Grow images in the generated trees

`Image` is the only widget in the repository whose size hint is a length in
pixels -- everything else hints a share, or nothing -- so it is the only one
that exercises a rule beside a hint, a box a widget knows before it is drawn,
and the answer the commit before this one changed. The generated trees had
none, which is why nothing there could reach that case.

`Kind::Image` is a fifth leaf, drawn one time in five, and it steps to a plain
rect when the shrinker reduces it: a picture measures nothing either, but its
length is its own, so the leaf that takes whatever it is given is the simpler
one. The picture is a 64x64 checkerboard of purple and black in 8 px cells,
committed at `src/assets/checkerboard.png` beside the generator that draws it
-- the way `examples/tabs` keeps its own -- and included rather than opened, so
that growing a tree does not depend on a working directory and one seed is one
tree whatever anything else does.

One upload per tree, however many images it grows: a `TextureHandle` is a
counted reference, so the first image in a tree uploads the checkerboard and
every one after it clones the handle. Measured: seed 1 at depth 4 grows 13
images and holds 1 texture, seed 6 grows none and holds none, and
`a_tree_of_images_uploads_one_texture` asserts it. `Image::new` is what a
caller holding a handle needs, since `image` uploads what it is given.

A seed names a tree only while the generator draws the same things in the same
order, so every seed now grows a different tree. The seed list in
`generated.rs` says so: 20 and 86 no longer grow the trees whose defects they
once caught, and both of those live on as shrunk fixtures in `unsettled.rs`,
which are trees rather than numbers. The seeds those fixtures name are
similarly historical, and their file says that too.

Format, clippy with and without layout-diagnostics, and the suite (135 + 19 +
13 + 4) are clean. The cold dump is a new baseline of 34,571 boxes over the 400
depth-5 trees, since the trees themselves changed; all three seed scans pass
over the new ones -- 400 at depth 5 in 62.79s, 1,000 at depth 6 in 160.20s,
2,000 at depth 4 in 299.58s -- which is what actually checks that images lay
out warm the way they do cold.

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

Cargo.toml

@@ -25,6 +25,12 @@ tokio = { workspace = true, features = ["sync", "rt", "rt-multi-thread", "time"]
 [workspace]
 members = ["core", "macro", "rig-input"]
 
+# Full debug info was the bulk of what the linker wrote here and almost none of
+# what anything read. `dev` keeps line tables and scopes, which is what stepping
+# through an example wants; the tests keep the line tables alone, which is what
+# a backtrace reads. Measured when the tests became one target: relinking them
+# went from 9.8 s to 7.7 s with these, and target/ from 45 GB to 13 GB with the
+# two changes together.
 [profile.dev]
 debug = 1
 

TODO

@@ -24,3 +24,17 @@ transforms on a move entry (scale + rotation)
     write and no redraw
     wanted for compose-style stretch at the end of a scroll area, and for
     rotation generally
+
+a prepare stage on Event, so Data has no placeholder field
+    run_sensors builds one CursorData per widget and has to put something in
+    `sense` before anything knows which sense matched, so it writes
+    CursorSense::Hovering and says in place that it means nothing;
+    should_run then clones the whole thing to overwrite that one field
+    the state is representable only because the type lets the caller say it:
+    what the caller supplies and what matching adds are two different things
+    wearing one struct
+    the awkward part is doing it without the generics getting annoying --
+    Data<'a> is already a GAT with a default, and splitting it in two adds
+    another associated type to every Event impl for the sake of one field
+    (Bryan, 2026-09-20; low priority, he wants a good answer rather than a
+    quick one)

core/src/fixed.rs

@@ -131,14 +131,6 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
         self.0 as f32 / Self::one().0 as f32
     }
 
-    /// The same value on another grid, rounded where the new one is coarser.
-    pub const fn to_scale<const TO: u32>(self) -> Fixed<TO> {
-        Fixed(match TO >= SHIFT {
-            true => self.0 << (TO - SHIFT),
-            false => shift_round(self.0 as i64, SHIFT - TO) as i32,
-        })
-    }
-
     pub const fn add(self, rhs: Self) -> Self {
         Self(self.0.wrapping_add(rhs.0))
     }
@@ -246,16 +238,6 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
     }
 }
 
-/// Back to a single step, rounding halves away from zero so that a value and
-/// its negation round to the same distance.
-const fn shift_round(v: i64, bits: u32) -> i64 {
-    let half = (1i64 << bits) >> 1;
-    match v < 0 {
-        true => -((-v + half) >> bits),
-        false => (v + half) >> bits,
-    }
-}
-
 const fn div_round(num: i64, den: i64) -> i64 {
     let (q, rem) = (num / den, num % den);
     match rem.unsigned_abs() * 2 >= den.unsigned_abs() {
@@ -531,15 +513,6 @@ mod tests {
         assert_eq!(Px::from_f32(-1e12), Px::MIN);
     }
 
-    #[test]
-    fn a_coarser_grid_rounds_and_a_finer_one_does_not() {
-        // A third, which neither grid holds exactly.
-        let third = Rel::ONE / Rel::from_int(3);
-        assert_eq!(third.to_scale::<6>(), Fixed::<6>::from_raw(21));
-        let coarse = Fixed::<6>::from_raw(21);
-        assert_eq!(coarse.to_scale::<24>().to_scale::<6>(), coarse);
-    }
-
     #[test]
     fn lerp_takes_the_fraction_as_the_receiver() {
         let (from, to) = (Px::from_int(10), Px::from_int(20));

core/src/layout_diagnostics.rs

@@ -15,7 +15,7 @@
 //! reuse, size, placement, and text events for one suspicious widget. The
 //! selection is a set and survives [`take`] until cleared.
 
-use crate::{Axis, LayoutLen, PxVec2, Size, UiRegion, WidgetId};
+use crate::{Axis, LayoutHolds, LayoutLen, PxVec2, Size, UiRegion, UiVec2, WidgetId};
 use std::{
     cell::RefCell,
     collections::{HashMap, HashSet},
@@ -23,100 +23,71 @@ use std::{
     time::Instant,
 };
 
-#[derive(Clone, Copy)]
-pub(crate) enum Counter {
-    Updates,
-    DrawRequests,
-    WidgetDraws,
-    RegionNodeDraws,
-    SizeReads,
-    HintHits,
-    HintMisses,
-    ReuseAttempts,
-    ReuseExact,
-    ReuseMoved,
-    ReuseDirty,
-    ReuseWrongParent,
-    ReuseRemapped,
-    ReuseOutside,
-    ReuseWrongLayer,
-    ReuseWrongNode,
-    QueuePops,
-    DepthReads,
-    LocalRedraws,
-    SizeChanges,
-    ReaderEdges,
-    PrimitiveWrites,
-    TextRenders,
-    TextShapeHits,
-    TextShapes,
-    TextBreaks,
-    GlyphPlacements,
-    OutsidePinnedLen,
-    OutsideRelBase,
-    OutsideRegion,
+/// Declares a counter or timer kind beside the name its report prints. Two
+/// lists in the same order was one list too many: a variant inserted without
+/// its label moving with it renames every total after it, and nothing says
+/// so.
+macro_rules! labelled {
+    ($(#[$meta:meta])* $vis:vis enum $Name:ident { $($variant:ident = $label:literal,)* }) => {
+        $(#[$meta])*
+        #[derive(Clone, Copy)]
+        $vis enum $Name { $($variant,)* }
+
+        impl $Name {
+            const COUNT: usize = [$($label,)*].len();
+            const NAMES: [&'static str; Self::COUNT] = [$($label,)*];
+        }
+    };
 }
 
-impl Counter {
-    const COUNT: usize = Self::OutsideRegion as usize + 1;
-
-    const NAMES: [&'static str; Self::COUNT] = [
-        "updates",
-        "draw requests",
-        "widget draws",
-        "region-node draws",
-        "draw-result size reads",
-        "hint hits",
-        "hint misses",
-        "reuse attempts",
-        "reuse exact",
-        "reuse moved",
-        "reuse: dirty",
-        "reuse: wrong parent",
-        "reuse remapped",
-        "reuse: outside what it holds for",
-        "reuse: another layer",
-        "reuse: region-node choice changed",
-        "redraw queue pops",
-        "depth reads",
-        "local redraws",
-        "size changes",
-        "reader edges",
-        "primitive writes",
-        "text renders",
-        "text shape hits",
-        "text shapes",
-        "text line breaks",
-        "glyph placements",
-        "reuse outside: the length it was pinned to",
-        "reuse outside: a rel base",
-        "reuse outside: a region length",
-    ];
+labelled! {
+    pub(crate) enum Counter {
+        Updates = "updates",
+        DrawRequests = "draw requests",
+        WidgetDraws = "widget draws",
+        RegionNodeDraws = "region-node draws",
+        SizeReads = "draw-result size reads",
+        HintHits = "hint hits",
+        HintMisses = "hint misses",
+        ReuseAttempts = "reuse attempts",
+        ReuseExact = "reuse exact",
+        ReuseMoved = "reuse moved",
+        ReuseDirty = "reuse: dirty",
+        ReuseUndrawn = "reuse: nothing drawn to keep",
+        ReuseWrongParent = "reuse: wrong parent",
+        ReuseRemapped = "reuse remapped",
+        ReuseOutside = "reuse: outside what it holds for",
+        ReuseWrongLayer = "reuse: another layer",
+        ReuseWrongNode = "reuse: region-node choice changed",
+        ReuseWrongMask = "reuse: a different inherited mask",
+        QueuePops = "redraw queue pops",
+        DepthReads = "depth reads",
+        LocalRedraws = "local redraws",
+        SizeChanges = "size changes",
+        ReaderEdges = "reader edges",
+        PrimitiveWrites = "primitive writes",
+        TextRenders = "text renders",
+        TextShapeHits = "text shape hits",
+        TextShapes = "text shapes",
+        TextBreaks = "text line breaks",
+        GlyphPlacements = "glyph placements",
+        OutsidePinnedLen = "reuse outside: the length it was pinned to",
+        OutsideWindow = "reuse outside: this window",
+        OutsideRelBase = "reuse outside: a rel base",
+        OutsideRegion = "reuse outside: a region length",
+    }
 }
 
-#[derive(Clone, Copy)]
-pub(crate) enum TimerKind {
-    Update,
-    FullLayout,
-    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",
-        "incremental layout",
-        "text render",
-        "text shape",
-        "text line break",
-        "glyph placement",
-    ];
+labelled! {
+    pub(crate) enum TimerKind {
+        Update = "update total",
+        FullLayout = "full layout",
+        IncrementalLayout = "incremental layout",
+        TextRender = "text render",
+        TextShape = "text shape",
+        TextBreak = "text line break",
+        GlyphPlacement = "glyph placement",
+    }
 }
 
 #[derive(Clone)]
@@ -251,6 +222,8 @@ pub enum ReuseOutcome {
     Dirty,
     WrongParent,
     WrongLayer,
+    WrongMask,
+    WrongNode,
     Remapped,
     Outside,
     Undrawn,
@@ -264,7 +237,7 @@ pub enum TraceEvent {
         id: WidgetId,
         parent: Option<WidgetId>,
         region: UiRegion,
-        pixel_size: PxVec2,
+        region_px: PxVec2,
         region_node: bool,
     },
     Reuse {
@@ -360,7 +333,7 @@ pub(crate) fn draw_request(
     id: WidgetId,
     parent: Option<WidgetId>,
     region: UiRegion,
-    pixel_size: PxVec2,
+    region_px: PxVec2,
     region_node: bool,
 ) {
     trace(
@@ -369,7 +342,7 @@ pub(crate) fn draw_request(
             id,
             parent,
             region,
-            pixel_size,
+            region_px,
             region_node,
         },
     );
@@ -379,6 +352,41 @@ pub(crate) fn reuse(id: WidgetId, outcome: ReuseOutcome) {
     trace(id, TraceEvent::Reuse { id, outcome });
 }
 
+/// A drawing that cannot be reused because the box on offer is outside what
+/// it holds for, and which of the four contracts said so. They overlap: a
+/// drawing can be outside two of them at once, and counting each is what
+/// says where a rel base redrawing more than it should is coming from.
+pub(crate) fn outside(
+    id: WidgetId,
+    holds: LayoutHolds,
+    region: UiRegion,
+    rel_base: UiVec2,
+    window: PxVec2,
+) {
+    for axis in Axis::BOTH {
+        let holds = holds[axis];
+        let len = region[axis].len();
+        let window = window[axis];
+        if holds.region_len.is_some_and(|pinned| pinned != len) {
+            bump(Counter::OutsidePinnedLen);
+        }
+        if !holds.window.contains(window) {
+            bump(Counter::OutsideWindow);
+        }
+        if holds
+            .rel_base
+            .is_some_and(|pinned| pinned != rel_base[axis])
+        {
+            bump(Counter::OutsideRelBase);
+        }
+        if !holds.region.contains(len.to_px(window)) {
+            bump(Counter::OutsideRegion);
+        }
+    }
+    bump(Counter::ReuseOutside);
+    reuse(id, ReuseOutcome::Outside);
+}
+
 pub(crate) fn size_reported(id: WidgetId, size: Size) {
     trace(id, TraceEvent::SizeReported { id, size });
 }

core/src/lib.rs

@@ -9,6 +9,7 @@
 #![feature(unsize)]
 #![feature(coerce_unsized)]
 #![feature(option_into_flat_iter)]
+#![feature(const_index)]
 
 #[cfg(feature = "layout-diagnostics")]
 pub mod layout_diagnostics;

core/src/orientation/align.rs

@@ -1,8 +1,9 @@
+use crate::util::impl_axis_index;
 use crate::{Px, Rel};
 
 use super::*;
 
-#[derive(Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub struct Align {
     pub x: Option<AxisAlign>,
     pub y: Option<AxisAlign>,
@@ -83,28 +84,6 @@ pub struct RegionAlign {
     pub y: AxisAlign,
 }
 
-impl RegionAlign {
-    /// Both axes at the near edge: the start of a box in its own orientation.
-    pub const NEAR: Self = Self {
-        x: AxisAlign::NEG,
-        y: AxisAlign::NEG,
-    };
-
-    pub fn axis(&self, axis: Axis) -> AxisAlign {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
-
-    pub fn axis_mut(&mut self, axis: Axis) -> &mut AxisAlign {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-}
-
 impl RegionAlign {
     pub const TOP_LEFT: Self = Self::new(AxisAlign::NEG, AxisAlign::NEG);
     pub const TOP_CENTER: Self = Self::new(AxisAlign::CENTER, AxisAlign::NEG);
@@ -172,13 +151,15 @@ impl Vec2 {
 }
 
 impl Len {
+    /// This length placed in the box it is measured in: the alignment names a
+    /// point along that box, and the two ends are that point less the part of
+    /// the length falling before it and plus the part falling after.
     pub const fn align(&self, align: AxisAlign) -> UiSpan {
         let rel = align.rel();
-        let rest = Rel::ONE.sub(rel);
         let at = Len::from_parts(rel, Px::ZERO);
         UiSpan {
-            start: Len::from_parts(at.rel.sub(self.rel.mul(rel)), at.px.sub(self.px.mul(rel))),
-            end: Len::from_parts(at.rel.add(self.rel.mul(rest)), at.px.add(self.px.mul(rest))),
+            start: at - self.scale(rel),
+            end: at + self.scale(Rel::ONE.sub(rel)),
         }
     }
 }
@@ -231,3 +212,6 @@ impl RegionAlign {
         UiVec2::from(self)
     }
 }
+
+impl_axis_index!(RegionAlign => AxisAlign);
+impl_axis_index!(Align => Option<AxisAlign>);

core/src/orientation/axis.rs

@@ -1,4 +1,5 @@
 use super::*;
+use crate::util::impl_axis_index;
 use crate::{Fixed, FixedVec2};
 
