf, other).span(Dir::RIGHT).add(&mut harness.rsc);
+    harness.set_root(root);
+    diagnostics::trace_widget(leaf.id());
+    let _ = diagnostics::take();
+
+    let _ = harness.rsc.widgets_mut().get_dyn_mut(root.id());
+    let _ = harness.rsc.widgets_mut().get_dyn_mut(leaf.id());
+    harness.frame();
+
+    let report = diagnostics::take();
+    assert!(
+        report
+            .traces()
+            .iter()
+            .any(|event| matches!(event, TraceEvent::Placed { id, .. } if *id == leaf.id()))
+    );
+    assert!(
+        report
+            .traces()
+            .iter()
+            .any(|event| matches!(event, TraceEvent::DrawRequest { id, .. } if *id == leaf.id()))
+    );
+    assert!(
+        report
+            .traces()
+            .iter()
+            .any(|event| matches!(event, TraceEvent::SizeRead { id, .. } if *id == leaf.id()))
+    );
+    assert!(
+        report
+            .traces()
+            .iter()
+            .any(|event| matches!(event, TraceEvent::SizeReported { id, .. } if *id == leaf.id()))
+    );
+    diagnostics::clear_traced_widgets();
+}
+
+fn env<T: std::str::FromStr>(name: &str, fallback: T) -> T {
+    std::env::var(name)
+        .ok()
+        .and_then(|value| value.parse().ok())
+        .unwrap_or(fallback)
+}
+
+#[cfg(feature = "layout-diagnostics")]
+fn trace_selected(tree: &Tree) {
+    let Ok(value) = std::env::var("IRIS_TRACE_INDEX") else {
+        return;
+    };
+    let index = value
+        .parse::<usize>()
+        .expect("IRIS_TRACE_INDEX must be a tree.ids index");
+    let id = tree.ids[index];
+    iris::core::layout_diagnostics::trace_widget(id);
+    println!("tracing tree.ids[{index}] = {id:?}");
+}
+
+#[cfg(not(feature = "layout-diagnostics"))]
+fn trace_selected(_: &Tree) {}
+
+fn warm(seed: u64, depth: usize) -> (Harness, Tree) {
+    let mut harness = Harness::new(OUTPUT);
+    let (root, tree) = grow(&mut harness.rsc, seed, depth, &Edits::default());
+    harness.state.root = Some(root);
+    harness.frame();
+    println!(
+        "fixture: seed {seed}, depth {depth}, {} widgets, {} active",
+        tree.ids.len(),
+        harness.render.active_widgets()
+    );
+    #[cfg(feature = "layout-diagnostics")]
+    let _ = iris::core::layout_diagnostics::take();
+    (harness, tree)
+}
+
+fn report(label: &str, mut elapsed: Vec<f64>, _harness: &Harness) {
+    elapsed.sort_by(|a, b| a.partial_cmp(b).unwrap());
+    let frames = elapsed.len();
+    // The worst frame is the stutter somebody sees, so it goes beside the
+    // median; p99 says whether it is the load or a single interruption.
+    println!(
+        "{label}: {frames} frame(s), min {:.3} ms, median {:.3} ms, p99 {:.3} ms, \
+         max {:.3} ms, total {:.1} ms",
+        elapsed[0],
+        elapsed[frames / 2],
+        elapsed[frames * 99 / 100],
+        elapsed[frames - 1],
+        elapsed.iter().sum::<f64>(),
+    );
+    #[cfg(feature = "layout-diagnostics")]
+    {
+        let diagnostics = iris::core::layout_diagnostics::take();
+        print!("{}", diagnostics.per_frame(frames));
+        for callsite in diagnostics.hot_text().iter().take(3) {
+            let mut ancestry = Vec::new();
+            let mut id = Some(callsite.id);
+            while let Some(widget) = id {
+                ancestry.push(_harness.rsc.widgets().label(widget).as_str());
+                id = _harness
+                    .render
+                    .active
+                    .get(&widget)
+                    .and_then(|active| active.parent);
+            }
+            println!("    text ancestry: {}", ancestry.join(" < "));
+        }
+    }
+}
+
+fn run(
+    label: &str,
+    frames: usize,
+    harness: &mut Harness,
+    mut change: impl FnMut(&mut Harness, usize),
+) {
+    let mut elapsed = Vec::with_capacity(frames);
+    for frame in 0..frames {
+        change(harness, frame);
+        let start = Instant::now();
+        harness.frame();
+        elapsed.push(start.elapsed().as_secs_f64() * 1_000.0);
+    }
+    report(label, elapsed, harness);
+}
+
+#[test]
+#[ignore = "measurement, not a check"]
+fn layout_cost() {
+    let seed = env("IRIS_SEED", 1_u64);
+    let depth = env("IRIS_DEPTH", 7_usize);
+    let frames = env("IRIS_FRAMES", 100_usize);
+    assert!(frames > 0, "IRIS_FRAMES must be greater than zero");
+    let phase = env("IRIS_PHASE", String::from("all"));
+    assert!(
+        ["all", "cold", "repaint", "many", "size", "scroll", "resize"].contains(&phase.as_str()),
+        "unknown IRIS_PHASE {phase:?}"
+    );
+    let selected = |name| phase == "all" || phase == name;
+
+    if selected("cold") {
+        let mut harness = Harness::new(OUTPUT);
+        let (root, tree) = grow(&mut harness.rsc, seed, depth, &Edits::default());
+        harness.state.root = Some(root);
+        println!(
+            "fixture: seed {seed}, depth {depth}, {} widgets",
+            tree.ids.len()
+        );
+        trace_selected(&tree);
+        #[cfg(feature = "layout-diagnostics")]
+        let _ = iris::core::layout_diagnostics::take();
+        run("cold", 1, &mut harness, |_, _| {});
+        drop(tree);
+    }
+
+    if selected("repaint") {
+        let (mut harness, tree) = warm(seed, depth);
+        trace_selected(&tree);
+        let leaf = tree.ids[0];
+        run("repaint", frames, &mut harness, move |harness, _| {
+            let _ = harness.rsc.widgets_mut().get_dyn_mut(leaf);
+        });
+    }
+
+    if selected("many") {
+        let (mut harness, tree) = warm(seed, depth);
+        trace_selected(&tree);
+        // Spread through the tree rather than taken from one subtree, so the
+        // dependency paths the frame settles overlap.
+        let wanted = env("IRIS_DIRTY", 32_usize).max(1);
+        let step = (tree.ids.len() / wanted).max(1);
+        let dirty: Vec<_> = tree.ids.iter().copied().step_by(step).collect();
+        println!("marking {} of {} widgets", dirty.len(), tree.ids.len());
+        run("many", frames, &mut harness, move |harness, _| {
+            for &id in &dirty {
+                harness.rsc.widgets_mut().get_dyn_mut(id);
+            }
+        });
+    }
+
+    if selected("size") {
+        let (mut harness, tree) = warm(seed, depth);
+        trace_selected(&tree);
+        let sized = tree.sized[0];
+        run("size", frames, &mut harness, move |harness, frame| {
+            harness.rsc[sized].x = Some(Len::px(100.0 + (frame % 2) as f32 * 40.0));
+        });
+    }
+
+    if selected("scroll") {
+        let (mut harness, tree) = warm(seed, depth);
+        trace_selected(&tree);
+        let scroll = tree.scrolls[0];
+        run("scroll", frames, &mut harness, move |harness, frame| {
+            harness.rsc[scroll].scroll(if frame % 2 == 0 { 12.0 } else { -12.0 });
+        });
+    }
+
+    if selected("resize") {
+        let (mut harness, tree) = warm(seed, depth);
+        trace_selected(&tree);
+        run("resize", frames, &mut harness, |harness, frame| {
+            harness.resize((OUTPUT.0 - ((frame + 1) % 2) as f32 * 8.0, OUTPUT.1));
+        });
+        drop(tree);
+    }
+}