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
@@ -8,14 +9,8 @@ pub enum Axis {
 }
 
 impl Axis {
-    /// A per-axis pair with `aligned` on this axis and `ortho` on the other,
-    /// which is what `from_axis` does for a vector.
-    pub fn pair<T>(self, aligned: T, ortho: T) -> [T; 2] {
-        match self {
-            Self::X => [aligned, ortho],
-            Self::Y => [ortho, aligned],
-        }
-    }
+    /// Both of them, for the layout code that asks the same question of each.
+    pub const BOTH: [Self; 2] = [Self::X, Self::Y];
 }
 
 impl std::ops::Not for Axis {
@@ -53,20 +48,6 @@ pub enum Sign {
 }
 
 impl<const SHIFT: u32> FixedVec2<SHIFT> {
-    pub const fn axis(&self, axis: Axis) -> Fixed<SHIFT> {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
-
-    pub const fn axis_mut(&mut self, axis: Axis) -> &mut Fixed<SHIFT> {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-
     pub const fn from_axis(axis: Axis, aligned: Fixed<SHIFT>, ortho: Fixed<SHIFT>) -> Self {
         match axis {
             Axis::X => Self::new(aligned, ortho),
@@ -76,20 +57,6 @@ impl<const SHIFT: u32> FixedVec2<SHIFT> {
 }
 
 impl Vec2 {
-    pub fn axis(&self, axis: Axis) -> f32 {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
-
-    pub fn axis_mut(&mut self, axis: Axis) -> &mut f32 {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-
     pub const fn from_axis(axis: Axis, aligned: f32, ortho: f32) -> Self {
         Self {
             x: match axis {
@@ -104,47 +71,5 @@ impl Vec2 {
     }
 }
 
-pub const trait AxisT {
-    fn get() -> Axis;
-}
-
-pub struct XAxis;
-const impl AxisT for XAxis {
-    fn get() -> Axis {
-        Axis::X
-    }
-}
-
-pub struct YAxis;
-const impl AxisT for YAxis {
-    fn get() -> Axis {
-        Axis::Y
-    }
-}
-
-#[derive(Clone, Copy, Debug, Default)]
-pub struct BothAxis<T> {
-    pub x: T,
-    pub y: T,
-}
-
-impl<T> BothAxis<T> {
-    pub const fn axis<A: const AxisT>(&mut self) -> &mut T {
-        match A::get() {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-    pub fn take_axis<A: const AxisT>(self) -> T {
-        match A::get() {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
-    pub fn axis_dyn(&mut self, axis: Axis) -> &mut T {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-}
+impl_axis_index!({const SHIFT: u32} FixedVec2<SHIFT> => Fixed<SHIFT>);
+impl_axis_index!(Vec2 => f32);

core/src/orientation/len.rs

@@ -1,4 +1,5 @@
 use super::*;
+use crate::util::impl_axis_index;
 use crate::{Px, PxVec2, Rel, UiNum, Weight, util::impl_op};
 
 #[derive(Debug, Default, Clone, Copy, PartialEq)]
@@ -118,20 +119,6 @@ impl Size {
             },
         }
     }
-
-    pub fn axis(&self, axis: Axis) -> LayoutLen {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
-
-    pub fn axis_mut(&mut self, axis: Axis) -> &mut LayoutLen {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
 }
 
 impl LayoutLen {
@@ -158,11 +145,39 @@ impl LayoutLen {
         Len::from_parts(self.rel.add(share), self.px)
     }
 
+    /// Only pixels: the same number of them whatever box it lands in, and
+    /// whatever anyone else in the row asks for. A length that is any part
+    /// of a box or of what is left over is not one.
+    pub fn is_px(&self) -> bool {
+        self.rel == Rel::ZERO && self.leftover == Weight::ZERO
+    }
+
+    /// Nothing but a claim on what is left over, so there is no length here
+    /// at all where nothing is.
+    pub fn is_only_leftover(&self) -> bool {
+        self.leftover > Weight::ZERO && self.without_leftover() == Len::ZERO
+    }
+
+    /// This as a length of a box, where it is one. `leftover` is not: a
+    /// share of what is left over is a length only to whoever divides one,
+    /// so it passes up in the reported size instead and is resolved there.
+    pub fn declared(&self) -> Option<Len> {
+        (self.leftover == Weight::ZERO).then(|| self.without_leftover())
+    }
+
+    /// What this takes whatever is left over: the reading of a length for
+    /// anyone not dividing a box between siblings, where a share is a claim
+    /// on someone else's room rather than a length of its own.
+    /// [`Self::apply_leftover`] is the opposite reading of the same value.
+    pub const fn without_leftover(&self) -> Len {
+        Len::from_parts(self.rel, self.px)
+    }
+
     /// This length, given as a part of a box `len` long, as a part of the
     /// box `len` is itself a part of. The share is untouched: it is a claim
     /// on whoever divides the room, not a fraction of anything.
     pub const fn within_len(self, len: Len) -> Self {
-        let part = Len::from_parts(self.rel, self.px).within_len(len);
+        let part = self.without_leftover().within_len(len);
         Self {
             px: part.px,
             rel: part.rel,
@@ -236,3 +251,5 @@ impl std::fmt::Display for LayoutLen {
         Ok(())
     }
 }
+
+impl_axis_index!(Size => LayoutLen);

core/src/orientation/pos.rs

@@ -1,3 +1,4 @@
+use crate::util::impl_axis_index;
 use std::{fmt::Display, marker::Destruct};
 
 use super::*;
@@ -61,20 +62,6 @@ impl UiVec2 {
         }
     }
 
-    pub fn axis_mut(&mut self, axis: Axis) -> &mut Len {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-
-    pub fn axis(&self, axis: Axis) -> Len {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
-
     /// Resolved against a box of `size`, which is where a fraction stops
     /// being one and becomes a place.
     pub fn to_px(&self, size: PxVec2) -> PxVec2 {
@@ -176,14 +163,6 @@ impl Len {
         Self::from_parts(Rel::ZERO, Px::from_f32(px))
     }
 
-    pub const fn rel_min() -> Self {
-        Self::ZERO
-    }
-
-    pub const fn rel_max() -> Self {
-        Self::FULL
-    }
-
     pub const fn max(&self, other: Self) -> Self {
         Self {
             rel: self.rel.max(other.rel),
@@ -226,10 +205,6 @@ impl Len {
         })
     }
 
-    pub fn select_len(&self, len: Len) -> Self {
-        len.within_len(*self)
-    }
-
     pub const fn flip(&mut self) {
         self.rel = Rel::ONE.sub(self.rel);
         self.px = self.px.neg();
@@ -294,17 +269,17 @@ impl UiSpan {
         }
     }
 
-    pub const fn len(&self) -> Len {
-        self.end - self.start
+    /// A box `len` long inside this one, on the side `align` says. Both must
+    /// be lengths of the same rel base: it subtracts one from the other
+    /// rather than composing it in, which is what keeps a fraction the same
+    /// fraction however long this box turns out to be.
+    pub const fn place(self, len: Len, align: AxisAlign) -> Self {
+        let start = self.start + (self.len() - len).scale(align.rel());
+        Self::new(start, start + len)
     }
 
-    /// Both ends by the same amount, which is what moving a box without
-    /// changing its length does to every part of it.
-    pub const fn translated(self, by: Len) -> Self {
-        Self {
-            start: self.start + by,
-            end: self.end + by,
-        }
+    pub const fn len(&self) -> Len {
+        self.end - self.start
     }
 }
 
@@ -316,17 +291,6 @@ pub struct UiRegion {
 }
 
 impl UiRegion {
-    /// Every part of the box by the same amount on each axis. Done to the
-    /// whole region rather than an end at a time, because that is what it is
-    /// -- and because four adds in a row are four adds, where four asked for
-    /// separately are four sequences.
-    pub const fn translated(self, x: Len, y: Len) -> Self {
-        Self {
-            x: self.x.translated(x),
-            y: self.y.translated(y),
-        }
-    }
-
     pub const FULL: Self = Self {
         x: UiSpan::FULL,
         y: UiSpan::FULL,
@@ -348,20 +312,6 @@ impl UiRegion {
             y: self.y.within(&parent.y),
         }
     }
-    pub const fn axis(&self, axis: Axis) -> &UiSpan {
-        match axis {
-            Axis::X => &self.x,
-            Axis::Y => &self.y,
-        }
-    }
-
-    pub const fn axis_mut(&mut self, axis: Axis) -> &mut UiSpan {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-
     pub const fn flip(&mut self, axis: Axis) {
         match axis {
             Axis::X => self.x.flip(),
@@ -462,3 +412,6 @@ impl Display for PixelRegion {
         write!(f, "{} -> {}", self.top_left, self.bot_right)
     }
 }
+
+impl_axis_index!(UiVec2 => Len);
+impl_axis_index!(UiRegion => UiSpan);

core/src/primitive/text.rs

@@ -133,10 +133,6 @@ impl TextBuffer {
         }
     }
 
-    pub fn new_empty() -> Self {
-        Self::new("")
-    }
-
     pub fn text(&self) -> &str {
         &self.text
     }

core/src/render/atlas.rs

@@ -167,10 +167,6 @@ impl GlyphAtlas {
     pub fn page_count(&self) -> u32 {
         self.pages.len() as u32
     }
-
-    pub fn glyph_count(&self) -> usize {
-        self.entries.len()
-    }
 }
 
 impl Page {

core/src/render/mod.rs

@@ -23,13 +23,22 @@ pub use primitive::*;
 const PRELUDE: &str = include_str!("./shader/prelude.wgsl");
 
 fn module_source(wgsl: &str) -> String {
-    // The steps come from the same constants the CPU counts in, rather than
-    // a second copy of them written into the shader: a grid the two disagree
-    // about puts every coordinate somewhere else.
+    // Every number both sides count in, written once here rather than a
+    // second time in the shader: a grid the two disagree about puts every
+    // coordinate somewhere else, and a sentinel they disagree about makes one
+    // of them walk a chain from a slot the other says is not there.
     format!(
-        "const PX_STEP: f32 = 1.0 / {}.0;\nconst REL_STEP: f32 = 1.0 / {}.0;\n{PRELUDE}\n{wgsl}",
+        "const PX_STEP: f32 = 1.0 / {}.0;\n\
+         const REL_STEP: f32 = 1.0 / {}.0;\n\
+         const MASK_NONE: u32 = {}u;\n\
+         const MOVE_NONE: u32 = {}u;\n\
+         const CHAIN_LIMIT: u32 = {}u;\n\
+         {PRELUDE}\n{wgsl}",
         1u32 << crate::PX_SHIFT,
         1u32 << crate::REL_SHIFT,
+        MaskIdx::NONE.idx(),
+        MoveIdx::NONE.idx(),
+        crate::CHAIN_LIMIT,
     )
 }
 

core/src/render/shader/prelude.wgsl

@@ -26,9 +26,11 @@ struct MoveOffset {
     parent: u32,
 }
 
-// `PX_STEP` and `REL_STEP` are prepended from `iris_core`'s own constants:
-// what it stores is a whole count of each, both powers of two, so decoding
-// is exact and the number here is the number the CPU decided.
+// `PX_STEP`, `REL_STEP`, `MASK_NONE`, `MOVE_NONE` and `CHAIN_LIMIT` are
+// prepended from `iris_core`'s own constants, so none of them is written
+// twice. What the CPU stores is a whole count of each step, and both steps
+// are powers of two, so decoding is exact and the number here is the number
+// the CPU decided.
 
 // Every coordinate the CPU decided is a whole count of `PX_STEP`, so one that
 // composes to within half a step of a pixel boundary is on that boundary and
@@ -70,12 +72,6 @@ struct Region {
     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;
-
 // The same expression `Len::within` uses, in floats rather than on the
 // CPU's grid: a move is resolved here so that scrolling a subtree writes one
 // entry instead of walking it. What has to hold is that this agrees with
@@ -171,7 +167,7 @@ fn vs_main(
 }
 
 fn masked(in: VertexOutput, color: vec4<f32>) -> vec4<f32> {
-    if in.mask_idx == 4294967295u {
+    if in.mask_idx == MASK_NONE {
         return color;
     }
     let mask = masks[in.mask_idx];

core/src/ui/active.rs

@@ -1,5 +1,5 @@
 use crate::{
-    LayerId, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, PlaceDesc, RegionAlign, RetainedPrimitive,
+    Declared, LayerId, LayoutHolds, MaskIdx, MoveIdx, PlaceDesc, RegionAlign, RetainedPrimitive,
     Size, TextureHandle, UiRegion, UiVec2, WidgetId,
 };
 
@@ -27,7 +27,7 @@ pub struct ActiveData {
     pub region: UiRegion,
     /// The measured answer and its dependencies. A hint-only dependency or
     /// a widget first encountered during placement has no measurement yet.
-    pub answer: Option<(Size, LayoutHolds)>,
+    pub answer: Option<Answer>,
     /// Asked more than once in its parent's last draw -- measured in one box
     /// and then asked in the one the parent decided. The parent's layout
     /// rests on the first answer and its drawing on the last, so only the
@@ -51,15 +51,13 @@ pub struct ActiveData {
     /// An owned mask holds one reference independently of its primitives.
     pub mask_region: Option<UiRegion>,
     pub children: Vec<WidgetId>,
-    /// The children whose size this widget read while drawing.
-    pub size_deps: Vec<WidgetId>,
     /// The movable region its primitives are positioned through: its own when
     /// opted in, otherwise the nearest ancestor's.
     pub move_idx: MoveIdx,
     /// The declared lengths whoever drew this widget resolved into its rel base.
     /// A change to one moves a box this widget cannot fix by drawing again,
     /// and comparing them is what says so.
-    pub declared: [Option<LayoutLen>; 2],
+    pub declared: Declared,
     /// Its alignment when it was last drawn, which a change to the property
     /// is found against.
     pub own_align: RegionAlign,
@@ -82,6 +80,20 @@ impl ActiveData {
     /// all. Not `size`, which is what its last drawing reported: a drawing
     /// re-expressed in the box that answer chose is not a second answer.
     pub fn measured(&self) -> Option<Size> {
-        self.answer.map(|(size, _)| size)
+        self.answer.map(|answer| answer.size)
+    }
+
+    /// Whether it owns a region node rather than sharing the one it was drawn
+    /// under, which is what its two move indices being different says.
+    pub fn is_region_node(&self) -> bool {
+        self.move_idx != self.parent_move
     }
 }
+
+/// What a widget answered when it was asked: the size it reported, and the
+/// boxes and windows that answer holds for.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Answer {
+    pub size: Size,
+    pub holds: LayoutHolds,
+}

core/src/ui/holds.rs

@@ -33,7 +33,13 @@ impl Holds {
         len.raw() >= self.lo.raw() && len.raw() <= self.hi.raw()
     }
 
-    pub const fn and(self, other: Self) -> Self {
+    /// Every length `other` holds for is one this holds for, so a drawing
+    /// made under this range is still good wherever `other` is.
+    pub const fn covers(&self, other: Self) -> bool {
+        self.lo.raw() <= other.lo.raw() && self.hi.raw() >= other.hi.raw()
+    }
+
+    pub const fn and(&self, other: Self) -> Self {
         Self {
             lo: self.lo.max(other.lo),
             hi: self.hi.min(other.hi),
@@ -51,7 +57,7 @@ impl Holds {
     /// boxes therefore give one length. That is a floor rather than an
     /// allowance: inverting it is two divisions and nothing else, and the
     /// whole of a box maps back to itself.
-    pub const fn through(self, len: Len) -> Self {
+    pub const fn through(&self, len: Len) -> Self {
         if self.lo.raw() == Px::MIN.raw() && self.hi.raw() == Px::MAX.raw() {
             return Self::ANY;
         }

core/src/ui/layout_holds.rs

@@ -1,10 +1,10 @@
-use crate::{Axis, Holds, Len, PxVec2, UiRegion, UiVec2};
+use crate::util::impl_axis_index;
+use crate::{Axis, Holds, Len, Px, PxVec2, UiRegion, UiVec2};
 