tests/remap.rs

@@ -1,27 +0,0 @@
-//! What a drawing can be taken out of, and what it cannot.
-
-use iris::core::{Remap, UiRegion, UiScalar, UiSpan};
-
-/// A box `size` tall whose top is `rel` of the way down the window.
-fn fixed(rel: f32, size: f32) -> UiRegion {
-    UiRegion::new(
-        UiSpan::FULL,
-        UiSpan::new(UiScalar { rel, abs: 0.0 }, UiScalar { rel, abs: size }),
-    )
-}
-
-#[test]
-fn a_fixed_box_can_be_carried_but_not_stretched() {
-    let from = fixed(0.0, 164.0);
-    assert!(Remap::new(from, UiRegion::FULL).is_none());
-    assert!(Remap::new(from, fixed(0.5, 164.0)).is_some());
-    assert!(Remap::new(from, fixed(0.0, 98.0)).is_none());
-}
-
-#[test]
-fn a_relative_box_can_be_stretched_to_any_other() {
-    let remap = Remap::new(UiRegion::FULL, fixed(0.0, 98.0)).expect("relative boxes remap");
-    // A part that filled the window keeps filling what replaced it, which is
-    // exactly what `outside` could not say for a box of a fixed length.
-    assert_eq!(remap.apply(UiRegion::FULL), fixed(0.0, 98.0));
-}

tests/replace_cost.rs

@@ -0,0 +1,43 @@
+//! What re-placing a subtree costs per frame, as a load for a counter rather
+//! than a check. A span of 200 fixed-height rows, five primitives each, with
+//! the row above them changing height every frame, so every row below is
+//! offered a box the same shape somewhere else.
+//!
+//!     cargo test --release --test replace_cost -- --ignored
+//!     perf stat -e instructions:u target/release/.../replace_cost-* --ignored
+//!
+//! Wall time is the wrong number here; see `draw_cost.rs`. Measured on
+//! 2026-09-14 at 1.98M instructions per frame, against 2.38M for rewriting
+//! each row's regions instead and 7.13M for redrawing them.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+const ROWS: usize = 200;
+const FRAMES: usize = 200;
+
+#[test]
+#[ignore = "measurement, not a check"]
+fn replacing_rows_every_frame() {
+    let mut h = Harness::new((1920, 1200));
+    let first = rect(Color::RED).height(40).add(&mut h.rsc);
+    let mut span = Span::empty(Dir::DOWN);
+    span.push(first.add_strong(&mut h.rsc));
+    for i in 0..ROWS {
+        let row = (
+            rect(Color::BLUE.darker(i as f32 / (ROWS * 2) as f32)),
+            rect(Color::GREEN).pad(2),
+            wtext("row").size(16).pad(2),
+        )
+            .span(Dir::RIGHT)
+            .pad(4)
+            .height(40)
+            .add(&mut h.rsc);
+        span.push(row.add_strong(&mut h.rsc));
+    }
+    h.set_root(span);
+    for i in 0..FRAMES {
+        h.rsc[first].y = Some(Len::px(40.0 + (i % 2) as f32));
+        h.frame();
+    }
+}

tests/retained.rs

@@ -79,10 +79,9 @@ fn a_leaf_that_depends_on_its_box_is_drawn_again_when_the_box_changes() {
     h.rsc[first].size = Size::from((150, 200));
     h.frame();
 
-    // Twice: once for the span to measure it, once for its real box. A child
-    // that can hint its length is spared the first, and a smaller number here
-    // means someone has made that cheaper rather than broken it.
-    assert_eq!(draws.get(), settled + 2);
+    // The preceding fixed child makes the remaining box this child's real
+    // box, so measuring it also draws it in its final place.
+    assert_eq!(draws.get(), settled + 1);
     assert_corners!(h, second, (150, 0), (400, 200));
 }
 
@@ -116,6 +115,22 @@ fn a_span_relays_out_when_a_child_it_measured_changes() {
     assert_corners!(h, second, (250, 0), (400, 200));
 }
 
+#[test]
+fn a_repaint_that_keeps_its_size_does_not_relay_out() {
+    let mut h = Harness::new((400, 200));
+    let (first, draws) = counted(&mut h, Size::from((100, 200)), OnResize::Translate);
+    let (second, _) = counted(&mut h, Size::REST, OnResize::Translate);
+    h.set_root((first, second).span(Dir::RIGHT));
+    let settled = draws.get();
+
+    // Taking mutable access is the ordinary content-change signal. This
+    // widget returns the same size, so the parent has nothing to lay out.
+    let _ = h.rsc.widgets_mut().get_dyn_mut(first.id());
+    h.frame();
+
+    assert_eq!(draws.get(), settled + 1);
+}
+
 #[test]
 fn a_placed_child_survives_the_next_frame() {
     let mut h = Harness::new((400, 200));
@@ -141,7 +156,7 @@ impl Widget for FromHint {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let len = painter.size_hint(&self.inner, Axis::Y).unwrap();
         let mut region = UiRegion::FULL;
-        region.y.end = region.y.start.offset(len.abs);
+        region.y.end = region.y.start.offset(len.px);
         painter.widget_within(&self.inner, region);
         Size::REST
     }
@@ -158,7 +173,7 @@ fn a_parent_that_only_read_a_hint_relays_out_when_the_hint_changes() {
     h.set_root(parent);
     assert_corners!(h, inner, (0, 0), (400, 80));
 
-    h.rsc[inner].y = Some(Len::abs(120));
+    h.rsc[inner].y = Some(Len::px(120));
     h.frame();
 
     assert_corners!(h, inner, (0, 0), (400, 120));
@@ -172,14 +187,34 @@ struct ReadsOutput {
 impl Widget for ReadsOutput {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
-        Size::abs(painter.output_size() / 4.0)
+        Size::px(painter.output_size() / 4.0)
+    }
+}
+
+/// Reads the output across one axis only, and says so: its drawing follows
+/// a taller box on its own, so only a wider one is worth a draw.
+struct ReadsWidth {
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for ReadsWidth {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        Size::px((painter.output_len(Axis::X) / 4.0, 20.0).into())
+    }
+
+    fn on_resize(&self, axis: Axis) -> OnResize {
+        match axis {
+            Axis::X => OnResize::Redraw,
+            Axis::Y => OnResize::Scale,
+        }
     }
 }
 
 #[test]
 fn a_resize_does_not_redraw_what_the_shader_can_move() {
     let mut h = Harness::new((400, 200));
-    let (leaf, draws) = counted(&mut h, Size::REST, OnResize::Redraw);
+    let (leaf, draws) = counted(&mut h, Size::REST, OnResize::Scale);
     h.set_root(leaf);
     let settled = draws.get();
 
@@ -190,11 +225,28 @@ fn a_resize_does_not_redraw_what_the_shader_can_move() {
     assert_eq!(
         draws.get(),
         settled,
-        "its box is the same fraction of a different output"
+        "a scaling drawing follows its box, and the output is one"
     );
     assert_corners!(h, leaf, (0, 0), (800, 100));
 }
 
+/// The output is the root of the box chain, so a resize is a box that changed
+/// length and `OnResize` answers for it -- there is not a second rule for the
+/// window. A drawing that does not scale is redrawn whichever box moved.
+#[test]
+fn a_resize_redraws_what_does_not_scale() {
+    let mut h = Harness::new((400, 200));
+    let (leaf, draws) = counted(&mut h, Size::REST, OnResize::Redraw);
+    h.set_root(leaf);
+    let settled = draws.get();
+
+    h.resize((800, 100));
+    h.frame();
+
+    assert_eq!(draws.get(), settled + 1, "its box is a different length");
+    assert_corners!(h, leaf, (0, 0), (800, 100));
+}
+
 #[test]
 fn a_resize_redraws_what_read_the_output() {
     let mut h = Harness::new((400, 200));
@@ -212,6 +264,83 @@ fn a_resize_redraws_what_read_the_output() {
     assert_eq!(draws.get(), settled + 1);
 }
 
+#[test]
+fn a_resize_only_redraws_read_output_axes() {
+    let mut h = Harness::new((400, 200));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = ReadsWidth {
+        draws: draws.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root(leaf);
+    let settled = draws.get();
+
+    h.resize((400, 300));
+    h.frame();
+    assert_eq!(draws.get(), settled, "height was never read");
+
+    h.resize((800, 300));
+    h.frame();
+    assert_eq!(draws.get(), settled + 1, "width changes its answer");
+}
+
+#[test]
+fn subpixel_resize_changes_accumulate_from_the_last_layout() {
+    let mut h = Harness::new((400, 200));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = ReadsWidth {
+        draws: draws.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root(leaf);
+    let settled = draws.get();
+
+    for width in [400.02, 400.04, 400.05] {
+        h.resize((width, 200.0));
+        h.frame();
+        assert_eq!(draws.get(), settled);
+    }
+
+    h.resize((400.06, 200.0));
+    h.frame();
+    assert_eq!(draws.get(), settled + 1);
+}
+
+#[test]
+fn subpixel_box_changes_accumulate_from_the_last_draw() {
+    let mut h = Harness::new((400, 200));
+    let (first, draws, _) = pair(&mut h, OnResize::Redraw);
+    let settled = draws.get();
+
+    for width in [100.02, 100.04, 100.05] {
+        h.rsc[first].size.x = Len::px(width);
+        h.frame();
+        assert_eq!(draws.get(), settled);
+    }
+
+    h.rsc[first].size.x = Len::px(100.06);
+    h.frame();
+    assert_eq!(draws.get(), settled + 1);
+}
+
+#[test]
+fn reporting_the_same_output_size_does_not_start_a_resize() {
+    let mut h = Harness::new((400, 200));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = ReadsOutput {
+        draws: draws.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root(leaf);
+    let settled = draws.get();
+
+    h.resize((400, 200));
+
+    assert!(!h.needs_redraw());
+    h.frame();
+    assert_eq!(draws.get(), settled);
+}
+
 #[test]
 fn narrowing_the_output_reflows_text_and_relays_out_around_it() {
     let mut h = Harness::new((600, 400));
@@ -239,14 +368,104 @@ fn a_change_two_levels_under_its_reader_still_reaches_it() {
     // Every wrapper up to the outer pad read the size below it, so the outer
     // pad is what draws again -- and the span it hands the box to is the same
     // size as before, which is what lets a draw reuse its way past the leaf.
-    let (leaf, _) = counted(&mut h, Size::abs((100, 100).into()), OnResize::Redraw);
+    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), OnResize::Redraw);
     let padded = leaf.pad(10).add(&mut h.rsc);
     let below = rect(Color::RED).add(&mut h.rsc);
     h.set_root((padded, below).span(Dir::DOWN).pad(12));
     assert_corners!(h, below, (12, 132), (388, 388));
 
-    h.rsc[leaf].size = Size::abs((100, 200).into());
+    h.rsc[leaf].size = Size::px((100, 200).into());
     h.frame();
 
     assert_corners!(h, below, (12, 232), (388, 388));
 }
+
+/// Claims its drawing survives its box changing length, and has a child so
+/// that the walk looking for what does not has one to reach.
+struct Stretchy {
+    inner: StrongWidget,
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for Stretchy {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        painter.widget(&self.inner).size()
+    }
+
+    fn on_resize(&self, _: Axis) -> OnResize {
+        OnResize::Scale
+    }
+}
+
+#[test]
+fn stretching_a_subtree_carries_the_children_in_it() {
+    let mut h = Harness::new((400, 400));
+    let first = rect(Color::RED).height(40).add(&mut h.rsc);
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let draws = Rc::new(Cell::new(0));
+    let outer = Stretchy {
+        inner: inner.add_strong(&mut h.rsc),
+        draws: draws.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root((first, outer).span(Dir::DOWN));
+    let settled = draws.get();
+    assert_corners!(h, inner, (0, 40), (400, 400));
+
+    h.rsc[first].y = Some(Len::px(80));
+    h.frame();
+
+    assert_eq!(
+        draws.get(),
+        settled,
+        "its drawing follows its box, rather than being made again"
+    );
+    assert_corners!(h, outer, (0, 80), (400, 400));
+    assert_corners!(h, inner, (0, 80), (400, 400));
+}
+
+#[test]
+fn a_widened_row_redraws_what_reads_its_length_and_nothing_else() {
+    let mut h = Harness::new((400, 200));
+    // What a transcript row is: something whose shaping depends on the width
+    // it is given, beside something that only has to be the right shape.
+    let (wraps, wrap_draws) = counted(&mut h, Size::REST, OnResize::Redraw);
+    let (backing, back_draws) = counted(&mut h, Size::REST, OnResize::Scale);
+    let row = (backing, wraps).span(Dir::RIGHT).add(&mut h.rsc);
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, row).span(Dir::RIGHT));
+    let (settled_wrap, settled_back) = (wrap_draws.get(), back_draws.get());
+
+    h.rsc[bar].x = Some(Len::px(200));
+    h.frame();
+
+    // The span reads every child's size, so redrawing one takes the span
+    // with it -- and the span then measures and places the redrawn child.
+    assert!(wrap_draws.get() > settled_wrap, "reads the width it got");
+    assert_eq!(back_draws.get(), settled_back, "only has to be the shape");
+    assert_corners!(h, backing, (200, 0), (300, 200));
+    assert_corners!(h, wraps, (300, 0), (400, 200));
+}
+
+#[test]
+fn a_declared_length_child_is_not_redrawn_when_the_box_around_it_grows() {
+    let mut h = Harness::new((400, 200));
+    // Its box is a fixed 80 wherever the row's edges end up, so drawing it
+    // again would be for a width it does not have. The declared width is what
+    // lets the span say that without drawing it: a width the span learnt by
+    // drawing the child in its own box is only an answer for that box.
+    let (counter, draws) = counted(&mut h, Size::from((80, 200)), OnResize::Redraw);
+    let fixed = counter.width(80).add(&mut h.rsc);
+    let (rest, _) = counted(&mut h, Size::REST, OnResize::Scale);
+    let row = (fixed, rest).span(Dir::RIGHT).add(&mut h.rsc);
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, row).span(Dir::RIGHT));
+    let settled = draws.get();
+
+    h.rsc[bar].x = Some(Len::px(200));
+    h.frame();
+
+    assert_eq!(draws.get(), settled, "its own length did not change");
+    assert_corners!(h, fixed, (200, 0), (280, 200));
+}