-const AXES: [Axis; 2] = [Axis::X, Axis::Y];
-
-/// What one evaluation of a widget depends on: the window lengths its reads
-/// hold for, the pixel lengths of its own box, and the symbolic lengths of
-/// that box and of its rel base where either one is what it was expressed in.
+/// What one evaluation of a widget depends on along one axis: the window
+/// lengths its reads hold for, the pixel lengths of its own box, and the
+/// symbolic lengths of that box and of its rel base where either one is what
+/// it was expressed in.
 ///
 /// The symbolic lengths are pins rather than ranges: a container places its
 /// children as lengths of its rel base measured from where its own box starts,
@@ -19,61 +19,90 @@ const AXES: [Axis; 2] = [Axis::X, Axis::Y];
 /// of the rel base that is only pixels is not one: it is that many pixels
 /// whatever the rel base turns out to be.
 #[derive(Clone, Copy, Debug, PartialEq)]
+pub struct AxisHolds {
+    pub window: Holds,
+    pub rel_base: Option<Len>,
+    pub region: Holds,
+    pub region_len: Option<Len>,
+}
+
+impl AxisHolds {
+    pub const ANY: Self = Self {
+        window: Holds::ANY,
+        rel_base: None,
+        region: Holds::ANY,
+        region_len: None,
+    };
+
+    pub fn and(&self, other: Self) -> Self {
+        // Two pins of the same length disagreeing would mean one drawing was
+        // a fraction of two different lengths at once.
+        debug_assert!(
+            self.region_len.is_none()
+                || other.region_len.is_none()
+                || self.region_len == other.region_len
+        );
+        debug_assert!(
+            self.rel_base.is_none() || other.rel_base.is_none() || self.rel_base == other.rel_base
+        );
+        Self {
+            window: self.window.and(other.window),
+            rel_base: self.rel_base.or(other.rel_base),
+            region: self.region.and(other.region),
+            region_len: self.region_len.or(other.region_len),
+        }
+    }
+
+    pub fn covers(&self, other: Self) -> bool {
+        self.window.covers(other.window)
+            && self.region.covers(other.region)
+            && self
+                .region_len
+                .is_none_or(|len| other.region_len == Some(len))
+            && self.rel_base.is_none_or(|len| other.rel_base == Some(len))
+    }
+
+    /// Whether a widget in a box `len` long, with that rel base, in that
+    /// window, is one this drawing holds for.
+    pub fn contains(&self, window: Px, rel_base: Len, len: Len) -> bool {
+        self.window.contains(window)
+            && self.rel_base.is_none_or(|pinned| pinned == rel_base)
+            && self.region.contains(len.to_px(window))
+            && self.region_len.is_none_or(|pinned| pinned == len)
+    }
+}
+
+/// [`AxisHolds`] on both axes. Every question asked of it is asked of one
+/// axis at a time, since a widget that read one length holds for any length
+/// of the other.
+#[derive(Clone, Copy, Debug, PartialEq)]
 pub struct LayoutHolds {
-    pub window: [Holds; 2],
-    pub rel_base: [Option<Len>; 2],
-    pub region: [Holds; 2],
-    pub region_len: [Option<Len>; 2],
+    pub x: AxisHolds,
+    pub y: AxisHolds,
 }
 
 impl LayoutHolds {
     pub const ANY: Self = Self {
-        window: [Holds::ANY; 2],
-        rel_base: [None; 2],
-        region: [Holds::ANY; 2],
-        region_len: [None; 2],
+        x: AxisHolds::ANY,
+        y: AxisHolds::ANY,
     };
 
-    pub fn and(self, other: Self) -> Self {
-        let mut result = Self::ANY;
-        for n in 0..2 {
-            result.window[n] = self.window[n].and(other.window[n]);
-            result.region[n] = self.region[n].and(other.region[n]);
-            debug_assert!(
-                self.region_len[n].is_none()
-                    || other.region_len[n].is_none()
-                    || self.region_len[n] == other.region_len[n]
-            );
-            debug_assert!(
-                self.rel_base[n].is_none()
-                    || other.rel_base[n].is_none()
-                    || self.rel_base[n] == other.rel_base[n]
-            );
-            result.region_len[n] = self.region_len[n].or(other.region_len[n]);
-            result.rel_base[n] = self.rel_base[n].or(other.rel_base[n]);
+    pub fn and(&self, other: Self) -> Self {
+        Self {
+            x: self.x.and(other.x),
+            y: self.y.and(other.y),
         }
-        result
     }
 
-    pub fn covers(self, other: Self) -> bool {
-        (0..2).all(|n| {
-            self.window[n].lo <= other.window[n].lo
-                && self.window[n].hi >= other.window[n].hi
-                && self.region[n].lo <= other.region[n].lo
-                && self.region[n].hi >= other.region[n].hi
-                && self.region_len[n].is_none_or(|len| other.region_len[n] == Some(len))
-                && self.rel_base[n].is_none_or(|len| other.rel_base[n] == Some(len))
-        })
+    pub fn covers(&self, other: Self) -> bool {
+        self.x.covers(other.x) && self.y.covers(other.y)
     }
 
-    pub fn contains(self, window: PxVec2, rel_base: UiVec2, region: UiRegion) -> bool {
-        AXES.into_iter().all(|axis| {
-            let n = axis as usize;
-            let len = region.axis(axis).len();
-            self.window[n].contains(window.axis(axis))
-                && self.rel_base[n].is_none_or(|pinned| pinned == rel_base.axis(axis))
-                && self.region[n].contains(len.to_px(window.axis(axis)))
-                && self.region_len[n].is_none_or(|pinned| pinned == len)
-        })
+    pub fn contains(&self, window: PxVec2, rel_base: UiVec2, region: UiRegion) -> bool {
+        Axis::BOTH
+            .into_iter()
+            .all(|axis| self[axis].contains(window[axis], rel_base[axis], region[axis].len()))
     }
 }
+
+impl_axis_index!(LayoutHolds => AxisHolds);

core/src/ui/mod.rs

@@ -20,7 +20,7 @@ pub use active::*;
 pub use holds::*;
 pub use layout_holds::*;
 pub use painter::{Painter, PrimitiveLike};
-pub use place::*;
+pub use place::{PlaceDesc, PlaceDescAxis, RetainedPrimitive};
 pub use render_state::*;
 
 #[derive(Default)]

core/src/ui/painter.rs

@@ -1,16 +1,18 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
-    Axis, Holds, LayoutHolds, LayoutLen, Len, PlaceDesc, Px, PxVec2, RegionAlign, Rel,
-    RenderedText, RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData,
-    TextureHandle, UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
+    Axis, Declared, Holds, LayoutHolds, LayoutLen, Len, PlaceDesc, Px, PxVec2, RegionAlign, Rel,
+    RenderedText, RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextureHandle,
+    UiRegion, UiRenderState, UiRsc, UiVec2, Weight, WidgetId, Widgets,
     render::{
         GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveInst, PrimitiveKind,
         TexturePrimitive,
     },
-    ui::render_state::{DrawInfo, Placing},
+    ui::{
+        place::{PlaceSpan, RelBase},
+        render_state::{DrawInfo, Placing},
+    },
 };
-const AXES: [Axis; 2] = [Axis::X, Axis::Y];
 
 /// makes your surfaces look pretty
 pub struct Painter<'a> {
@@ -155,13 +157,17 @@ impl<'a> Painter<'a> {
     /// where that is this widget's own narrowed the way the region is. An
     /// axis the region leaves whole is not read at all, so a wrapper that
     /// only moves its child does not pin its drawing to a rel base.
-    fn state_rel_base(&mut self, mut place: PlaceDesc) -> PlaceDesc {
-        for axis in AXES {
-            if let Some(span) = place.axis(axis).narrows_rel_base() {
+    fn resolve_rel_base(&mut self, mut place: PlaceDesc) -> PlaceDesc {
+        for axis in Axis::BOTH {
+            let at = &mut place[axis];
+            let (RelBase::WithRegion, PlaceSpan::Within(span)) = (at.rel_base, at.span) else {
+                continue;
+            };
             let len = span.len();
-                let stated = (len != Len::FULL).then(|| len.within_len(self.rel_base(axis)));
-                *place.axis_mut(axis) = place.axis(axis).with_rel_base(stated);
-            }
+            at.rel_base = match len == Len::FULL {
+                true => RelBase::Inherit,
+                false => RelBase::Len(len.within_len(self.rel_base(axis))),
+            };
         }
         place
     }
@@ -181,12 +187,22 @@ impl<'a> Painter<'a> {
         id: &'s StrongWidget<W>,
         place: impl Into<PlaceDesc>,
     ) -> DrawResult<'s, 'a, W> {
-        let place = self.state_rel_base(place.into());
+        let mut place = self.resolve_rel_base(place.into());
         let region_node = self.rsc.widgets().is_region_node(id.id());
-        let declared = self.declared_lens(id);
         let align = self.rsc.widgets().alignment(id.id());
+        // A share fills what the pixels and fraction beside it leave of the
+        // box and overflows where they are longer, which is the rule a span
+        // follows with one child. Only the overflow is a box of the child's
+        // own: a share that fits is the box it was given, which is what this
+        // place already says.
+        for axis in Axis::BOTH {
+            if let Some(len) = self.share_past_the_offer(id.id(), place, align, axis) {
+                place[axis] = len.as_desc().fills();
+            }
+        }
+        let declared = self.declared_lens(id);
         let (rel_base, region) =
-            rel_base_and_region(self.region, self.rel_base, place, declared, align);
+            place.rel_base_and_region(self.region, self.rel_base, declared, align);
         #[cfg(feature = "layout-diagnostics")]
         if region_node {
             diag::bump(Counter::RegionNodeDraws);
@@ -197,8 +213,7 @@ impl<'a> Painter<'a> {
         if !re_asked {
             self.children.push(id.id());
         }
-        let px = rel_base.to_px(self.window);
-        let (size, answer_holds, holds) = self.state.draw_inner(
+        let drawn = self.state.draw_inner(
             id.id(),
             DrawInfo {
                 layer: self.layer,
@@ -212,13 +227,12 @@ impl<'a> Painter<'a> {
                 placed: place,
                 asked: place,
                 re_asked,
-                px,
             },
             None,
             self.rsc,
         );
-        let holds = self.in_parent(holds, region, place, declared);
-        let answer_holds = self.in_parent(answer_holds, region, place, declared);
+        let holds = self.in_parent(drawn.drawing_holds, region, place, declared);
+        let answer_holds = self.in_parent(drawn.answer.holds, region, place, declared);
         match self.under.iter_mut().find(|(child, _)| *child == id.id()) {
             Some((_, kept)) => *kept = holds,
             None => self.under.push((id.id(), holds)),
@@ -226,7 +240,7 @@ impl<'a> Painter<'a> {
         DrawResult {
             child: id,
             painter: self,
-            size,
+            size: drawn.answer.size,
             answer_holds,
         }
     }
@@ -255,10 +269,10 @@ impl<'a> Painter<'a> {
         id: &'s StrongWidget<W>,
         place: impl Into<PlaceDesc>,
     ) -> DrawResult<'s, 'a, W> {
-        let place = self.state_rel_base(place.into());
-        let states_rel_base = AXES
+        let place = self.resolve_rel_base(place.into());
+        let states_rel_base = Axis::BOTH
             .iter()
-            .any(|&axis| place.axis(axis).stated_rel_base().is_some());
+            .any(|&axis| matches!(place[axis].rel_base, RelBase::Len(_)));
         if states_rel_base || !self.children.contains(&id.id()) {
             return self.widget_at(id, place);
         }
@@ -281,19 +295,57 @@ impl<'a> Painter<'a> {
             id: self.id,
             region: self.region,
             rel_base: self.rel_base,
-            window: self.window,
             depth: self.depth,
             move_idx: self.move_idx,
             mask: self.mask,
         }
     }
 
-    /// What a widget's rules declare its lengths to be, which whoever draws
-    /// it resolves into its rel base. Reading them depends on nothing -- the box
+    /// What a rule or a hint declares a widget's lengths to be, which whoever
+    /// draws it resolves into its rel base. Reading them depends on nothing -- the box
     /// that comes of them is kept on the child, and `redraw` compares it
     /// there.
-    fn declared_lens<W: ?Sized>(&self, id: &StrongWidget<W>) -> [Option<LayoutLen>; 2] {
-        declared_lens(self.rsc.widgets(), id.id())
+    fn declared_lens<W: ?Sized>(&self, id: &StrongWidget<W>) -> Declared {
+        self.rsc.widgets().declared_lens(id.id())
+    }
+
+    /// The box a child's own share asks for where that is longer than the box
+    /// `place` gives it, and nothing where the share fits.
+    ///
+    /// A share is a length only to whoever divides one, and nothing divides a
+    /// box handed to one child: what is left of it after the pixels and the
+    /// fraction beside the share is what the share takes, so the length comes
+    /// to the whole box until those are longer than it and to them once they
+    /// are. Only that second case is a box this widget did not give, and the
+    /// crossing between them is a question in pixels, so this widget's drawing
+    /// holds for the windows on one side of it. Narrowed rather than stated,
+    /// because this widget may have read its own box as well, and a range it
+    /// pinned for that still holds.
+    fn share_past_the_offer(
+        &mut self,
+        id: WidgetId,
+        place: PlaceDesc,
+        align: RegionAlign,
+        axis: Axis,
+    ) -> Option<Len> {
+        // A place that is the child's placement outright is a box its parent
+        // decided, and a parent that divides one has already given the share
+        // whatever it was owed. Only an offer -- a box with the answer still
+        // to be placed inside it -- is a box a share reads.
+        if place[axis].fills {
+            return None;
+        }
+        // A share with nothing beside it is the box whatever the box is, so
+        // there is no comparison to make and no range to keep for one.
+        let stated = self.rsc.widgets().exact_len(id, axis)?;
+        if stated.leftover == Weight::ZERO || stated.is_only_leftover() {
+            return None;
+        }
+        let fixed = stated
+            .without_leftover()
+            .within_len(place.base(axis, self.rel_base));
+        let offer = place.of(self.region, align)[axis].len();
+        self.longer_than(fixed, offer, axis).then_some(fixed)
     }
 
     /// What a child says its length is without being drawn, if it can say,
@@ -301,15 +353,8 @@ impl<'a> Painter<'a> {
     /// against this widget's rel base, which is the rel base a child asked with
     /// nothing narrowed gets. Asking counts as reading its size.
     pub fn size_hint<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<LayoutLen> {
-        let widgets = self.rsc.widgets();
-        // A rule is the answer where there is one: it wins over whatever the
-        // widget would draw, so it has to win over what the widget says too.
-        let hint = widgets.size_rules(id.id()).axis(axis).exact().or_else(|| {
-            widgets
-                .get_dyn(id.id())
-                .and_then(|widget| widget.size_hint(axis))
-        });
-        let rel_base = self.rel_base.axis(axis);
+        let hint = self.rsc.widgets().exact_len(id.id(), axis);
+        let rel_base = self.rel_base[axis];
         let resolved = hint.map(|hint| hint.within_len(rel_base));
         #[cfg(feature = "layout-diagnostics")]
         {
@@ -326,7 +371,7 @@ impl<'a> Painter<'a> {
             // the child's own: resolved against a rel base of pixels, none is
             // left to see it by.
             if hint.rel != Rel::ZERO {
-                self.own.rel_base[axis as usize] = Some(rel_base);
+                self.own[axis].rel_base = Some(rel_base);
             }
         }
         resolved
@@ -392,8 +437,8 @@ impl<'a> Painter<'a> {
     /// again. One axis at a time, because a container that divides one axis
     /// holds for any length of the other.
     pub fn region_len(&mut self, axis: Axis) -> Len {
-        let len = self.region.axis(axis).len();
-        self.own.region_len[axis as usize] = Some(len);
+        let len = self.region[axis].len();
+        self.own[axis].region_len = Some(len);
         len
     }
 
@@ -403,8 +448,8 @@ impl<'a> Painter<'a> {
     /// Reading it pins the drawing to that rel base, the way
     /// [`Self::region_len`] pins it to the box.
     pub fn rel_base(&mut self, axis: Axis) -> Len {
-        let len = self.rel_base.axis(axis);
-        self.own.rel_base[axis as usize] = Some(len);
+        let len = self.rel_base[axis];
+        self.own[axis].rel_base = Some(len);
         len
     }
 
@@ -425,10 +470,7 @@ impl<'a> Painter<'a> {
     /// worth anything, since reading one is also what makes its own size
     /// depend on it.
     pub fn has_exact_size(&self, axis: Axis) -> bool {
-        self.rsc
-            .widgets()
-            .size_rules(self.id)
-            .axis(axis)
+        self.rsc.widgets().size_rules(self.id)[axis]
             .exact()
             .is_some()
     }
@@ -442,9 +484,9 @@ impl<'a> Painter<'a> {
     /// One axis of this widget's own box in pixels. Prefer this to
     /// [`Self::px_size`] when the other axis cannot affect the drawing.
     pub fn px_len(&mut self, axis: Axis) -> Px {
-        let len = self.region.axis(axis).len();
-        let px = len.to_px(self.window.axis(axis));
-        let own = &mut self.own.region[axis as usize];
+        let len = self.region[axis].len();
+        let px = len.to_px(self.window[axis]);
+        let own = &mut self.own[axis].region;
         if *own == Holds::ANY {
             *own = Holds::at(px);
         }
@@ -456,15 +498,15 @@ impl<'a> Painter<'a> {
     /// of the box, and the same reported size. A widget that read its length
     /// in pixels holds for that one alone until it says otherwise.
     pub fn holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
-        let len = self.region.axis(axis).len();
+        let len = self.region[axis].len();
         let holds = holds.into();
         debug_assert!(
-            holds.contains(len.to_px(self.window.axis(axis))),
+            holds.contains(len.to_px(self.window[axis])),
             "'{}' ({:?}) says its drawing holds for lengths that leave out its own box",
             self.label(),
             self.id
         );
-        self.own.region[axis as usize] = holds;
+        self.own[axis].region = holds;
     }
 
     /// A window length in pixels, which is what every length in layout is
@@ -472,9 +514,9 @@ impl<'a> Painter<'a> {
     /// length is a fraction of it; one that is only pixels is that many
     /// pixels in any window and pins nothing.
     pub fn to_px(&mut self, len: Len, axis: Axis) -> Px {
-        let window = self.window.axis(axis);
+        let window = self.window[axis];
         if len.rel != Rel::ZERO {
-            let own = &mut self.own.window[axis as usize];
+            let own = &mut self.own[axis].window;
             if *own == Holds::ANY {
                 *own = Holds::at(window);
             }
@@ -482,6 +524,40 @@ impl<'a> Painter<'a> {
         len.to_px(window)
     }
 
+    /// Whether `len` is longer than `than`, kept as the windows that comparison
+    /// comes out the same way on: a drawing that took one of two lengths holds
+    /// where the same one is the longer, and nowhere else.
+    ///
+    /// Which is longer is a question in pixels -- `rel(0.5)` is longer than 300
+    /// px at a box of 600 and shorter at 400 -- and it is asked of the
+    /// difference and answered back through that same difference, so the
+    /// boundary is the drawing's own rather than a second way of finding it.
+    /// Narrowed rather than stated, because whatever else this widget read
+    /// about the window is a reason its drawing holds where it does too.
+    ///
+    /// This is the one operation a length that is the longer of two needs: the
+    /// room a container has left for the shares it divides, and a share that
+    /// overflows the box it was given because the pixels beside it are longer
+    /// than the box.
+    pub fn longer_than(&mut self, len: Len, than: Len, axis: Axis) -> bool {
+        let over = len - than;
+        let window = self.window[axis];
+        let longer = over.to_px(window) > Px::ZERO;
+        let side = match longer {
+            true => Px::STEP..=Px::MAX,
+            false => Px::MIN..=Px::ZERO,
+        };
+        let holds = Holds::from(side).through(over);
+        debug_assert!(
+            holds.contains(window),
+            "'{}' ({:?}) compared two lengths and kept a range without this window",
+            self.label(),
+            self.id
+        );
+        self.own[axis].window = self.own[axis].window.and(holds);
+        longer
+    }
+
     /// The windows this drawing holds for, stated rather than taken: a
     /// container that branched on a length in pixels says which side of the
     /// boundary it was on, which is wider than the one window reading that
@@ -489,16 +565,12 @@ impl<'a> Painter<'a> {
     pub fn window_holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
         let holds = holds.into();
         debug_assert!(
-            holds.contains(self.window.axis(axis)),
+            holds.contains(self.window[axis]),
             "'{}' ({:?}) says its drawing holds for windows that leave out this one",
             self.label(),
             self.id
         );
-        self.own.window[axis as usize] = holds;
-    }
-
-    pub fn text_data(&mut self) -> &mut TextData {
-        &mut self.rsc.ui_mut().text
+        self.own[axis].window = holds;
     }
 
     pub fn child_layer(&mut self) {
@@ -553,7 +625,7 @@ impl<W: ?Sized> DrawResult<'_, '_, W> {
     }
 
     pub fn len(self, axis: Axis) -> LayoutLen {
-        self.size().axis(axis)
+        self.size()[axis]
     }
 }
 
@@ -582,10 +654,10 @@ impl PrimitiveLike for &TextureHandle {
     }
 }
 
-/// Moves what a child depends on into this widget's own terms: this
-/// method's `impl` block is where a `Painter`'s own boxes are, so it takes
-/// only what the child was asked with.
 impl Painter<'_> {
+    /// Moves what a child depends on into this widget's own terms, taking
+    /// only what the child was asked with.
+    ///
     /// Window ranges are already about the one unit and combine directly.
     /// A rel base pin becomes this widget's own rel base wherever a length of it
     /// is what reached the child; where only pixels did, no length of this
@@ -602,20 +674,26 @@ impl Painter<'_> {
         holds: LayoutHolds,
         region: UiRegion,
         place: PlaceDesc,
-        declared: [Option<LayoutLen>; 2],
+        declared: Declared,
     ) -> LayoutHolds {
         let mut result = LayoutHolds::ANY;
-        for axis in AXES {
-            let n = axis as usize;
+        for axis in Axis::BOTH {
+            let declared = declared[axis];
+            let holds = holds[axis];
+            let at = place[axis];
+            let result = &mut result[axis];
             // Every read became pixels against the window, so a range on
             // it is already in this widget's terms.
-            result.window[n] = holds.window[n];
-            let at = *place.axis(axis);
-            let reaches = at.stated_rel_base().is_none()
-                && !at.is_sized()
-                && declared[n].is_none_or(|len| len.rel != Rel::ZERO);
-            result.rel_base[n] = holds.rel_base[n].and(reaches.then(|| self.rel_base.axis(axis)));
-            match (at.within_span(), declared[n].is_some()) {
+            result.window = holds.window;
+            // A length this widget named -- a resolved share, a box a sibling
+            // decided, a box it sized outright, which is its own base -- is
+            // not a length of this widget's rel base, so a pin on it stops
+            // here. So does a declaration in pixels: no length of either base
+            // is in it to see.
+            let reaches = !matches!(at.rel_base, RelBase::Len(_))
+                && declared.is_none_or(|len| len.rel != Rel::ZERO);
+            result.rel_base = holds.rel_base.and(reaches.then(|| self.rel_base[axis]));
+            match (at.span, declared) {
                 // Its box is a part of this widget's own box, in that box's
                 // own lengths, so what it holds for maps back through that
                 // part into a range on this widget's box. A length it pinned
@@ -623,12 +701,12 @@ impl Painter<'_> {
                 // part is the whole of the box less pixels, which is the one
                 // shape that inverts exactly; any other part pins this
                 // widget's own length.
-                (Some(span), false) => {
+                (PlaceSpan::Within(span), None) => {
                     let part_len = span.len();
-                    result.region[n] = holds.region[n].through(part_len);
-                    result.region_len[n] = holds.region_len[n].map(|pinned| match part_len.rel {
+                    result.region = holds.region.through(part_len);
+                    result.region_len = holds.region_len.map(|pinned| match part_len.rel {
                         Rel::ONE => pinned - Len::from_parts(Rel::ZERO, part_len.px),
-                        _ => self.region.axis(axis).len(),
+                        _ => self.region[axis].len(),
                     });
                 }
                 // Its box is a length this widget decided, from its own
@@ -636,8 +714,7 @@ impl Painter<'_> {
                 // widget's box reaches it, so what it holds for is a range
                 // on the window and none of it on that box.
                 _ => {
-                    result.window[n] =
-                        result.window[n].and(holds.region[n].through(region.axis(axis).len()));
+                    result.window = result.window.and(holds.region.through(region[axis].len()));
                 }
             }
         }
@@ -645,103 +722,101 @@ impl Painter<'_> {
     }
 }
 
-/// What a widget declares a length of its box to be. `leftover` is not one: a
-/// share of what is left over is only a length to the widget dividing one,
-/// so it passes up in the size instead.
-pub(crate) fn declared_lens(widgets: &Widgets, id: WidgetId) -> [Option<LayoutLen>; 2] {
-    let rules = widgets.size_rules(id);
-    let widget = widgets.get_dyn(id);
-    AXES.map(|axis| {
-        rules.axis(axis).declared().or_else(|| {
+impl Widgets {
+    /// What says a widget's length on one axis without drawing it, if anything
+    /// does. A rule is the answer where there is one: it wins over whatever the
+    /// widget would draw, so it has to win over what the widget says too -- a
+    /// share included, since a share is a length only to whoever divides one,
+    /// and that is the parent rather than this widget.
+    fn exact_len(&self, id: WidgetId, axis: Axis) -> Option<LayoutLen> {
+        self.size_rules(id)[axis].exact().or_else(|| {
             // A hint still narrows the box where no rule does, which is how a
             // widget with a natural pixel size -- an image, a gap -- gets that
             // size rather than the whole offer. That is the offer's business
             // rather than a declaration's, and this falls away once a widget
             // occupies its reported size inside the box it was offered.
-            widget
-                .and_then(|widget| widget.size_hint(axis))
-                .filter(|len| len.leftover == Weight::ZERO)
-        })
+            self.get_dyn(id)?.size_hint(axis)
         })
+    }
+
+    /// What a widget's box is where a rule or its own hint gives one outright,
+    /// rather than a share for whoever draws it to divide.
+    pub(super) fn declared_lens(&self, id: WidgetId) -> Declared {
+        Declared::from_axes(|axis| self.exact_len(id, axis)?.declared())
+    }
 }
 
-/// Whether what a widget reported along an axis is the whole of the box it
-/// is in rather than a part to be placed inside it. A share fills, because a
-/// share is a length only to whoever divides one, and whoever did is the one
-/// that handed down this box. A declared axis does too: the rule already gave
-/// the region its length, and the rule's length is what the widget reports
-/// there. And an axis the parent decided from the answer is
-/// the answer already.
-pub(crate) fn fills(reported: LayoutLen, declared: Option<LayoutLen>, decided: bool) -> bool {
-    reported.leftover != Weight::ZERO || declared.is_some() || decided
+impl LayoutLen {
+    /// Whether what a widget reported along an axis is the whole of the box
+    /// it is in rather than a part to be placed inside it. A share fills,
+    /// because a share is a length only to whoever divides one, and whoever
+    /// did is the one that handed down this box. A declared axis does too:
+    /// the rule already gave the region its length, and the rule's length is
+    /// what the widget reports there. And an axis the parent decided from
+    /// the answer is the answer already.
+    pub(super) fn fills(&self, declared: Option<Len>, decided: bool) -> bool {
+        self.leftover != Weight::ZERO || declared.is_some() || decided
+    }
 }
 
-/// Where a widget's drawing goes inside the part its parent gave it: what
-/// it reported, on the side of the part its alignment says, and the whole
-/// part wherever the answer fills it.
-///
-/// The length it reported is a length of its rel base, and the part is one too,
-/// so this takes one from the other rather than composing it into the part.
-/// That is what makes a fraction the same fraction wherever the part it is
-/// placed in sits and however long it is -- the fraction is resolved once,
-/// here, against the rel base it was reported of.
-pub(crate) fn placement(
+impl PlaceDesc {
+    /// Where a widget's drawing goes inside the part its parent gave it: what
+    /// it reported, on the side of the part its alignment says, and the whole
+    /// part wherever the answer fills it.
+    ///
+    /// The length it reported is a length of its rel base, and the part is one
+    /// too, so this takes one from the other rather than composing it into the
+    /// part. That is what makes a fraction the same fraction wherever the part
+    /// it is placed in sits and however long it is -- the fraction is resolved
+    /// once, here, against the rel base it was reported of.
+    pub(super) fn placement(
+        self,
         region: UiRegion,
         size: Size,
-    declared: [Option<LayoutLen>; 2],
-    place: PlaceDesc,
+        declared: Declared,
         align: RegionAlign,
-) -> UiRegion {
+    ) -> UiRegion {
         let mut placed = region;
-    for axis in AXES {
-        let n = axis as usize;
-        let reported = size.axis(axis);
-        if fills(reported, declared[n], place.axis(axis).does_fill()) {
+        for axis in Axis::BOTH {
+            let reported = size[axis];
+            if reported.fills(declared[axis], self[axis].fills) {
                 continue;
             }
-        let len = Len::from_parts(reported.rel, reported.px);
-        let span = placed.axis_mut(axis);
-        span.start += (span.len() - len).scale(align.axis(axis).rel());
-        span.end = span.start + len;
+            placed[axis] = placed[axis].place(reported.without_leftover(), align[axis]);
         }
         placed
-}
+    }
 
-/// The rel base length and the box a child is asked in, in the coordinates the
-/// widget asking draws in.
-///
-/// `own` is that widget's own box, and `place` what of it the child is
-/// given, including any rel base it states -- a row's slot, or padding's rel
-/// base less its pixels. That is a window length, like every other length
-/// here, since a slot of a row is not a fraction of anything the row can
-/// name. The child's declaration is a fraction of whichever reached it, and
-/// is the only one that also places the box: a box the caller decided is
-/// what `place` names.
-pub(crate) fn rel_base_and_region(
+    /// The rel base length and the box a child is asked in, in the coordinates the
+    /// widget asking draws in.
+    ///
+    /// `own` is that widget's own box, and `place` what of it the child is
+    /// given, including any rel base it states -- a row's slot, or padding's rel
+    /// base less its pixels. That is a window length, like every other length
+    /// here, since a slot of a row is not a fraction of anything the row can
+    /// name. The child's declaration is a fraction of whichever reached it, and
+    /// is the only one that also places the box: a box the caller decided is
+    /// what `place` names.
+    pub(super) fn rel_base_and_region(
+        self,
         own: UiRegion,
         parent_rel_base: UiVec2,
-    place: PlaceDesc,
-    declared: [Option<LayoutLen>; 2],
+        declared: Declared,
         align: RegionAlign,
-) -> (UiVec2, UiRegion) {
-    let given = place.of(own, align);
+    ) -> (UiVec2, UiRegion) {
+        let given = self.of(own, align);
         let mut rel_base = parent_rel_base;
         let mut region = given;
-    for axis in AXES {
-        let n = axis as usize;
-        let base = place
-            .axis(axis)
-            .stated_rel_base()
-            .unwrap_or_else(|| parent_rel_base.axis(axis));
-        let len = declared[n]
-            .map(|len| Len::from_parts(len.rel, len.px).within_len(base))
+        for axis in Axis::BOTH {
+            let base = self.base(axis, parent_rel_base);
+            let len = declared[axis]
+                .map(|len| len.within_len(base))
                 .unwrap_or(base);
-        *rel_base.axis_mut(axis) = len;
-        if declared[n].is_some() {
-            let slot = given.axis(axis);
-            let start = slot.start + (slot.len() - len).scale(align.axis(axis).rel());
-            *region.axis_mut(axis) = UiSpan::new(start, start + len);
+            rel_base[axis] = len;
+            if declared[axis].is_some() {
+                region[axis] = given[axis].place(len, align[axis]);
             }
         }
         (rel_base, region)
+    }
 }

core/src/ui/place.rs

@@ -1,4 +1,5 @@
-use crate::{Axis, AxisAlign, Len, PrimitiveHandle, RegionAlign, UiRegion, UiSpan};
+use crate::util::impl_axis_index;
+use crate::{Axis, AxisAlign, Len, PrimitiveHandle, RegionAlign, UiRegion, UiSpan, UiVec2};
 