tests/revision_cost.rs

@@ -0,0 +1,199 @@
+//! What a resize frame costs and what it holds, on a tree the revision before
+//! #16 also builds.
+//!
+//! Deliberately written in the API subset `43ce8c7` and this branch share, so
+//! the same source can be dropped into an old worktree and measured there:
+//! that is the only like-for-like comparison with the code the retained
+//! layout replaced. The random tree cannot carry one, because the generator
+//! itself changed with the work.
+//!
+//!     ROWS=40 FRAMES=500 cargo test --release --test revision_cost \
+//!       -- --ignored --nocapture resize_cost
+//!     ROWS=2000 cargo test --release --test revision_cost \
+//!       -- --ignored --nocapture text_memory
+//!
+//! Wall time on this machine varies with CPU frequency; take the number from
+//! `perf stat -e instructions:u` on the test binary directly.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+use std::time::Instant;
+
+/// xorshift64, so one seed is one set of paragraphs on any machine.
+struct Rng(u64);
+
+impl Rng {
+    fn bits(&mut self) -> u64 {
+        self.0 ^= self.0 << 13;
+        self.0 ^= self.0 >> 7;
+        self.0 ^= self.0 << 17;
+        self.0
+    }
+
+    fn below(&mut self, n: usize) -> usize {
+        (self.bits() % n as u64) as usize
+    }
+}
+
+const WORDS: [&str; 24] = [
+    "wrapping",
+    "shapes",
+    "one",
+    "source",
+    "into",
+    "as",
+    "many",
+    "lines",
+    "as",
+    "the",
+    "box",
+    "leaves",
+    "room",
+    "for",
+    "paragraph",
+    "height",
+    "answer",
+    "setting",
+    "container",
+    "width",
+    "before",
+    "knows",
+    "measured",
+    "again",
+];
+
+/// A run of its own words, so nothing here is fast for two texts being the
+/// same string.
+fn words(rng: &mut Rng, least: usize, most: usize) -> String {
+    let words = least + rng.below(most - least);
+    let mut out = String::new();
+    for _ in 0..words {
+        if !out.is_empty() {
+            out.push(' ');
+        }
+        out.push_str(WORDS[rng.below(WORDS.len())]);
+    }
+    out
+}
+
+const OUTPUT: (f32, f32) = (900.0, 1200.0);
+
+fn env<T: std::str::FromStr>(name: &str, fallback: T) -> T {
+    std::env::var(name)
+        .ok()
+        .and_then(|value| value.parse().ok())
+        .unwrap_or(fallback)
+}
+
+/// A row of a fixed-width rect beside a column of one wrapping and one
+/// overflowing text: the shape that makes a container measure a child in a
+/// box it will not keep.
+fn build(h: &mut Harness, rows: usize) -> Vec<WidgetId> {
+    let mut rng = Rng(1);
+    let mut paragraphs = Vec::new();
+    let mut col = Span::empty(Dir::DOWN);
+    for _ in 0..rows {
+        let mut row = Span::empty(Dir::RIGHT);
+        row.push(rect(Color::RED).width(Len::px(40.0)).add_strong(&mut h.rsc));
+        let mut body = Span::empty(Dir::DOWN);
+        let para = wtext(words(&mut rng, 12, 52))
+            .size(16)
+            .wrap(true)
+            .add_strong(&mut h.rsc);
+        paragraphs.push(para.id());
+        body.push(para);
+        body.push(
+            // Short, or its unwrapped width decides the row and the
+            // paragraph beside it never wraps.
+            wtext(words(&mut rng, 2, 6))
+                .size(16)
+                .wrap(false)
+                .add_strong(&mut h.rsc),
+        );
+        row.push(body.add_strong(&mut h.rsc));
+        col.push(row.add_strong(&mut h.rsc));
+    }
+    let root = col.add(&mut h.rsc);
+    h.set_root(root);
+    paragraphs
+}
+
+#[test]
+#[ignore = "measurement, not a check"]
+fn resize_cost() {
+    let rows = env("ROWS", 40_usize);
+    let frames = env("FRAMES", 500_usize);
+    let mut h = Harness::new(OUTPUT);
+    let paragraphs = build(&mut h, rows);
+    // What it cost is only half the comparison: the old code is cheaper
+    // partly because it wraps at the container's whole width rather than the
+    // part left beside the rect, and draws past the edge of the output.
+    println!("output width {}", OUTPUT.0);
+    for (at, id) in paragraphs.iter().enumerate().take(3) {
+        println!("paragraph {at}: {:?}", h.region(id));
+    }
+
+    // Two widths in turn is the friendly case for anything that remembers an
+    // answer, so `SWEEP=1` never repeats one -- a drag rather than a toggle.
+    let sweep = env("SWEEP", 0_usize) != 0;
+    let mut elapsed = Vec::with_capacity(frames);
+    for frame in 0..frames {
+        let narrower = match sweep {
+            true => (frame % 256) as f32,
+            false => ((frame + 1) % 2) as f32 * 8.0,
+        };
+        h.resize((OUTPUT.0 - narrower, OUTPUT.1));
+        let start = Instant::now();
+        h.frame();
+        elapsed.push(start.elapsed().as_secs_f64() * 1000.0);
+    }
+    elapsed.sort_by(|a, b| a.partial_cmp(b).unwrap());
+    println!(
+        "resize: {frames} frames, min {:.3} ms, median {:.3} ms, p99 {:.3} ms, \
+         max {:.3} ms, total {:.1} ms",
+        elapsed[0],
+        elapsed[frames / 2],
+        elapsed[frames * 99 / 100],
+        elapsed[frames - 1],
+        elapsed.iter().sum::<f64>()
+    );
+}
+
+fn kb(field: &str) -> u64 {
+    std::fs::read_to_string("/proc/self/status")
+        .unwrap()
+        .lines()
+        .find(|line| line.starts_with(field))
+        .and_then(|line| line.split_whitespace().nth(1)?.parse().ok())
+        .unwrap()
+}
+
+fn report(label: &str) {
+    println!(
+        "{label:24} rss {:>7} kB   peak {:>7} kB",
+        kb("VmRSS:"),
+        kb("VmHWM:")
+    );
+}
+
+/// Run this one on its own: the figures are the whole process's.
+#[test]
+#[ignore = "measurement, not a check"]
+fn text_memory() {
+    let rows = env("ROWS", 2000_usize);
+    report("before");
+    let mut h = Harness::new(OUTPUT);
+    let paragraphs = build(&mut h, rows);
+    report("after cold frame");
+    for frame in 0..40 {
+        h.resize((OUTPUT.0 - ((frame + 1) % 2) as f32 * 8.0, OUTPUT.1));
+        h.frame();
+    }
+    report("after 40 resizes");
+    // Settled: the output holds still and one leaf repaints per frame.
+    for _ in 0..10 {
+        let _ = h.rsc.widgets_mut().get_dyn_mut(paragraphs[0]);
+        h.frame();
+    }
+    report("after settling");
+}

tests/shrink.rs

@@ -0,0 +1,525 @@
+//! A property test that shrinks its own counterexample.
+//!
+//! `generated.rs` reproduces a failure from a seed, but a seed is not a lead
+//! anybody can read: the tree is hundreds of widgets, and reconstructing the
+//! part that matters by hand has failed every time it has been tried. This
+//! grows trees it can take apart, so a failure is reduced to the smallest
+//! tree that still shows it and printed as something to write a fast test
+//! from.
+//!
+//!     cargo test --release --test shrink -- --ignored --nocapture
+//!
+//! `SHRINK_SEEDS` how many trees to try, `SHRINK_DEPTH` how deep to grow
+//! them, `SHRINK_CASE` which scenario. It is a fuzzer: run it once the
+//! ordinary tests pass, and turn what it finds into a test of its own rather
+//! than leaving a seed as the record.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+use iris::random::{Branch, Rng};
+
+/// The same two leaves `iris::random` grows, since only one of them reads the
+/// width it is given and that is the difference that matters.
+const WORDS: &[&str] = &[
+    "Wrapping",
+    "shapes",
+    "one",
+    "source",
+    "into",
+    "as",
+    "many",
+    "lines",
+    "as",
+    "the",
+    "box",
+    "leaves",
+    "room",
+    "for,",
+    "so",
+    "a",
+    "paragraph's",
+    "height",
+    "is",
+    "an",
+    "answer",
+    "and",
+    "not",
+    "a",
+    "setting.",
+];
+
+const ONE_LINE: &str = "one line, overflowing whatever it is given";
+
+const OUTER: (f32, f32) = (1920.0, 1200.0);
+const INNER: (f32, f32) = (640.0, 900.0);
+
+#[derive(Clone, Debug, PartialEq)]
+enum Node {
+    /// Words taken from [`WORDS`], and whether it wraps.
+    Text(usize, bool),
+    /// The leaf that overflows whatever box it is given rather than wrapping.
+    OneLine,
+    Rect,
+    /// Direction, gap, children in creation order, and the order they are
+    /// attached in -- separate so a tree that reorders its children
+    /// still makes the same widgets in the same order, and two
+    /// builds line up index for index.
+    Span(bool, f32, Vec<Node>, Vec<usize>),
+    Stack(Vec<Node>),
+    Pad(f32, Box<Node>),
+    Aligned(u8, u8, Box<Node>),
+    Sized(Option<Len>, Option<Len>, Box<Node>),
+    Scroll(bool, Box<Node>),
+    Branch(Box<Node>, Box<Node>, Box<Node>, f32),
+}
+
+fn axis_align(v: u8) -> Option<AxisAlign> {
+    match v % 4 {
+        0 => None,
+        1 => Some(AxisAlign::Neg),
+        2 => Some(AxisAlign::Center),
+        _ => Some(AxisAlign::Pos),
+    }
+}
+
+fn dir(down: bool) -> Dir {
+    if down { Dir::DOWN } else { Dir::RIGHT }
+}
+
+impl Node {
+    /// Builds into `h`, pushing every id in tree order, so two builds of one
+    /// node line up index for index and their boxes can be compared.
+    fn build(
+        &self,
+        h: &mut Harness,
+        out: &mut Vec<WidgetId>,
+        spans: &mut Vec<WeakWidget<Span>>,
+    ) -> StrongWidget {
+        let id: StrongWidget = match self {
+            Node::Text(words, wrap) => {
+                let n = (*words).clamp(1, WORDS.len());
+                wtext(WORDS[..n].join(" "))
+                    .size(16)
+                    .wrap(*wrap)
+                    .add_strong(&mut h.rsc)
+            }
+            Node::OneLine => wtext(ONE_LINE).size(16).wrap(false).add_strong(&mut h.rsc),
+            Node::Rect => rect(Color::RED).add_strong(&mut h.rsc),
+            Node::Span(down, gap, kids, order) => {
+                let mut built: Vec<_> = kids.iter().map(|k| Some(k.build(h, out, spans))).collect();
+                // `order` is a permutation, so each is taken exactly once.
+                let children = order
+                    .iter()
+                    .map(|&i| built[i].take().expect("order repeats an index"))
+                    .collect();
+                let handle = Span {
+                    children,
+                    dir: dir(*down),
+                    gap: *gap,
+                }
+                .add(&mut h.rsc);
+                spans.push(handle);
+                handle.add_strong(&mut h.rsc)
+            }
+            Node::Stack(kids) => {
+                let children = kids.iter().map(|k| k.build(h, out, spans)).collect();
+                Stack {
+                    children,
+                    size: StackSize::Child(0),
+                }
+                .add_strong(&mut h.rsc)
+            }
+            Node::Pad(p, kid) => {
+                let inner = kid.build(h, out, spans);
+                Pad {
+                    padding: Padding {
+                        left: *p,
+                        right: *p,
+                        top: *p,
+                        bottom: *p,
+                    },
+                    inner,
+                }
+                .add_strong(&mut h.rsc)
+            }
+            Node::Aligned(x, y, kid) => {
+                let inner = kid.build(h, out, spans);
+                Aligned {
+                    inner,
+                    align: Align {
+                        x: axis_align(*x),
+                        y: axis_align(*y),
+                    },
+                }
+                .add_strong(&mut h.rsc)
+            }
+            Node::Sized(x, y, kid) => {
+                let inner = kid.build(h, out, spans);
+                SetSize {
+                    inner,
+                    x: *x,
+                    y: *y,
+                }
+                .add_strong(&mut h.rsc)
+            }
+            Node::Scroll(down, kid) => {
+                let inner = kid.build(h, out, spans);
+                let axis = if *down { Axis::Y } else { Axis::X };
+                Scroll::new(inner, axis).add_strong(&mut h.rsc)
+            }
+            Node::Branch(probe, a, b, at) => {
+                let probe = probe.build(h, out, spans);
+                let wide = a.build(h, out, spans);
+                let narrow = b.build(h, out, spans);
+                Branch {
+                    probe,
+                    wide,
+                    narrow,
+                    threshold: *at,
+                }
+                .add_strong(&mut h.rsc)
+            }
+        };
+        out.push(id.id());
+        id
+    }
+
+    fn size(&self) -> usize {
+        1 + match self {
+            Node::Text(..) | Node::OneLine | Node::Rect => 0,
+            Node::Span(_, _, kids, _) | Node::Stack(kids) => kids.iter().map(Node::size).sum(),
+            Node::Pad(_, k)
+            | Node::Aligned(_, _, k)
+            | Node::Sized(_, _, k)
+            | Node::Scroll(_, k) => k.size(),
+            Node::Branch(p, a, b, _) => p.size() + a.size() + b.size(),
+        }
+    }
+
+    /// Every one-step simplification: a wrapper replaced by what it wrapped, a
+    /// child dropped, a length or a word count reduced. Ordered cheapest-first
+    /// so the greedy walk takes the biggest bites early.
+    fn smaller(&self) -> Vec<Node> {
+        let mut out = Vec::new();
+        let leaf = Node::Rect;
+        match self {
+            Node::Text(words, wrap) => {
+                if *words > 1 {
+                    out.push(Node::Text(words / 2, *wrap));
+                    out.push(Node::Text(words - 1, *wrap));
+                }
+                if *wrap {
+                    out.push(Node::Text(*words, false));
+                }
+                out.push(leaf);
+            }
+            Node::OneLine => out.push(Node::Rect),
+            Node::Rect => {}
+            Node::Span(down, gap, kids, order) => {
+                out.extend(order.iter().map(|&i| kids[i].clone()));
+                for i in 0..kids.len() {
+                    if kids.len() > 1 {
+                        let mut less = kids.clone();
+                        less.remove(i);
+                        let order = (0..less.len()).collect();
+                        out.push(Node::Span(*down, *gap, less, order));
+                    }
+                }
+                if *gap != 0.0 {
+                    out.push(Node::Span(*down, 0.0, kids.clone(), order.clone()));
+                }
+                for (i, kid) in kids.iter().enumerate() {
+                    for small in kid.smaller() {
+                        let mut next = kids.clone();
+                        next[i] = small;
+                        out.push(Node::Span(*down, *gap, next, order.clone()));
+                    }
+                }
+            }
+            Node::Stack(kids) => {
+                out.extend(kids.iter().cloned());
+                for i in 0..kids.len() {
+                    if kids.len() > 1 {
+                        let mut less = kids.clone();
+                        less.remove(i);
+                        out.push(Node::Stack(less));
+                    }
+                }
+                for (i, kid) in kids.iter().enumerate() {
+                    for small in kid.smaller() {
+                        let mut next = kids.clone();
+                        next[i] = small;
+                        out.push(Node::Stack(next));
+                    }
+                }
+            }
+            Node::Pad(p, kid) => {
+                out.push((**kid).clone());
+                if *p != 0.0 {
+                    out.push(Node::Pad(0.0, kid.clone()));
+                }