 /// How a child's region along one axis comes from the region of the widget
 /// asking, and what its fractions are of.
@@ -10,21 +11,23 @@ use crate::{Axis, AxisAlign, Len, PrimitiveHandle, RegionAlign, UiRegion, UiSpan
 /// it differently, so it is said here.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub struct PlaceDescAxis {
-    span: PlaceSpan,
-    fills: bool,
-    rel_base: RelBase,
+    pub span: PlaceSpan,
+    pub fills: bool,
+    pub rel_base: RelBase,
 }
 
 #[derive(Clone, Copy, Debug, PartialEq)]
-enum PlaceSpan {
+pub enum PlaceSpan {
     Within(UiSpan),
     Shifted(UiSpan),
     Sized(Len),
 }
 
-/// What a child's fractions are of, where the caller has not named a length.
+/// What a child's fractions are of. [`PlaceSpan::Sized`] is a length the
+/// caller named, which is always its own base, so nothing here constructs one
+/// beside anything but [`Self::Len`].
 #[derive(Clone, Copy, Debug, PartialEq)]
-enum RelBase {
+pub enum RelBase {
     /// The caller's own, unchanged.
     Inherit,
     /// The caller's own, narrowed the way the region is.
@@ -35,53 +38,7 @@ enum RelBase {
 
 impl PlaceDescAxis {
     /// The whole of the caller's box.
-    pub const WHOLE: Self = Self::within(UiSpan::FULL);
-
-    /// `span` composed into the caller's own box, so it moves and scales
-    /// with it: [`UiSpan::within`], which is what a container that insets
-    /// one speaks. Taking eleven pixels off the end needs no length, where
-    /// saying the same thing in window lengths would make the container read
-    /// its own box -- and a box chosen from its own answer then feeds back
-    /// into the answer.
-    ///
-    /// The child's rel base is narrowed the same way, so padding takes its
-    /// pixels off both and `rel(1)` under it fills the caller rather than
-    /// overflowing it.
-    pub const fn within(span: UiSpan) -> Self {
-        Self {
-            span: PlaceSpan::Within(span),
-            fills: false,
-            rel_base: RelBase::WithRegion,
-        }
-    }
-
-    /// `span` shifted to where the caller's own box starts: window lengths
-    /// along a cursor, which is what a container dividing room among its
-    /// children speaks. A child's report is a window length, so the cursor
-    /// that sums those reports is one too, and a moved box re-places every
-    /// child by re-adding its start, exactly.
-    ///
-    /// The child's rel base passes through: how far along the cursor a child
-    /// sits says nothing about what a fraction under it is of.
-    pub const fn shifted(span: UiSpan) -> Self {
-        Self {
-            span: PlaceSpan::Shifted(span),
-            fills: false,
-            rel_base: RelBase::Inherit,
-        }
-    }
-
-    /// A box this long, placed in the caller's own by the child's alignment:
-    /// the rule that places an answer, with the length given from above
-    /// rather than reported. What a stack's sizing child decides for the
-    /// rest. It is the child's rel base too.
-    pub const fn sized(len: Len) -> Self {
-        Self {
-            span: PlaceSpan::Sized(len),
-            fills: false,
-            rel_base: RelBase::Len(len),
-        }
-    }
+    pub const WHOLE: Self = UiSpan::FULL.within_desc();
 
     /// This region is the child's placement: its answer is not placed inside
     /// it again. A container uses it where it hands back exactly what the
@@ -91,6 +48,14 @@ impl PlaceDescAxis {
         self
     }
 
+    /// This along `axis`, and the whole of the caller's box across it: what
+    /// a container dividing one axis says, since nothing divides the other.
+    /// [`PlaceDesc::from_axis`] says the across one where it is not the
+    /// whole.
+    pub const fn on_axis(self, axis: Axis) -> PlaceDesc {
+        PlaceDesc::from_axis(axis, self, Self::WHOLE)
+    }
+
     /// What the child's fractions are of, as a length of the window: a
     /// resolved share, or a box a sibling's answer decided.
     pub const fn rel_base(mut self, len: Len) -> Self {
@@ -98,70 +63,17 @@ impl PlaceDescAxis {
         self
     }
 
-    /// Whether the region is the placement outright, rather than a box the
-    /// answer is placed inside.
-    pub(crate) const fn does_fill(self) -> bool {
-        self.fills
-    }
-
     /// Where it lands in the coordinates `own` is in.
-    pub(crate) fn of(self, own: UiSpan, align: AxisAlign) -> UiSpan {
+    pub fn of(self, own: UiSpan, align: AxisAlign) -> UiSpan {
         match self.span {
             PlaceSpan::Within(span) => span.within(&own),
             PlaceSpan::Shifted(mut span) => {
                 span.shift(own.start);
                 span
             }
-            PlaceSpan::Sized(len) => {
-                let start = own.start + (own.len() - len).scale(align.rel());
-                UiSpan::new(start, start + len)
+            PlaceSpan::Sized(len) => own.place(len, align),
         }
     }
-    }
-
-    /// The child's rel base, where this says one outright. `None` forwards
-    /// the caller's own, and [`RelBase::WithRegion`] is resolved by whoever
-    /// can read that rel base, so it does not reach here.
-    pub(crate) const fn stated_rel_base(self) -> Option<Len> {
-        match self.rel_base {
-            RelBase::Len(len) => Some(len),
-            _ => None,
-        }
-    }
-
-    /// The length this narrows the caller's rel base by, where it does.
-    /// `None` leaves that rel base alone, and reading it is then a
-    /// dependency the caller does not take.
-    pub(crate) const fn narrows_rel_base(self) -> Option<UiSpan> {
-        match (self.rel_base, self.span) {
-            (RelBase::WithRegion, PlaceSpan::Within(span)) => Some(span),
-            _ => None,
-        }
-    }
-
-    /// The span it composes into the caller's box, where that is what it
-    /// does: the one case whose validity maps back through the part.
-    pub(crate) const fn within_span(self) -> Option<UiSpan> {
-        match self.span {
-            PlaceSpan::Within(span) => Some(span),
-            _ => None,
-        }
-    }
-
-    /// Whether the caller decided this length rather than a place along its
-    /// own box, which is what stops its length reaching the child at all.
-    pub(crate) const fn is_sized(self) -> bool {
-        matches!(self.span, PlaceSpan::Sized(_))
-    }
-
-    /// The same, with its rel base stated outright.
-    pub(crate) const fn with_rel_base(mut self, len: Option<Len>) -> Self {
-        self.rel_base = match len {
-            Some(len) => RelBase::Len(len),
-            None => RelBase::Inherit,
-        };
-        self
-    }
 }
 
 /// Where a child is asked, on both axes. A [`UiRegion`] converts into the
@@ -185,29 +97,21 @@ impl PlaceDesc {
         Self { x: place, y: place }
     }
 
+    /// A description per axis, where the two differ and neither is the
+    /// axis a container divides.
+    pub fn from_axes(f: impl Fn(Axis) -> PlaceDescAxis) -> Self {
+        Self::new(f(Axis::X), f(Axis::Y))
+    }
+
     /// `aligned` on `axis` and `ortho` on the other, which is how a
     /// container that divides one axis says what it is doing.
-    pub fn from_axis(axis: Axis, aligned: PlaceDescAxis, ortho: PlaceDescAxis) -> Self {
+    pub const fn from_axis(axis: Axis, aligned: PlaceDescAxis, ortho: PlaceDescAxis) -> Self {
         match axis {
             Axis::X => Self::new(aligned, ortho),
             Axis::Y => Self::new(ortho, aligned),
         }
     }
 
-    pub const fn axis(&self, axis: Axis) -> &PlaceDescAxis {
-        match axis {
-            Axis::X => &self.x,
-            Axis::Y => &self.y,
-        }
-    }
-
-    pub const fn axis_mut(&mut self, axis: Axis) -> &mut PlaceDescAxis {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-
     /// Both regions are the child's placement. See [`PlaceDescAxis::fills`].
     pub const fn fills(self) -> Self {
         Self::new(self.x.fills(), self.y.fills())
@@ -215,22 +119,81 @@ impl PlaceDesc {
 
     /// The child's rel base on one axis. See [`PlaceDescAxis::rel_base`].
     pub const fn rel_base(mut self, axis: Axis, len: Len) -> Self {
-        *self.axis_mut(axis) = self.axis(axis).rel_base(len);
+        self[axis] = self[axis].rel_base(len);
         self
     }
 
+    /// What a child's fractions on one axis are of, as a length of the
+    /// window: a length this place names, or the rel base of the widget
+    /// giving it, which is `parent_rel_base`.
+    pub(super) fn base(&self, axis: Axis, parent_rel_base: UiVec2) -> Len {
+        match self[axis].rel_base {
+            RelBase::Len(len) => len,
+            RelBase::Inherit | RelBase::WithRegion => parent_rel_base[axis],
+        }
+    }
+
     /// The box each axis names, in the coordinates `own` is in.
-    pub(crate) fn of(self, own: UiRegion, align: RegionAlign) -> UiRegion {
+    pub fn of(self, own: UiRegion, align: RegionAlign) -> UiRegion {
         UiRegion::new(self.x.of(own.x, align.x), self.y.of(own.y, align.y))
     }
 }
 
+impl UiSpan {
+    /// This span composed into the caller's own box, so it moves and scales
+    /// with it: [`UiSpan::within`], which is what a container that insets
+    /// one speaks. Taking eleven pixels off the end needs no length, where
+    /// saying the same thing in window lengths would make the container read
+    /// its own box -- and a box chosen from its own answer then feeds back
+    /// into the answer.
+    ///
+    /// The child's rel base is narrowed the same way, so padding takes its
+    /// pixels off both and `rel(1)` under it fills the caller rather than
+    /// overflowing it.
+    pub const fn within_desc(self) -> PlaceDescAxis {
+        PlaceDescAxis {
+            span: PlaceSpan::Within(self),
+            fills: false,
+            rel_base: RelBase::WithRegion,
+        }
+    }
+
+    /// This span shifted to where the caller's own box starts: window
+    /// lengths along a cursor, which is what a container dividing room among
+    /// its children speaks. A child's report is a window length, so the
+    /// cursor that sums those reports is one too, and a moved box re-places
+    /// every child by re-adding its start, exactly.
+    ///
+    /// The child's rel base passes through: how far along the cursor a child
+    /// sits says nothing about what a fraction under it is of. The same span
+    /// says [`Self::within_desc`] as a part of that box instead, and which is
+    /// meant cannot be read off the numbers.
+    pub const fn shifted_desc(self) -> PlaceDescAxis {
+        PlaceDescAxis {
+            span: PlaceSpan::Shifted(self),
+            fills: false,
+            rel_base: RelBase::Inherit,
+        }
+    }
+}
+
+impl Len {
+    /// A box this long, placed in the caller's own by the child's alignment:
+    /// the rule that places an answer, with the length given from above
+    /// rather than reported. What a stack's sizing child decides for the
+    /// rest. It is the child's rel base too.
+    pub const fn as_desc(self) -> PlaceDescAxis {
+        PlaceDescAxis {
+            span: PlaceSpan::Sized(self),
+            fills: false,
+            rel_base: RelBase::Len(self),
+        }
+    }
+}
+
 impl From<UiRegion> for PlaceDesc {
     fn from(region: UiRegion) -> Self {
-        Self::new(
-            PlaceDescAxis::within(region.x),
-            PlaceDescAxis::within(region.y),
-        )
+        Self::new(region.x.within_desc(), region.y.within_desc())
     }
 }
 
@@ -247,3 +210,5 @@ pub struct RetainedPrimitive {
     pub handle: PrimitiveHandle,
     pub region: UiRegion,
 }
+
+impl_axis_index!(PlaceDesc => PlaceDescAxis);

core/src/ui/render_state.rs

@@ -1,15 +1,12 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
-use crate::ui::painter::{declared_lens, placement, rel_base_and_region};
 use crate::{
-    ActiveData, Axis, DrawLayers, IdLike, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx, Moves,
-    Painter, PixelRegion, PlaceDesc, PxVec2, Rel, Size, StrongWidget, UiRegion, UiRsc, UiSpan,
-    UiVec2, Weight, WidgetId, Widgets,
+    ActiveData, Answer, Axis, Declared, DrawLayers, IdLike, LayoutHolds, LayoutLen, Len, MaskIdx,
+    MoveIdx, Moves, Painter, PixelRegion, PlaceDesc, PxVec2, Rel, Size, StrongWidget, UiRegion,
+    UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
     util::{HashMap, Vec2},
 };
 
-const AXES: [Axis; 2] = [Axis::X, Axis::Y];
-
 /// Where a widget is drawn: what its parent decides about the draw besides
 /// the boxes themselves.
 #[derive(Clone, Copy)]
@@ -33,8 +30,14 @@ pub(super) struct DrawInfo {
     pub asked: PlaceDesc,
     /// Whether the parent already asked about this widget in this draw.
     pub re_asked: bool,
-    /// The rel base in pixels, resolved once against the window.
-    pub px: PxVec2,
+}
+
+/// What one draw of a widget came to: the answer it gave, and the boxes and
+/// windows the drawing that gave it holds for. The two are separate ranges --
+/// a drawing can be invalid where its answer still stands.
+pub(super) struct Drawn {
+    pub answer: Answer,
+    pub drawing_holds: LayoutHolds,
 }
 
 /// What a widget's children are placed in: its own box, the coordinates its
@@ -43,7 +46,6 @@ pub(super) struct Placing {
     pub id: WidgetId,
     pub region: UiRegion,
     pub rel_base: UiVec2,
-    pub window: PxVec2,
     pub depth: usize,
     pub move_idx: MoveIdx,
     pub mask: MaskIdx,
@@ -114,7 +116,7 @@ impl UiRenderState {
             // it will ask either again.
             let answer = active
                 .answer
-                .is_some_and(|(_, holds)| holds.contains(size, active.rel_base, active.region));
+                .is_some_and(|answer| answer.holds.contains(size, active.rel_base, active.region));
             answer && active.holds.contains(size, active.rel_base, active.region)
         });
         if !stands {
@@ -125,7 +127,6 @@ impl UiRenderState {
     /// The root is asked about in the output. Its own rules narrow both its
     /// rel base and box; nothing above it chose a different one.
     fn root_info(&self, rel_base: UiVec2, region: UiRegion) -> DrawInfo {
-        let px = rel_base.to_px(self.output_size);
         DrawInfo {
             layer: 0,
             parent: None,
@@ -138,7 +139,6 @@ impl UiRenderState {
             placed: PlaceDesc::WHOLE,
             asked: PlaceDesc::WHOLE,
             re_asked: false,
-            px,
         }
     }
 