+                out.extend(
+                    kid.smaller()
+                        .into_iter()
+                        .map(|k| Node::Pad(*p, Box::new(k))),
+                );
+            }
+            Node::Aligned(x, y, kid) => {
+                out.push((**kid).clone());
+                for (nx, ny) in [(0, *y), (*x, 0)] {
+                    if (nx, ny) != (*x, *y) {
+                        out.push(Node::Aligned(nx, ny, kid.clone()));
+                    }
+                }
+                out.extend(
+                    kid.smaller()
+                        .into_iter()
+                        .map(|k| Node::Aligned(*x, *y, Box::new(k))),
+                );
+            }
+            Node::Sized(x, y, kid) => {
+                out.push((**kid).clone());
+                if x.is_some() {
+                    out.push(Node::Sized(None, *y, kid.clone()));
+                }
+                if y.is_some() {
+                    out.push(Node::Sized(*x, None, kid.clone()));
+                }
+                out.extend(
+                    kid.smaller()
+                        .into_iter()
+                        .map(|k| Node::Sized(*x, *y, Box::new(k))),
+                );
+            }
+            Node::Scroll(down, kid) => {
+                out.push((**kid).clone());
+                out.extend(
+                    kid.smaller()
+                        .into_iter()
+                        .map(|k| Node::Scroll(*down, Box::new(k))),
+                );
+            }
+            Node::Branch(p, a, b, at) => {
+                out.push((**p).clone());
+                out.push((**a).clone());
+                out.push((**b).clone());
+                for small in p.smaller() {
+                    out.push(Node::Branch(Box::new(small), a.clone(), b.clone(), *at));
+                }
+                for small in a.smaller() {
+                    out.push(Node::Branch(p.clone(), Box::new(small), b.clone(), *at));
+                }
+                for small in b.smaller() {
+                    out.push(Node::Branch(p.clone(), a.clone(), Box::new(small), *at));
+                }
+            }
+        }
+        out
+    }
+}
+
+/// A declared size over about half the tree, the way `iris::random` puts them
+/// in: on the way into every child rather than as a node kind of its own, so
+/// readers of a size are dense rather than occasional.
+fn sized(rng: &mut Rng, inner: Node) -> Node {
+    if !rng.chance() {
+        return inner;
+    }
+    let len = |rng: &mut Rng| match rng.below(4) {
+        0 => Some(Len::px(20.0 + rng.below(180) as f32)),
+        1 => Some(Len::REST),
+        _ => None,
+    };
+    Node::Sized(len(rng), len(rng), Box::new(inner))
+}
+
+fn grow(rng: &mut Rng, depth: usize) -> Node {
+    if depth == 0 {
+        return match rng.below(4) {
+            0 => Node::Text(1 + rng.below(WORDS.len()), true),
+            1 => Node::OneLine,
+            _ => Node::Rect,
+        };
+    }
+    let len = |rng: &mut Rng| match rng.below(4) {
+        0 => Some(Len::px(20.0 + rng.below(180) as f32)),
+        1 => Some(Len::REST),
+        2 => Some(Len::rel(0.25 + rng.below(3) as f32 * 0.25)),
+        _ => None,
+    };
+    let kid = |rng: &mut Rng| {
+        let inner = grow(rng, depth - 1);
+        sized(rng, inner)
+    };
+    match rng.below(8) {
+        0 => Node::Scroll(rng.chance(), Box::new(kid(rng))),
+        1 => Node::Aligned(rng.below(4) as u8, rng.below(4) as u8, Box::new(kid(rng))),
+        2 => Node::Pad(rng.below(24) as f32, Box::new(kid(rng))),
+        3 => Node::Sized(len(rng), len(rng), Box::new(kid(rng))),
+        4 => Node::Branch(
+            Box::new(kid(rng)),
+            Box::new(kid(rng)),
+            Box::new(kid(rng)),
+            rng.below(500) as f32,
+        ),
+        5 => Node::Stack((0..2 + rng.below(2)).map(|_| kid(rng)).collect()),
+        _ => {
+            let kids: Vec<_> = (0..2 + rng.below(3)).map(|_| kid(rng)).collect();
+            let order = (0..kids.len()).collect();
+            Node::Span(rng.chance(), rng.below(3) as f32 * 4.0, kids, order)
+        }
+    }
+}
+
+#[derive(Clone, Copy, PartialEq)]
+enum Case {
+    Resize,
+    Repaint,
+    ResizeRepaint,
+    Reorder,
+}
+
+/// Every span's children rotated by one, as a tree rather than as a change:
+/// what a warm frame reaches by moving them has to be where growing them that
+/// way lands.
+fn reordered(node: &Node) -> Node {
+    match node {
+        Node::Span(down, gap, kids, order) => {
+            let kids = kids.iter().map(reordered).collect::<Vec<_>>();
+            let mut order = order.clone();
+            order.rotate_left(1);
+            Node::Span(*down, *gap, kids, order)
+        }
+        Node::Stack(kids) => Node::Stack(kids.iter().map(reordered).collect()),
+        Node::Pad(p, k) => Node::Pad(*p, Box::new(reordered(k))),
+        Node::Aligned(x, y, k) => Node::Aligned(*x, *y, Box::new(reordered(k))),
+        Node::Sized(x, y, k) => Node::Sized(*x, *y, Box::new(reordered(k))),
+        Node::Scroll(d, k) => Node::Scroll(*d, Box::new(reordered(k))),
+        Node::Branch(p, a, b, at) => Node::Branch(
+            Box::new(reordered(p)),
+            Box::new(reordered(a)),
+            Box::new(reordered(b)),
+            *at,
+        ),
+        leaf => leaf.clone(),
+    }
+}
+
+/// Runs one scenario warm and cold and says where they disagree.
+fn diverges(node: &Node, case: Case) -> Option<String> {
+    let resizes = matches!(case, Case::Resize | Case::ResizeRepaint);
+    let repaints = matches!(case, Case::Repaint | Case::ResizeRepaint);
+    let start = if resizes { OUTER } else { INNER };
+    let mut warm = Harness::new(start);
+    let mut warm_ids = Vec::new();
+    let mut warm_spans = Vec::new();
+    let root = node.build(&mut warm, &mut warm_ids, &mut warm_spans);
+    warm.state.root = Some(root);
+    // The frame that makes it warm: without it there is nothing retained and
+    // the comparison is two cold starts agreeing with each other.
+    warm.frame();
+    if resizes {
+        warm.resize(INNER);
+        warm.frame();
+    }
+    if repaints {
+        for &id in &warm_ids {
+            warm.rsc.widgets_mut().get_dyn_mut(id);
+        }
+        warm.frame();
+    }
+    if case == Case::Reorder {
+        for span in &warm_spans {
+            warm.rsc[*span].children.rotate_left(1);
+        }
+        warm.frame();
+    }
+
+    // What the warm tree was moved into, grown that way from the start.
+    let want = match case {
+        Case::Reorder => reordered(node),
+        _ => node.clone(),
+    };
+    let mut cold = Harness::new(INNER);
+    let mut cold_ids = Vec::new();
+    let mut cold_spans = Vec::new();
+    let root = want.build(&mut cold, &mut cold_ids, &mut cold_spans);
+    cold.state.root = Some(root);
+    cold.frame();
+
+    for (i, (&w, &c)) in warm_ids.iter().zip(&cold_ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        let same = match (got, want) {
+            (Some(g), Some(c)) => {
+                let d = |a: f32, b: f32| (a - b).abs() <= 0.05;
+                d(g.top_left.x, c.top_left.x)
+                    && d(g.top_left.y, c.top_left.y)
+                    && d(g.bot_right.x, c.bot_right.x)
+                    && d(g.bot_right.y, c.bot_right.y)
+            }
+            (None, None) => true,
+            _ => false,
+        };
+        if !same {
+            return Some(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    None
+}
+
+/// Takes the first simplification that still fails, until none does.
+fn shrink(mut node: Node, case: Case) -> Node {
+    loop {
+        let Some(next) = node
+            .smaller()
+            .into_iter()
+            .find(|small| diverges(small, case).is_some())
+        else {
+            return node;
+        };
+        node = next;
+    }
+}
+
+fn env<T: std::str::FromStr>(name: &str, fallback: T) -> T {
+    std::env::var(name)
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(fallback)
+}
+
+#[test]
+#[ignore = "a fuzzer; run it once the ordinary tests pass"]
+fn no_grown_tree_lays_out_differently_warm_than_cold() {
+    let seeds: u64 = env("SHRINK_SEEDS", 400);
+    let depth: usize = env("SHRINK_DEPTH", 5);
+    let case = match env("SHRINK_CASE", String::from("resize")).as_str() {
+        "repaint" => Case::Repaint,
+        "resize-repaint" => Case::ResizeRepaint,
+        "reorder" => Case::Reorder,
+        _ => Case::Resize,
+    };
+
+    for seed in 1..=seeds {
+        let node = grow(&mut Rng::new(seed), depth);
+        let Some(how) = diverges(&node, case) else {
+            continue;
+        };
+        let small = shrink(node.clone(), case);
+        println!(
+            "seed {seed}: {how}\ngrown {} widgets, shrank to {}\n{small:#?}",
+            node.size(),
+            small.size()
+        );
+        panic!("seed {seed} lays out differently warm than cold");
+    }
+    let sizes: Vec<usize> = (1..=seeds)
+        .map(|seed| grow(&mut Rng::new(seed), depth).size())
+        .collect();
+    let total: usize = sizes.iter().sum();
+    println!(
+        "{seeds} trees at depth {depth} agree: {} widgets total, largest {}",
+        total,
+        sizes.iter().max().copied().unwrap_or(0)
+    );
+}

tests/trace_unsettled.rs

@@ -0,0 +1,155 @@
+//! Traces the six-widget tree in `unsettled.rs`, to see what box its text is
+//! actually drawn in on a first frame against a settled one.
+
+#![cfg(feature = "layout-diagnostics")]
+
+use iris::core::layout_diagnostics::{self as diag, TraceEvent};
+use iris::harness::Harness;
+use iris::prelude::*;
+
+fn plant(h: &mut Harness) -> Vec<WidgetId> {
+    let plain = wtext("Wrapping").size(16).wrap(false).add(&mut h.rsc);
+    let wrapped = wtext("Wrapping shapes").size(16).wrap(true).add(&mut h.rsc);
+    let sized = SetSize {
+        inner: wrapped.add_strong(&mut h.rsc),
+        x: Some(Len::px(76.0)),
+        y: None,
+    }
+    .add(&mut h.rsc);
+    let aligned = Aligned {
+        inner: sized.add_strong(&mut h.rsc),
+        align: Align {
+            x: Some(AxisAlign::Pos),
+            y: Some(AxisAlign::Pos),
+        },
+    }
+    .add(&mut h.rsc);
+    let stack = Stack {
+        children: vec![plain.add_strong(&mut h.rsc), aligned.add_strong(&mut h.rsc)],
+        size: StackSize::Child(0),
+    }
+    .add(&mut h.rsc);
+    let root = (stack,).span(Dir::RIGHT).add(&mut h.rsc);
+    h.state.root = Some(root.add_strong(&mut h.rsc));
+    vec![
+        plain.id(),
+        wrapped.id(),
+        sized.id(),
+        aligned.id(),
+        stack.id(),
+        root.id(),
+    ]
+}
+
+fn dump(label: &str, report: &diag::Report, text: WidgetId) {
+    println!("--- {label} ---");
+    for event in report.traces() {
+        match event {
+            TraceEvent::DrawRequest {
+                id,
+                region,
+                pixel_size,
+                ..
+            } if *id == text => {
+                println!(
+                    "  draw in {:.2}x{:.2}  region {region:?}",
+                    pixel_size.x, pixel_size.y
+                )
+            }
+            TraceEvent::SizeReported { id, size } if *id == text => {
+                println!("  reported {size}")
+            }
+            TraceEvent::SizeRead { id, reader, size } if *id == text => {
+                println!("  size read by {reader:?}: {size}")
+            }
+            TraceEvent::Placed { id, parent, region } if *id == text => {
+                println!("  placed by {parent:?} at {region:?}")
+            }
+            TraceEvent::Reuse { id, outcome } if *id == text => println!("  reuse: {outcome:?}"),
+            _ => {}
+        }
+    }
+}
+
+#[test]
+#[ignore = "a diagnostic, not a check"]
+fn what_box_the_text_is_drawn_in() {
+    diag::clear_traced_widgets();
+    let _ = diag::take();
+    let mut h = Harness::new((640, 900));
+    let ids = plant(&mut h);
+    let text = ids[1];
+    diag::trace_widget(text);
+    let _ = diag::take();
+
+    h.frame();
+    dump("first frame", &diag::take(), text);
+
+    for _ in 0..2 {
+        for &id in &ids {
+            h.rsc.widgets_mut().get_dyn_mut(id);
+        }
+        let _ = diag::take();
+        h.frame();
+        dump("repaint", &diag::take(), text);
+    }
+    diag::clear_traced_widgets();
+}
+
+fn plant_fixed(h: &mut Harness) -> Vec<WidgetId> {
+    let words = "Wrapping shapes one source into as many lines as the box leaves";
+    let text = wtext(words).size(16).wrap(true).add(&mut h.rsc);
+    let aligned = Aligned {
+        inner: text.add_strong(&mut h.rsc),
+        align: Align {
+            x: Some(AxisAlign::Neg),
+            y: None,
+        },
+    }
+    .add(&mut h.rsc);
+    let inner = (aligned,).span(Dir::RIGHT).add(&mut h.rsc);
+    let sized = SetSize {
+        inner: inner.add_strong(&mut h.rsc),
+        x: Some(Len::px(189.0)),
+        y: Some(Len::px(176.0)),
+    }
+    .add(&mut h.rsc);
+    let filler = rect(Color::RED).add(&mut h.rsc);
+    let root = (filler, sized).span(Dir::RIGHT).add(&mut h.rsc);
+    h.state.root = Some(root.add_strong(&mut h.rsc));
+    vec![
+        text.id(),
+        aligned.id(),
+        inner.id(),
+        sized.id(),
+        filler.id(),
+        root.id(),
+    ]
+}
+
+#[test]
+#[ignore = "a diagnostic, not a check"]
+fn what_box_the_fixed_text_is_drawn_in() {
+    diag::clear_traced_widgets();
+    let _ = diag::take();
+    let mut h = Harness::new((1920, 1200));
+    let ids = plant_fixed(&mut h);
+    let text = ids[0];
+    diag::trace_widget(text);
+    let _ = diag::take();
+
+    h.frame();
+    dump("first frame at 1920", &diag::take(), text);
+    h.resize((640, 900));
+    h.frame();
+    dump("after resize to 640", &diag::take(), text);
+
+    let mut cold = Harness::new((640, 900));
+    let cids = plant_fixed(&mut cold);
+    diag::clear_traced_widgets();
+    diag::trace_widget(cids[0]);
+    let _ = diag::take();
+    cold.frame();
+    dump("cold at 640", &diag::take(), cids[0]);
+    diag::clear_traced_widgets();
+}