@@ -194,11 +194,10 @@ impl UiRenderState {
     /// rules. Nothing above it narrowed anything or chose where it goes, so
     /// its declaration is the whole of what decides either.
     fn root_layout(id: WidgetId, widgets: &Widgets) -> (UiVec2, UiRegion) {
-        rel_base_and_region(
+        PlaceDesc::WHOLE.rel_base_and_region(
             UiRegion::FULL,
             UiVec2::FULL_SIZE,
-            PlaceDesc::WHOLE,
-            declared_lens(widgets, id),
+            widgets.declared_lens(id),
             widgets.alignment(id),
         )
     }
@@ -209,7 +208,7 @@ impl UiRenderState {
         info: DrawInfo,
         mut old: Option<ActiveData>,
         rsc: &mut dyn UiRsc,
-    ) -> (Size, LayoutHolds, LayoutHolds) {
+    ) -> Drawn {
         let old_parent = old
             .as_ref()
             .or_else(|| self.active.get(&id))
@@ -218,10 +217,16 @@ impl UiRenderState {
         #[cfg(feature = "layout-diagnostics")]
         {
             diag::bump(Counter::DrawRequests);
-            diag::draw_request(id, info.parent, region, info.px, info.region_node);
+            diag::draw_request(
+                id,
+                info.parent,
+                region,
+                region.to_px(self.output_size).size(),
+                info.region_node,
+            );
         }
         let align = rsc.widgets().alignment(id);
-        let declared = declared_lens(rsc.widgets(), id);
+        let declared = rsc.widgets().declared_lens(id);
         // Nothing this widget measured can be dirty while it draws: layout is
         // one bottom-up walk, so anything deeper has settled or deferred to
         // its own parent, and a deferred one leaves that parent marked.
@@ -235,9 +240,9 @@ impl UiRenderState {
             .then(|| self.retained_answer(id, region, info))
             .flatten()
             .and_then(|answer| {
-                let placed = placement(region, answer.0, declared, info.placed, align);
+                let placed = info.placed.placement(region, answer.size, declared, align);
                 self.try_reuse(id, region, placed, info, rsc)
-                    .map(|()| answer)
+                    .then_some(answer)
             });
         let answer = reused.unwrap_or_else(|| {
             if old.is_none() {
@@ -247,7 +252,7 @@ impl UiRenderState {
             // Where the drawing goes: the part its parent gave it, with the
             // answer placed inside that part on any axis the parent left
             // open.
-            let placed = placement(region, answer.0, declared, info.placed, align);
+            let placed = info.placed.placement(region, answer.size, declared, align);
             if placed != region {
                 self.relocate(id, placed, info, rsc);
             }
@@ -275,7 +280,10 @@ impl UiRenderState {
         {
             old_parent.children.retain(|child| *child != id);
         }
-        (answer.0, answer.1, drawing_holds)
+        Drawn {
+            answer,
+            drawing_holds,
+        }
     }
 
     /// Calls a widget's `draw` and keeps what it drew in `region`.
@@ -286,14 +294,14 @@ impl UiRenderState {
         info: DrawInfo,
         old: Option<ActiveData>,
         rsc: &mut dyn UiRsc,
-    ) -> (Size, LayoutHolds) {
+    ) -> Answer {
         let rel_base = info.rel_base;
         let (move_idx, region, retired_move) = match info.region_node {
             // A node entry is only a translation. Its local box keeps the
             // same window-unit length as the box in its parent's node.
             true => (
-                self.move_slot(id, info.parent_move, translation(region)),
-                local_region(region),
+                self.move_slot(id, info.parent_move, region.as_translation()),
+                region.at_origin(),
                 None,
             ),
             // Keep the old entry alive until every descendant has migrated.
@@ -305,8 +313,6 @@ impl UiRenderState {
             .and_then(|old| old.mask_region.map(|_| old.mask));
         let old_children = old.map_or_else(Vec::new, |old| old.children);
         rsc.widgets_mut().needs_redraw.remove(&id);
-        let px = info.px;
-
         let window = self.output_size;
         let mut painter = Painter {
             state: self,
@@ -377,14 +383,14 @@ impl UiRenderState {
         // resolved into the rel base when the child was asked, and resolving it
         // again here would take the fraction of a fraction.
         let rules = rsc.widgets().size_rules(id);
-        let ruled = |axis: Axis, reported: LayoutLen| match rules.axis(axis).exact() {
+        let ruled = |axis: Axis, reported: LayoutLen| match rules[axis].exact() {
             None => reported,
             Some(len) if len.leftover == Weight::ZERO => LayoutLen {
-                rel: info.rel_base.axis(axis).rel,
-                px: info.rel_base.axis(axis).px,
+                rel: info.rel_base[axis].rel,
+                px: info.rel_base[axis].px,
                 leftover: Weight::ZERO,
             },
-            Some(len) => len.within_len(info.rel_base.axis(axis)),
+            Some(len) => len.within_len(info.rel_base[axis]),
         };
         let size = Size {
             x: ruled(Axis::X, size.x),
@@ -397,11 +403,14 @@ impl UiRenderState {
         // was offered reports the height it needs.
         debug_assert!(
             mask == info.mask
-                || AXES
-                    .into_iter()
-                    .all(|axis| within_box(size, region, self.output_size, axis)),
-            "'{}' ({id:?}) clips to {px:?} and reports {size}",
+                || Axis::BOTH.into_iter().all(|axis| size.within_box(
+                    region,
+                    self.output_size,
+                    axis
+                )),
+            "'{}' ({id:?}) clips to {} and reports {size}",
             rsc.widgets().label(id),
+            region.to_px(window),
         );
         for c in &old_children {
             if !children.contains(c) {
@@ -418,25 +427,22 @@ impl UiRenderState {
         // rel base's own length, so the answer is that rel base's and not just
         // that many pixels of this window -- the same pin a widget that read
         // its rel base took for its drawing.
-        let rel_base = AXES.map(|axis| {
-            let fraction = rules
-                .axis(axis)
-                .exact()
-                .is_some_and(|len| len.rel != Rel::ZERO);
-            match fraction {
-                true => Some(info.rel_base.axis(axis)),
-                false => own.rel_base[axis as usize],
+        let mut own_holds = own;
+        for axis in Axis::BOTH {
+            let fraction = rules[axis].exact().is_some_and(|len| len.rel != Rel::ZERO);
+            if fraction {
+                own_holds[axis].rel_base = Some(info.rel_base[axis]);
+            }
         }
-        });
-        let own_holds = LayoutHolds { rel_base, ..own };
         let answer_holds = own_holds.and(answer_under);
         let holds = under
             .into_iter()
             .fold(answer_holds, |holds, (_, child)| holds.and(child));
         debug_assert!(
             holds.contains(self.output_size, info.rel_base, region),
-            "'{}' ({id:?}) drew in {px:?}, outside the ranges it reported: {holds:?}",
+            "'{}' ({id:?}) drew in {}, outside the ranges it reported: {holds:?}",
             rsc.widgets().label(id),
+            region.to_px(window),
         );
         // What it asked about and did not draw is still something it asked,
         // and a change there has to reach it. Asking answered whatever mark
@@ -457,7 +463,6 @@ impl UiRenderState {
                         placed: PlaceDesc::WHOLE,
                         asked: PlaceDesc::WHOLE,
                         re_asked: false,
-                        px,
                     },
                     rsc,
                 );
@@ -484,8 +489,7 @@ impl UiRenderState {
             primitives,
             mask_region,
             children,
-            size_deps,
-            declared: declared_lens(rsc.widgets(), id),
+            declared: rsc.widgets().declared_lens(id),
             own_align: rsc.widgets().alignment(id),
             move_idx,
             parent_move: info.parent_move,
@@ -495,7 +499,10 @@ impl UiRenderState {
         };
         rsc.on_draw(&active);
         self.active.insert(id, active);
-        (size, answer_holds)
+        Answer {
+            size,
+            holds: answer_holds,
+        }
     }
 
     /// Keeps a region node's entry across redraws because descendants retain
@@ -522,23 +529,17 @@ impl UiRenderState {
     /// drawing ended up. Alignment is exactly that case: the first box is the
     /// question and the smaller placed box holds the drawing. Whether the
     /// answer is stale at all is its caller's question, asked once there.
-    fn retained_answer(
-        &self,
-        id: WidgetId,
-        region: UiRegion,
-        info: DrawInfo,
-    ) -> Option<(Size, LayoutHolds)> {
+    fn retained_answer(&self, id: WidgetId, region: UiRegion, info: DrawInfo) -> Option<Answer> {
         let active = self.active.get(&id)?;
-        let has_region_node = active.move_idx != active.parent_move;
         if !active.drawn
-            || has_region_node != info.region_node
+            || active.is_region_node() != info.region_node
             || active.parent_move != info.parent_move
         {
             return None;
         }
         let answer = active.answer?;
         answer
-            .1
+            .holds
             .contains(self.output_size, info.rel_base, region)
             .then_some(answer)
     }
@@ -552,7 +553,7 @@ impl UiRenderState {
         placed: UiRegion,
         info: DrawInfo,
         rsc: &mut dyn UiRsc,
-    ) -> Option<()> {
+    ) -> bool {
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::ReuseAttempts);
         if rsc.widgets().needs_redraw.contains(&id) {
@@ -561,19 +562,26 @@ impl UiRenderState {
                 diag::bump(Counter::ReuseDirty);
                 diag::reuse(id, ReuseOutcome::Dirty);
             }
-            return None;
+            return false;
         }
-        let active = self.active.get(&id)?;
+        let Some(active) = self.active.get(&id) else {
+            return false;
+        };
         if !active.drawn {
             #[cfg(feature = "layout-diagnostics")]
+            {
+                diag::bump(Counter::ReuseUndrawn);
                 diag::reuse(id, ReuseOutcome::Undrawn);
-            return None;
             }
-        let has_region_node = active.move_idx != active.parent_move;
-        if has_region_node != info.region_node {
+            return false;
+        }
+        if active.is_region_node() != info.region_node {
             #[cfg(feature = "layout-diagnostics")]
+            {
                 diag::bump(Counter::ReuseWrongNode);
-            return None;
+                diag::reuse(id, ReuseOutcome::WrongNode);
+            }
+            return false;
         }
         // Drawn on another layer: the drawing sits in that layer's list and
         // paints at its moment, which no amount of geometry says. A container
@@ -586,10 +594,18 @@ impl UiRenderState {
                 diag::bump(Counter::ReuseWrongLayer);
                 diag::reuse(id, ReuseOutcome::WrongLayer);
             }
-            return None;
+            return false;
         }
+        // Its primitives name the mask it inherited, and a masking parent
+        // that redrew pushed another: keeping them would clip them by one
+        // nothing updates again.
         if active.parent_mask != info.mask {
-            return None;
+            #[cfg(feature = "layout-diagnostics")]
+            {
+                diag::bump(Counter::ReuseWrongMask);
+                diag::reuse(id, ReuseOutcome::WrongMask);
+            }
+            return false;
         }
         // Drawn somewhere else in the tree: its box is in coordinates it no
         // longer sits in, and its slot names the wrong parent.
@@ -599,7 +615,7 @@ impl UiRenderState {
                 diag::bump(Counter::ReuseWrongParent);
                 diag::reuse(id, ReuseOutcome::WrongParent);
             }
-            return None;
+            return false;
         }
         // In pixels, because the box is a fraction of the window and that
         // may be what changed -- an unchanged fraction of a window half the
@@ -609,35 +625,11 @@ impl UiRenderState {
             .contains(self.output_size, info.rel_base, region)
         {
             #[cfg(feature = "layout-diagnostics")]
-            {
-                // Which of the three said no, so a rel base that redraws more
-                // than it should says where to look. They overlap: a drawing
-                // can be outside two of them at once.
-                let holds = active.holds;
-                for axis in AXES {
-                    let n = axis as usize;
-                    if holds.region_len[n].is_some_and(|pinned| pinned != region.axis(axis).len()) {
-                        diag::bump(Counter::OutsidePinnedLen);
-                    }
-                    if !holds.window[n].contains(self.output_size.axis(axis))
-                        || holds.rel_base[n]
-                            .is_some_and(|pinned| pinned != info.rel_base.axis(axis))
-                    {
-                        diag::bump(Counter::OutsideRelBase);
-                    }
-                    if !holds.region[n]
-                        .contains(region.axis(axis).len().to_px(self.output_size.axis(axis)))
-                    {
-                        diag::bump(Counter::OutsideRegion);
-                    }
-                }
-                diag::bump(Counter::ReuseOutside);
-                diag::reuse(id, ReuseOutcome::Outside);
-            }
-            return None;
+            diag::outside(id, active.holds, region, info.rel_base, self.output_size);
+            return false;
         }
         self.relocate(id, placed, info, rsc);
-        Some(())
+        true
     }
 
     /// Puts a retained drawing where its parent now has it, without drawing:
@@ -650,15 +642,15 @@ impl UiRenderState {
             "'{}' ({id:?}) placed while marked to draw",
             rsc.widgets().label(id)
         );
-        let has_region_node = active.move_idx != active.parent_move;
-        let local = match has_region_node {
-            true => local_region(placed),
+        let is_region_node = active.is_region_node();
+        let local = match is_region_node {
+            true => placed.at_origin(),
             false => placed,
         };
         let moved = active.placement != local;
         let slot = active.move_idx;
-        if has_region_node {
-            self.moves.set(slot, translation(placed));
+        if is_region_node {
+            self.moves.set(slot, placed.as_translation());
         }
         if moved {
             self.reposition(id, local, info, rsc);
@@ -669,23 +661,13 @@ impl UiRenderState {
         active.placed = info.placed;
         #[cfg(feature = "layout-diagnostics")]
         {
-            match (moved, has_region_node) {
-                (true, true) => diag::bump(Counter::ReuseMoved),
-                (true, false) => diag::bump(Counter::ReuseRemapped),
-                (false, _) => diag::bump(Counter::ReuseExact),
-            }
-            diag::reuse(
-                id,
-                if moved {
-                    if has_region_node {
-                        ReuseOutcome::Moved
-                    } else {
-                        ReuseOutcome::Remapped
-                    }
-                } else {
-                    ReuseOutcome::Exact
-                },
-            );
+            let (counter, outcome) = match (moved, is_region_node) {
+                (true, true) => (Counter::ReuseMoved, ReuseOutcome::Moved),
+                (true, false) => (Counter::ReuseRemapped, ReuseOutcome::Remapped),
+                (false, _) => (Counter::ReuseExact, ReuseOutcome::Exact),
+            };
+            diag::bump(counter);
+            diag::reuse(id, outcome);
         }
     }
 
@@ -707,11 +689,10 @@ impl UiRenderState {
     ) {
         let active = &self.active[&child];
         let (rel_base, region) = Self::ask_again(active, at, place);
-        let placed = placement(
+        let placed = place.placement(
             region,
             active.measured().unwrap_or(active.size),
             active.declared,
-            place,
             active.own_align,
         );
         let info = DrawInfo {
@@ -719,14 +700,13 @@ impl UiRenderState {
             parent: Some(at.id),
             depth: at.depth + 1,
             parent_move: at.move_idx,
-            region_node: active.move_idx != active.parent_move,
+            region_node: active.is_region_node(),
             mask: at.mask,
             rel_base,
             region,
             placed: place,
             asked: active.asked,
             re_asked: active.re_asked,
-            px: rel_base.to_px(at.window),
         };
         self.relocate(child, placed, info, rsc);
     }
@@ -736,13 +716,7 @@ impl UiRenderState {
     /// it declared are its own record's, so both are resolved against that
     /// parent's rel base again exactly as the first ask resolved them.
     fn ask_again(active: &ActiveData, at: &Placing, place: PlaceDesc) -> (UiVec2, UiRegion) {
-        rel_base_and_region(
-            at.region,
-            at.rel_base,
-            place,
-            active.declared,
-            active.own_align,
-        )
+        place.rel_base_and_region(at.region, at.rel_base, active.declared, active.own_align)
     }
 
     /// Re-places everything inside a widget whose own box moved. Every child
@@ -763,21 +737,22 @@ impl UiRenderState {
             id,
             region: placed,
             rel_base: info.rel_base,
-            window: self.output_size,
             depth: info.depth,
             move_idx: active.move_idx,
             mask: active.mask,
         };
-        let children = active.children.len();
-        for index in 0..children {
-            let child = self.active[&id].children[index];
+        // Taken out and put back so that placing a child can borrow the state
+        // it needs; nothing on that path reads this widget's own child list.
+        let children = std::mem::take(&mut self.active.get_mut(&id).unwrap().children);
+        for &child in &children {
             self.place_child(child, &at, rsc);
         }
+        self.active.get_mut(&id).unwrap().children = children;
     }
 
-    /// A reused subtree keeps its shape, so every widget in it moves by the
-    /// same amount -- and where the top of it did not move, none of it did,
-    /// which is what makes this free in the ordinary case.
+    /// A reused subtree keeps its shape, so each widget in it keeps its depth
+    /// under the top -- and where the top's own depth did not change, none of
+    /// them did, which is what makes this free in the ordinary case.
     fn redepth(&mut self, id: WidgetId, depth: usize) {
         let Some(active) = self.active.get_mut(&id) else {
             return;
@@ -786,22 +761,22 @@ impl UiRenderState {
             return;
         }
         active.depth = depth;
-        let children = active.children.len();
-        for index in 0..children {
-            let child = self.active[&id].children[index];
+        // Taken out and put back so the walk can borrow the state it needs;
+        // it only ever goes further down, so it reads no list but its own.
+        let children = std::mem::take(&mut active.children);
+        for &child in &children {
             self.redepth(child, depth + 1);
         }
+        self.active.get_mut(&id).unwrap().children = children;
     }
 
     fn hints_agree(id: WidgetId, size: Size, rsc: &dyn UiRsc) -> bool {
         let Some(widget) = rsc.widgets().get_dyn(id) else {
             return true;
         };
-        AXES.into_iter().all(|axis| {
-            widget
-                .size_hint(axis)
-                .is_none_or(|hint| hint == size.axis(axis))
-        })
+        Axis::BOTH
+            .into_iter()
+            .all(|axis| widget.size_hint(axis).is_none_or(|hint| hint == size[axis]))
     }
 
     /// Takes a widget's record out and frees what it drew.
@@ -838,7 +813,6 @@ impl UiRenderState {
         }
         // After the descendants, whose slots name this one as their parent.
         self.drop_slot(id);
-        active.size_deps.clear();
         active.drawn = false;
         self.active.insert(id, active);
     }
@@ -882,9 +856,8 @@ impl UiRenderState {
                 primitives: Vec::new(),
                 mask_region: None,
                 children: Vec::new(),
-                size_deps: Vec::new(),
                 move_idx: info.parent_move,
-                declared: [None; 2],
+                declared: Declared::NONE,
                 own_align: rsc.widgets().alignment(id),
                 parent_move: info.parent_move,
                 mask: info.mask,
@@ -1074,7 +1047,7 @@ impl UiRenderState {
         // to draw -- with the mark left on, so the parent draws it rather
         // than keeping it. So is a widget the parent asked twice: its
         // layout rests on an answer this widget cannot give again alone.
-        let declared_changed = declared_lens(rsc.widgets(), id) != active.declared;
+        let declared_changed = rsc.widgets().declared_lens(id) != active.declared;
         let alignment_changed = rsc.widgets().alignment(id) != active.own_align;
         if let Some(parent) = active.parent
             && (declared_changed
@@ -1128,25 +1101,30 @@ impl UiRenderState {
             placed: active.asked,
             asked: active.asked,
             re_asked: false,
-            px: rel_base.to_px(self.output_size),
         };
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::LocalRedraws);
 
         let old = self.remove(id, false, rsc);
-        let answer = self.draw_inner(id, info, old, rsc);
+        let drawn = self.draw_inner(id, info, old, rsc);
         let active = self.active.get_mut(&id).unwrap();
+        // Against the box it was asked in, which is what both contracts are
+        // about. Where the answer put the drawing is shorter than that
+        // wherever the widget reported less than it was offered.
+        let (window, rel_base, region) = (self.output_size, active.rel_base, active.region);
         // A wider contract does not invalidate the guarantee the parent kept.
-        // Retain that guarantee so widening and narrowing back do not churn it.
-        if let Some((size, holds)) = was_answer
-            && answer.0 == size
-            && answer.1.covers(holds)
+        // Retain that guarantee so widening and narrowing back do not churn
+        // it -- but only where the narrower range still holds here: one this
+        // window is outside is refused by the parent's next ask, and refusing
+        // it throws away the drawing this one just made.
+        if let Some(was) = was_answer
+            && drawn.answer.size == was.size
+            && drawn.answer.holds.covers(was.holds)
+            && was.holds.contains(window, rel_base, region)
         {
             active.answer = was_answer;
         }
-        if active.holds.covers(was_holds)
-            && was_holds.contains(self.output_size, active.rel_base, active.placement)
-        {
+        if active.holds.covers(was_holds) && was_holds.contains(window, rel_base, region) {
             active.holds = was_holds;
         }
         if active.answer != was_answer || active.holds != was_holds {
@@ -1177,7 +1155,6 @@ impl UiRenderState {
             id,
             region,
             rel_base: active.rel_base,
-            window: self.output_size,
             depth: active.depth,
             move_idx: active.move_idx,
             mask: active.mask,
@@ -1185,33 +1162,37 @@ impl UiRenderState {
     }
 }
 
-/// Whether what a widget reports along `axis` is inside the box it drew in.
-/// Both are lengths of the window, so the comparison is in its pixels. A
-/// share is a length only to whoever divides one, so it is not a claim about
-/// this box and cannot exceed it.
-fn within_box(size: Size, region: UiRegion, window: PxVec2, axis: Axis) -> bool {
-    let len = size.axis(axis);
-    let window = window.axis(axis);
+impl Size {
+    /// Whether what a widget reports along `axis` is inside the box it drew
+    /// in. Both are lengths of the window, so the comparison is in its
+    /// pixels. A share is a length only to whoever divides one, so it is not
+    /// a claim about this box and cannot exceed it.
+    fn within_box(&self, region: UiRegion, window: PxVec2, axis: Axis) -> bool {
+        let len = self[axis];
+        let window = window[axis];
         len.leftover != Weight::ZERO
-        || Len::from_parts(len.rel, len.px).to_px(window) <= region.axis(axis).len().to_px(window)
+            || len.without_leftover().to_px(window) <= region[axis].len().to_px(window)
+    }
 }
 
-/// A box in a fresh region node keeps its window-unit length and starts at
-/// that node's origin.
-fn local_region(region: UiRegion) -> UiRegion {
-    let size = region.size();
+impl UiRegion {
+    /// A box in a fresh region node keeps its window-unit length and starts
+    /// at that node's origin.
+    fn at_origin(self) -> UiRegion {
+        let size = self.size();
         UiRegion::new(
             UiSpan::new(Len::ZERO, size.x),
             UiSpan::new(Len::ZERO, size.y),
         )
-}
+    }
 
-/// A region node changes only the origin. A full relative span anchored at
-/// the box start composes as that translation in both the CPU and shader.
-fn translation(region: UiRegion) -> UiRegion {
+    /// A region node changes only the origin. A full relative span anchored at
+    /// the box start composes as that translation in both the CPU and shader.
+    fn as_translation(self) -> UiRegion {
         UiRegion {
-        x: UiSpan::new(region.x.start, region.x.start + Len::FULL),
-        y: UiSpan::new(region.y.start, region.y.start + Len::FULL),
+            x: UiSpan::new(self.x.start, self.x.start + Len::FULL),
+            y: UiSpan::new(self.y.start, self.y.start + Len::FULL),
+        }
     }
 }
 

core/src/util/arena.rs

@@ -35,7 +35,7 @@ impl<T, I: IdNum> Arena<T, I> {
         self.data[i]
     }
 
-    pub(crate) fn get_mut(&mut self, id: Id<I>) -> &mut T {
+    pub fn get_mut(&mut self, id: Id<I>) -> &mut T {
         &mut self.data[id.idx()]
     }
 }

core/src/util/math.rs

@@ -93,3 +93,31 @@ macro_rules! impl_op {
 }
 
 pub(crate) use impl_op;
+
+/// `Index<Axis>` for a pair, which is how every pair here is read by axis.
+/// The generics clause is given in braces where the type has one.
+macro_rules! impl_axis_index {
+    ($({$($gen:tt)*})? $T:ty => $Out:ty) => {
+        const impl $(<$($gen)*>)? std::ops::Index<crate::Axis> for $T {
+            type Output = $Out;
+
+            fn index(&self, axis: crate::Axis) -> &$Out {
+                match axis {
+                    crate::Axis::X => &self.x,
+                    crate::Axis::Y => &self.y,
+                }
+            }
+        }
+
+        const impl $(<$($gen)*>)? std::ops::IndexMut<crate::Axis> for $T {
+            fn index_mut(&mut self, axis: crate::Axis) -> &mut $Out {
+                match axis {
+                    crate::Axis::X => &mut self.x,
+                    crate::Axis::Y => &mut self.y,
+                }
+            }
+        }
+    };
+}
+
+pub(crate) use impl_axis_index;

core/src/widget/size_rule.rs

@@ -1,4 +1,5 @@
-use crate::{Axis, LayoutLen, Weight};
+use crate::util::impl_axis_index;
+use crate::{Axis, LayoutLen, Len};
 
 /// What a widget's length on one axis is, as a rule its parent applies where
 /// it draws it rather than an answer the widget gives about itself.
@@ -19,14 +20,9 @@ pub enum SizeRule {
 
 impl SizeRule {
     /// The length this rule gives without the widget being drawn, if it can
-    /// give one. `leftover` is never among them: a share is a length only to
-    /// whoever divides one, so it passes up in the reported size instead and
-    /// is resolved there.
-    pub fn declared(&self) -> Option<LayoutLen> {
-        match self {
-            Self::Exact(len) if len.leftover == Weight::ZERO => Some(*len),
-            _ => None,
-        }
+    /// give one.
+    pub fn declared(&self) -> Option<Len> {
+        self.exact().and_then(|len| len.declared())
     }
 
     /// The length this rule gives outright, whatever the widget reports --
@@ -40,14 +36,6 @@ impl SizeRule {
             Self::Exact(len) => Some(*len),
         }
     }
-
-    /// The length a widget reporting `reported` ends up with.
-    pub fn apply(&self, reported: LayoutLen) -> LayoutLen {
-        match self {
-            Self::Free => reported,
-            Self::Exact(len) => *len,
-        }
-    }
 }
 
 impl From<LayoutLen> for SizeRule {
@@ -70,18 +58,29 @@ pub struct SizeRules {
     pub y: SizeRule,
 }
 
-impl SizeRules {
-    pub fn axis(&self, axis: Axis) -> SizeRule {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
+impl_axis_index!(SizeRules => SizeRule);
 
-    pub fn axis_mut(&mut self, axis: Axis) -> &mut SizeRule {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
+/// What a widget's box is on each axis where something says so outright,
+/// before it is drawn: a rule beside it, or a hint it gives about itself.
+/// Whoever draws the widget resolves these against its rel base.
+///
+/// A [`Len`] rather than a [`LayoutLen`], because a share can never be one
+/// -- see [`LayoutLen::declared`].
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Declared {
+    pub x: Option<Len>,
+    pub y: Option<Len>,
+}
+
+impl Declared {
+    pub const NONE: Self = Self { x: None, y: None };
+
+    pub fn from_axes(f: impl Fn(Axis) -> Option<Len>) -> Self {
+        Self {
+            x: f(Axis::X),
+            y: f(Axis::Y),
         }
     }
 }
+
+impl_axis_index!(Declared => Option<Len>);

core/src/widget/widgets.rs

@@ -30,6 +30,14 @@ impl Widgets {
         !self.needs_redraw.is_empty()
     }
 
+    /// Marks this widget for the next frame to draw again, with nothing about
+    /// it changed. Taking a widget mutably marks it too, which is the ordinary
+    /// content-change signal; this is for a change the borrow cannot express,
+    /// and for asking for the same tree over again.
+    pub fn mark_for_redraw(&mut self, id: impl IdLike) {
+        self.needs_redraw.insert(id.id());
+    }
+
     pub fn get_dyn(&self, id: WidgetId) -> Option<&dyn Widget> {
         Some(self.vec.get(id)?.widget.as_ref())
     }
@@ -41,14 +49,14 @@ impl Widgets {
 
     /// get_dyn but dynamic borrow checking of widgets
     /// lets you do recursive (tree) operations, like the painter does
-    pub(crate) fn get_dyn_dynamic<'a>(&self, id: WidgetId) -> WidgetWrapper<'a> {
+    pub(crate) fn get_dyn_dynamic<'a>(&self, id: WidgetId) -> DynBorrower<'a, dyn Widget> {
         // SAFETY: must guarantee no other mutable references to this widget exist
         // done through the borrow variable
         let data = unsafe { forget_mut(to_mut(self.vec.get(id).unwrap())) };
         if data.borrowed {
             panic!("tried to mutably borrow the same widget twice");
         }
-        WidgetWrapper::new(data.widget.as_mut(), &mut data.borrowed)
+        DynBorrower::new(data.widget.as_mut(), &mut data.borrowed)
     }
 
     pub fn get<I: IdLike>(&self, id: &I) -> Option<&I::Widget>
@@ -130,10 +138,10 @@ impl Widgets {
     pub fn set_size_rule(&mut self, id: impl IdLike, axis: Axis, rule: SizeRule) {
         let id = id.id();
         let data = self.data_mut(id).unwrap();
-        if *data.size.axis_mut(axis) == rule {
+        if data.size[axis] == rule {
             return;
         }
-        *data.size.axis_mut(axis) = rule;
+        data.size[axis] = rule;
         self.needs_redraw.insert(id);
     }
 
@@ -147,14 +155,14 @@ impl Widgets {
     pub fn set_alignment(&mut self, id: impl IdLike, axis: Axis, align: AxisAlign) {
         let id = id.id();
         let data = self.data_mut(id).unwrap();
-        if *data.align.axis_mut(axis) == align {
+        if data.align[axis] == align {
             return;
         }
-        *data.align.axis_mut(axis) = align;
+        data.align[axis] = align;
         self.needs_redraw.insert(id);
     }
 
-    /// Both axes at once, for a caller holding a pair.
+    /// Both axes at once.
     pub fn set_size_rules(
         &mut self,
         id: impl IdLike,
@@ -188,8 +196,6 @@ impl Default for Widgets {
     }
 }
 
-pub type WidgetWrapper<'a> = DynBorrower<'a, dyn Widget>;
-
 impl<I: IdLike> std::ops::Index<I> for Widgets
 where
     I::Widget: Sized + Widget,

scripts/run-headless.sh

@@ -106,11 +106,16 @@ export WAYLAND_DISPLAY
 
 echo "run-headless: $WAYLAND_DISPLAY (sway $(swaymsg -t get_version --raw | sed -n 's/.*"human_readable":"\([^"]*\)".*/\1/p'))" >&2
 
-swaymsg output HEADLESS-1 mode "$mode" >/dev/null
-# The extent `replay-touch` positions against, so a script's coordinates
-# are the output's own pixels.
-out_w=${mode%x*}
-out_h=${mode#*x}; out_h=${out_h%@*}
+# The extent `replay-touch` positions against, so a script's coordinates are
+# the output's own pixels. Set beside every mode change, since a gesture
+# scaled against a mode the output no longer has lands somewhere else and
+# still looks like a run that worked.
+set_mode() {
+    swaymsg output HEADLESS-1 mode "$1" >/dev/null
+    out_w=${1%x*}
+    out_h=${1#*x}; out_h=${out_h%@*}
+}
+set_mode "$mode"
 
 # Built before the app starts, so a compile error is not reported as a
 # window that failed to move.
@@ -149,7 +154,7 @@ while [ $i -lt "$((seconds * 2))" ]; do
 done
 
 if [ -n "$resize" ] && kill -0 "$pid" 2>/dev/null; then
-    swaymsg output HEADLESS-1 mode "$resize" >/dev/null
+    set_mode "$resize"
     echo "run-headless: resized to $resize" >&2
     sleep 2
 fi

src/default/attr.rs

@@ -15,9 +15,10 @@ where
             let region = ctx.data.render.window_region(&id).unwrap();
             let id_pos = region.top_left;
             let container_pos = ctx.data.render.window_region(&container).unwrap().top_left;
-            // The pointer arrives from the platform in floats; everything
-            // it is compared against is on the grid.
-            let pos = (PxVec2::from_f32(ctx.data.pos) + container_pos - id_pos).to_f32();
+            // The two regions are on the grid and the pointer is not, so the
+            // step between them is taken there and the pointer keeps the
+            // precision the platform gave it.
+            let pos = ctx.data.pos + (container_pos - id_pos).to_f32();
             let size = region.size().to_f32();
             select(
                 rsc,

src/default/render.rs

@@ -22,12 +22,12 @@ impl UiRenderer {
     }
 
     pub fn draw(&mut self) {
-        let output = match self.surface.get_current_texture() {
-            CurrentSurfaceTexture::Success(texture) => texture,
-            CurrentSurfaceTexture::Suboptimal(texture) => {
-                self.surface.configure(&self.device, &self.config);
-                texture
-            }
+        let (output, suboptimal) = match self.surface.get_current_texture() {
+            CurrentSurfaceTexture::Success(texture) => (texture, false),
+            // Used for this frame, and the swapchain rebuilt after it has
+            // been presented: configuring the surface while a texture it
+            // handed out is still alive panics.
+            CurrentSurfaceTexture::Suboptimal(texture) => (texture, true),
             CurrentSurfaceTexture::Outdated | CurrentSurfaceTexture::Lost => {
                 self.surface.configure(&self.device, &self.config);
                 return;
@@ -60,6 +60,9 @@ impl UiRenderer {
         self.queue.submit(std::iter::once(encoder.finish()));
         self.window.pre_present_notify();
         self.queue.present(output);
+        if suboptimal {
+            self.surface.configure(&self.device, &self.config);
+        }
     }
 
     pub fn resize(&mut self, size: &PhysicalSize<u32>) {

src/random.rs

@@ -8,23 +8,16 @@
 use crate::prelude::*;
 use std::collections::HashMap;
 