tests/unsettled.rs

@@ -0,0 +1,298 @@
+//! The smallest trees that laid out differently warm than cold, each shrunk
+//! by `tests/shrink.rs` from hundreds of widgets. The first two are a cold
+//! frame that had not settled: a wrapping text shaped at a width it was
+//! measured in rather than the one it was given. The rest are a widget
+//! measured again in a box its own answer had decided, where the old answer
+//! is a fixed point whatever the content now says.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+/// Six widgets, shrunk from a 402-widget tree the fuzzer found. Nothing about
+/// the tree changes -- every widget is marked for redraw and the frame is
+/// taken again -- so no box may move, and a warm frame has to land where a
+/// cold one does.
+fn plant(h: &mut Harness) -> Vec<WidgetId> {
+    let plain = wtext("Wrapping").size(16).wrap(false).add(&mut h.rsc);
+    let wrapped = wtext("Wrapping shapes").size(16).wrap(true).add(&mut h.rsc);
+    let sized = SetSize {
+        inner: wrapped.add_strong(&mut h.rsc),
+        x: Some(Len::px(76.0)),
+        y: None,
+    }
+    .add(&mut h.rsc);
+    let aligned = Aligned {
+        inner: sized.add_strong(&mut h.rsc),
+        align: Align {
+            x: Some(AxisAlign::Pos),
+            y: Some(AxisAlign::Pos),
+        },
+    }
+    .add(&mut h.rsc);
+    let stack = Stack {
+        children: vec![plain.add_strong(&mut h.rsc), aligned.add_strong(&mut h.rsc)],
+        size: StackSize::Child(0),
+    }
+    .add(&mut h.rsc);
+    let root = (stack,).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root(root);
+    vec![
+        plain.id(),
+        wrapped.id(),
+        sized.id(),
+        aligned.id(),
+        stack.id(),
+        root.id(),
+    ]
+}
+
+/// The first frame does not reach the layout a second one does, so "cold" is
+/// not a fixed point and comparing against it compares against a tree that
+/// has not settled.
+#[test]
+fn one_frame_is_enough() {
+    let mut h = Harness::new((640, 900));
+    let ids = plant(&mut h);
+    let first = h.region(&ids[1]).unwrap();
+    for _ in 0..3 {
+        for &id in &ids {
+            h.rsc.widgets_mut().get_dyn_mut(id);
+        }
+        h.frame();
+    }
+    let settled = h.region(&ids[1]).unwrap();
+    println!(
+        "first frame {} tall, settled {} tall",
+        first.bot_right.y - first.top_left.y,
+        settled.bot_right.y - settled.top_left.y
+    );
+    assert_eq!(
+        first.bot_right.y - first.top_left.y,
+        settled.bot_right.y - settled.top_left.y,
+        "the first frame had not finished laying out"
+    );
+}
+
+#[test]
+fn repainting_everything_moves_nothing() {
+    let mut warm = Harness::new((640, 900));
+    let ids = plant(&mut warm);
+    for &id in &ids {
+        warm.rsc.widgets_mut().get_dyn_mut(id);
+    }
+    warm.frame();
+
+    let mut cold = Harness::new((640, 900));
+    let cold_ids = plant(&mut cold);
+
+    let mut wrong = Vec::new();
+    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        if got != want {
+            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+}
+
+/// Six widgets, shrunk from 905. Everything inside the declared 189x176 box
+/// is the same size whatever the output is, so a resize may not change any of
+/// it -- but the text comes out 3.92px narrower warm than cold.
+fn plant_fixed(h: &mut Harness) -> Vec<WidgetId> {
+    let words = "Wrapping shapes one source into as many lines as the box leaves";
+    let text = wtext(words).size(16).wrap(true).add(&mut h.rsc);
+    let aligned = Aligned {
+        inner: text.add_strong(&mut h.rsc),
+        align: Align {
+            x: Some(AxisAlign::Neg),
+            y: None,
+        },
+    }
+    .add(&mut h.rsc);
+    let inner = (aligned,).span(Dir::RIGHT).add(&mut h.rsc);
+    let sized = SetSize {
+        inner: inner.add_strong(&mut h.rsc),
+        x: Some(Len::px(189.0)),
+        y: Some(Len::px(176.0)),
+    }
+    .add(&mut h.rsc);
+    let filler = rect(Color::RED).add(&mut h.rsc);
+    let root = (filler, sized).span(Dir::RIGHT).add(&mut h.rsc);
+    h.state.root = Some(root.add_strong(&mut h.rsc));
+    vec![
+        text.id(),
+        aligned.id(),
+        inner.id(),
+        sized.id(),
+        filler.id(),
+        root.id(),
+    ]
+}
+
+#[test]
+fn a_resize_does_not_reach_inside_a_box_of_declared_pixels() {
+    let mut warm = Harness::new((1920, 1200));
+    let ids = plant_fixed(&mut warm);
+    warm.frame();
+    warm.resize((640, 900));
+    warm.frame();
+
+    let mut cold = Harness::new((640, 900));
+    let cold_ids = plant_fixed(&mut cold);
+    cold.frame();
+
+    let mut wrong = Vec::new();
+    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        if got != want {
+            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+}
+
+/// Four widgets, shrunk from 486. A span's two children are swapped: warm by
+/// moving them, cold by growing them that way. Same widgets, same sizes, one
+/// ends up 29.9px from where the other does.
+fn plant_pair(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, WeakWidget<Span>) {
+    let wrapped = wtext("Wrapping shapes one source into as many lines")
+        .size(16)
+        .wrap(true)
+        .add(&mut h.rsc);
+    let plain = wtext("one line, overflowing whatever it is given")
+        .size(16)
+        .wrap(false)
+        .add(&mut h.rsc);
+    let first: StrongWidget = wrapped.add_strong(&mut h.rsc);
+    let second: StrongWidget = plain.add_strong(&mut h.rsc);
+    let children = match swapped {
+        true => vec![second, first],
+        false => vec![first, second],
+    };
+    let span = Span {
+        children,
+        dir: Dir::RIGHT,
+        gap: 0.0,
+    }
+    .add(&mut h.rsc);
+    let span_handle = span;
+    let aligned = Aligned {
+        inner: span.add_strong(&mut h.rsc),
+        align: Align {
+            x: Some(AxisAlign::Center),
+            y: None,
+        },
+    }
+    .add(&mut h.rsc);
+    h.state.root = Some(aligned.add_strong(&mut h.rsc));
+    (
+        vec![wrapped.id(), plain.id(), span.id(), aligned.id()],
+        span_handle,
+    )
+}
+
+#[test]
+fn swapping_two_children_lands_where_growing_them_that_way_does() {
+    let mut warm = Harness::new((640, 900));
+    let (ids, span) = plant_pair(&mut warm, false);
+    warm.frame();
+    warm.rsc[span].children.rotate_left(1);
+    warm.frame();
+
+    let mut cold = Harness::new((640, 900));
+    let (cold_ids, _) = plant_pair(&mut cold, true);
+    cold.frame();
+
+    let mut wrong = Vec::new();
+    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        if got != want {
+            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+}
+
+/// Eight widgets, shrunk from 80. The scroll decides how wide to make its
+/// content from what the content says, and hands that box down through a
+/// pass-through; the span under it was placed once, in that box, so nothing
+/// at its own edge says the box was its own answer.
+fn plant_scrolled(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget<Span>; 2]) {
+    let words = "Wrapping shapes one source into as many lines as the box leaves room for,";
+    let text = wtext(words).size(16).wrap(true).add(&mut h.rsc);
+    let filler = rect(Color::RED).add(&mut h.rsc);
+    let mut inner_children: Vec<StrongWidget> =
+        vec![text.add_strong(&mut h.rsc), filler.add_strong(&mut h.rsc)];
+    if swapped {
+        inner_children.rotate_left(1);
+    }
+    let inner = Span {
+        children: inner_children,
+        dir: Dir::RIGHT,
+        gap: 0.0,
+    }
+    .add(&mut h.rsc);
+    let block = rect(Color::RED).add(&mut h.rsc);
+    let fixed = SetSize {
+        inner: block.add_strong(&mut h.rsc),
+        x: Some(Len::px(87.0)),
+        y: None,
+    }
+    .add(&mut h.rsc);
+    let mut outer_children: Vec<StrongWidget> =
+        vec![fixed.add_strong(&mut h.rsc), inner.add_strong(&mut h.rsc)];
+    if swapped {
+        outer_children.rotate_left(1);
+    }
+    let outer = Span {
+        children: outer_children,
+        dir: Dir::RIGHT,
+        gap: 0.0,
+    }
+    .add(&mut h.rsc);
+    let through = SetSize {
+        inner: outer.add_strong(&mut h.rsc),
+        x: None,
+        y: None,
+    }
+    .add(&mut h.rsc);
+    let scroll = Scroll::new(through.add_strong(&mut h.rsc), Axis::X).add(&mut h.rsc);
+    h.state.root = Some(scroll.add_strong(&mut h.rsc));
+    (
+        vec![
+            text.id(),
+            filler.id(),
+            inner.id(),
+            block.id(),
+            fixed.id(),
+            outer.id(),
+            through.id(),
+            scroll.id(),
+        ],
+        [inner, outer],
+    )
+}
+
+#[test]
+fn a_span_placed_once_in_a_box_its_answer_decided() {
+    let mut warm = Harness::new((640, 900));
+    let (ids, spans) = plant_scrolled(&mut warm, false);
+    warm.frame();
+    for span in spans {
+        warm.rsc[span].children.rotate_left(1);
+    }
+    warm.frame();
+
+    let mut cold = Harness::new((640, 900));
+    let (cold_ids, _) = plant_scrolled(&mut cold, true);
+    cold.frame();
+
+    let mut wrong = Vec::new();
+    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        if got != want {
+            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+}