-/// The declared lengths of one widget carrying a size rule, by axis.
-pub type Lens = [Option<LayoutLen>; 2];
-
-/// Where one widget carrying an alignment sits, by axis. `None` uses the
-/// centered default.
-pub type Aligns = [Option<AxisAlign>; 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 rules were put on.
-    pub sizes: HashMap<usize, Lens>,
+    pub sizes: HashMap<usize, SizeRules>,
     /// Which children a span has, by the order the spans were made.
     pub spans: HashMap<usize, SpanEdit>,
     /// Alignments, by the order they were put on.
-    pub aligns: HashMap<usize, Aligns>,
+    pub aligns: HashMap<usize, Align>,
     /// Which widgets own a movable region, by the order they were offered
     /// one. Region nodes change what a move writes and how deep a primitive's
     /// chain is, so a tree that never grows one leaves both untested.
@@ -118,9 +111,9 @@ pub struct Branch {
 impl Widget for Branch {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
-        let top = PlaceDescAxis::shifted(UiSpan::new(Len::ZERO, cut));
+        let top = UiSpan::new(Len::ZERO, cut).shifted_desc();
         let measured = painter
-            .widget_at(&self.probe, PlaceDesc::new(PlaceDescAxis::WHOLE, top))
+            .widget_at(&self.probe, top.on_axis(Axis::Y))
             .len(Axis::X);
         let len = measured.apply_leftover();
         let px = painter.to_px(len, Axis::X);
@@ -134,8 +127,8 @@ impl Widget for Branch {
         };
         painter.window_holds(Axis::X, holds.through(len));
 
-        let below = PlaceDescAxis::shifted(UiSpan::new(cut, painter.region_len(Axis::Y)));
-        let place = PlaceDesc::new(PlaceDescAxis::WHOLE, below);
+        let below = UiSpan::new(cut, painter.region_len(Axis::Y)).shifted_desc();
+        let place = below.on_axis(Axis::Y);
         match px > threshold {
             true => painter.widget_at(&self.wide, place),
             false => painter.widget_at(&self.narrow, place),
@@ -176,9 +169,11 @@ pub struct Plan {
     /// it one and the offer is taken or declined; a second offer to the same
     /// widget is dropped, because two rules on one widget would settle in the
     /// order they were applied rather than in grow order.
-    pub size: Option<Lens>,
-    /// The alignment it carries, under the same one-offer rule.
-    pub align: Option<Aligns>,
+    pub size: Option<SizeRules>,
+    /// The alignment it carries, under the same one-offer rule. An axis left
+    /// out takes the centered default, which is what [`RegionAlign`] reads it
+    /// as.
+    pub align: Option<Align>,
     /// Whether it was offered a movable region of its own and what it
     /// answered. `Some(false)` is an offer declined, which still uses up the
     /// one offer, where `None` is an offer never made.
@@ -195,6 +190,9 @@ pub enum Kind {
         color: usize,
         alpha: u8,
     },
+    /// The one leaf whose own length is a number of pixels it knows before it
+    /// is drawn, which is the hint a rule beside it has to win over.
+    Image,
     /// Scrolling reads the pixel length of its box, which nothing else here
     /// does, and gives its child a box longer than its own.
     Scroll {
@@ -448,9 +446,11 @@ impl Kind {
         }
         match self {
             // The one leaf that reads the width it is given, then the one
-            // that does not, then the one that measures nothing at all.
+            // that does not, then the one that measures nothing at all. A
+            // picture measures nothing either, but its length is its own, so
+            // it steps to the leaf that takes whatever it is given.
             Kind::Wrapped => out.push(Kind::OneLine),
-            Kind::OneLine => out.push(Kind::Rect {
+            Kind::OneLine | Kind::Image => out.push(Kind::Rect {
                 color: 0,
                 alpha: 255,
             }),
@@ -632,9 +632,10 @@ struct Sow<'a> {
 
 impl Sow<'_> {
     fn leaf(&mut self) -> Plan {
-        Plan::bare(match self.rng.below(4) {
+        Plan::bare(match self.rng.below(5) {
             0 => Kind::Wrapped,
             1 => Kind::OneLine,
+            2 => Kind::Image,
             _ => {
                 let color = self.rng.below(COLORS.len());
                 let alpha = (self.rng.below(5) * 63) as u8;
@@ -651,7 +652,7 @@ impl Sow<'_> {
         }
     }
 
-    fn align(&mut self) -> Aligns {
+    fn align(&mut self) -> Align {
         let axis = |s: &mut Self| match s.rng.below(4) {
             0 => None,
             1 => Some(AxisAlign::NEG),
@@ -661,15 +662,21 @@ impl Sow<'_> {
         let (x, y) = (axis(self), axis(self));
         // Aligning on neither axis leaves the branch unexercised.
         match x.is_none() && y.is_none() {
-            true => [Some(AxisAlign::CENTER), y],
-            false => [x, y],
+            true => Align {
+                x: Some(AxisAlign::CENTER),
+                y,
+            },
+            false => Align { x, y },
         }
     }
 
     /// A declared size over half the tree, kept where a test can change it.
     fn sized(&mut self, inner: &mut Plan) {
         let take = self.rng.chance();
-        let lens = [self.len(), self.len()];
+        let lens = SizeRules {
+            x: self.len().into(),
+            y: self.len().into(),
+        };
         if !take || inner.size.is_some() {
             return;
         }
@@ -773,10 +780,6 @@ impl Sow<'_> {
         self.spans += 1;
         let edit = self.edits.spans.get(&idx).cloned().unwrap_or_default();
         let dir = self.rng.below(4);
-        // A row takes the height it is given rather than its tallest child,
-        // which is a rule beside it. Derived from an existing choice and
-        // consuming no randomness: a seed must keep growing the same tree
-        // when the generator gains another configuration.
         let gap = self.rng.below(3) as i32 * 4;
         let grown: Vec<usize> = (0..children.len()).collect();
         let order = span_edited(&grown, children.len(), spares.len(), &edit);
@@ -796,6 +799,7 @@ pub fn build<Rsc: UiRsc + 'static>(rsc: &mut Rsc, plan: &Plan) -> (StrongWidget,
     let mut build = Build {
         rsc,
         tree: Tree::default(),
+        checkerboard: None,
     };
     let root = build.node(plan);
     (root, build.tree)
@@ -804,6 +808,10 @@ pub fn build<Rsc: UiRsc + 'static>(rsc: &mut Rsc, plan: &Plan) -> (StrongWidget,
 struct Build<'a, Rsc> {
     rsc: &'a mut Rsc,
     tree: Tree,
+    /// The checkerboard, uploaded when the first image in this tree is built.
+    /// A handle is a reference to the texture, so every image after that one
+    /// clones this rather than uploading the same picture again.
+    checkerboard: Option<TextureHandle>,
 }
 
 impl<Rsc: UiRsc + 'static> Build<'_, Rsc> {
@@ -811,16 +819,14 @@ impl<Rsc: UiRsc + 'static> Build<'_, Rsc> {
         let built = self.kind(&plan.kind);
         let id = built.id();
         if let Some(lens) = plan.size {
-            self.rsc
-                .ui_mut()
-                .widgets
-                .set_size_rules(id, lens[0], lens[1]);
+            self.rsc.ui_mut().widgets.set_size_rules(id, lens.x, lens.y);
             self.tree.sized.push(id);
         }
         if let Some(align) = plan.align {
+            let resolved = RegionAlign::from(align);
             let widgets = &mut self.rsc.ui_mut().widgets;
-            for (axis, align) in [Axis::X, Axis::Y].into_iter().zip(align) {
-                widgets.set_alignment(id, axis, align.unwrap_or_default());
+            for axis in Axis::BOTH {
+                widgets.set_alignment(id, axis, resolved[axis]);
             }
             self.tree.aligned.push(id);
         }
@@ -831,6 +837,20 @@ impl<Rsc: UiRsc + 'static> Build<'_, Rsc> {
         built
     }
 
+    /// The one picture the generated trees draw: a 64x64 checkerboard of purple
+    /// and black in 8 px cells. Committed rather than drawn here, so that one
+    /// seed is one tree whatever anything else does, and included rather than
+    /// opened, so that growing a tree does not depend on a working directory.
+    fn checkerboard(&mut self) -> TextureHandle {
+        if self.checkerboard.is_none() {
+            let image = include_bytes!("assets/checkerboard.png")
+                .get_image()
+                .expect("the checkerboard is committed beside this file");
+            self.checkerboard = Some(self.rsc.ui_mut().textures.add(image));
+        }
+        self.checkerboard.clone().unwrap()
+    }
+
     fn kind(&mut self, kind: &Kind) -> StrongWidget {
         let id: StrongWidget = match kind {
             Kind::Wrapped => wtext(WORDS).size(16).wrap(true).add_strong(self.rsc),
@@ -839,6 +859,7 @@ impl<Rsc: UiRsc + 'static> Build<'_, Rsc> {
                 .wrap(false)
                 .add_strong(self.rsc),
             Kind::Rect { color, alpha } => rect(COLORS[*color].alpha(*alpha)).add_strong(self.rsc),
+            Kind::Image => Image::new(self.checkerboard()).add_strong(self.rsc),
             Kind::Scroll { axis, inner } => {
                 let inner = self.node(inner);
                 let id = Scroll::new(inner, *axis).add(self.rsc);
@@ -915,6 +936,10 @@ impl<Rsc: UiRsc + 'static> Build<'_, Rsc> {
                     gap: Px::from_int(*gap),
                 }
                 .add(self.rsc);
+                // A row takes the height it is given rather than its tallest
+                // child, which is a rule beside the span rather than anything
+                // it draws. Derived from `dir` rather than stored, so a plan
+                // that says the direction says this too.
                 if dir.axis == Axis::X {
                     self.rsc
                         .widgets_mut()

src/widget/image.rs

@@ -12,7 +12,16 @@ impl Widget for Image {
     }
 
     fn size_hint(&self, axis: Axis) -> Option<LayoutLen> {
-        Some(LayoutLen::px(self.handle.size().axis(axis)))
+        Some(LayoutLen::px(self.handle.size()[axis]))
+    }
+}
+
+impl Image {
+    /// One texture already uploaded, for a caller holding its handle: [`image`]
+    /// uploads what it is given, and several widgets showing one picture want
+    /// one upload and one slot between them.
+    pub fn new(handle: TextureHandle) -> Self {
+        Self { handle }
     }
 }
 

src/widget/position/scroll.rs

@@ -16,45 +16,46 @@ impl Widget for Scroll {
         let answer_len = painter
             .widget_at(&self.inner, PlaceDesc::WHOLE.fills())
             .len(self.axis);
-        let fixed = painter.to_px(Len::from_parts(answer_len.rel, answer_len.px), self.axis);
+        let answer_px = painter.to_px(answer_len.without_leftover(), self.axis);
         self.container_len = container_len;
-        self.content_len = fixed.max(container_len);
+        self.content_len = answer_px.max(container_len);
 
         if self.snap_end {
             self.amt = self.content_len - self.container_len;
         }
         self.update_amt();
-        let align = painter.alignment().axis(self.axis);
-        // Content of a fixed length that fits sits at the start of any box it
-        // fits in -- but only anchored there. Anywhere else it is a part of
-        // the room left over, so it moves with every length the box takes and
-        // the drawing holds for that length alone. One scrolled part way sits
-        // where it is until the box shrinks past what is left of it. Kept to
-        // the end, it moves with every length.
-        let fixed_len = answer_len.rel == Rel::ZERO && answer_len.leftover == Weight::ZERO;
-        if fixed_len && self.content_len <= self.container_len && align == AxisAlign::NEG {
-            painter.holds(self.axis, fixed..=Px::MAX);
-        } else if fixed_len && !self.snap_end {
+        // Reading the box in pixels above holds this drawing to that one
+        // length, so these two say where it holds more widely.
+        //
+        // Content of a fixed length that fits is handed the whole box below,
+        // and nothing here reads the box again, so every longer box gives the
+        // same drawing: it holds from the length the content needs upwards,
+        // and shrinking past that is what changes it. Where it sits in a box
+        // longer than itself is not this widget's to say -- placing its
+        // answer in the whole box is its own alignment, and that placement is
+        // a fraction of the box, so it holds at every length too.
+        //
+        // One scrolled part way sits where it is until the box shrinks past
+        // what is left of it. Kept to the end, it moves with every length.
+        let answer_is_px = answer_len.is_px();
+        if answer_is_px && self.content_len <= self.container_len {
+            painter.holds(self.axis, answer_px..=Px::MAX);
+        } else if answer_is_px && !self.snap_end {
             let left = self.content_len - self.amt;
             painter.holds(self.axis, Px::MIN..=left);
         }
 
-        // Content shorter than the viewport has room to sit in, and where it
-        // sits is this widget's own alignment -- the same property that would
-        // have placed the whole scroll in a box longer than it.
-        let slack = (self.container_len - self.content_len).max(Px::ZERO);
-        let anchor = slack.mul(align.rel());
-        // Content that fills the viewport and has not been scrolled is the
-        // viewport, and is handed back as it came. Writing the same box as
-        // its own length in pixels is the same box in another form, and the
-        // two do not round alike: a part centred in `rel 1` lands a step from
-        // one centred in `px 900`, since halving a difference is not halving
-        // each part of it.
-        let moved = anchor != Px::ZERO || self.amt != Px::ZERO;
-        let content = match moved || self.content_len != self.container_len {
+        // Content that fills the viewport is the viewport, and is handed back
+        // as it came -- it has nothing to scroll through, so the clamp above
+        // has already put `amt` at zero. Writing the same box as its own
+        // length in pixels is the same box in another form, and the two do
+        // not round alike: a part centred in `rel 1` lands a step from one
+        // centred in `px 900`, since halving a difference is not halving each
+        // part of it.
+        let content = match self.content_len > self.container_len {
             true => {
-                let start = Len::from_parts(Rel::ZERO, anchor - self.amt);
-                PlaceDescAxis::shifted(UiSpan::new(start, start.offset(self.content_len)))
+                let start = Len::from_parts(Rel::ZERO, -self.amt);
+                UiSpan::new(start, start.offset(self.content_len)).shifted_desc()
             }
             false => PlaceDescAxis::WHOLE,
         };
@@ -62,10 +63,7 @@ impl Widget for Scroll {
         // reports is a fraction of what is on screen rather than of the
         // content box its own answer decided. Where it goes is the content
         // box, scrolled: its drawing moved there, not made again there.
-        painter.place_at(
-            &self.inner,
-            PlaceDesc::from_axis(self.axis, content.fills(), PlaceDescAxis::WHOLE.fills()),
-        );
+        painter.place_at(&self.inner, content.on_axis(self.axis).fills());
         // What it occupies is its box, on both axes: it clips its content to
         // that box, so it can neither take less of one nor honestly ask for
         // more. The content's length is what it scrolls through, not what it

src/widget/position/span.rs

@@ -10,38 +10,32 @@ pub struct Span {
 impl Widget for Span {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let axis = self.dir.axis;
-        // The row: this span's own box, as a length of the rel base its children
-        // are laid out against. Its start is nothing's business -- a slot is
-        // a length from it -- so what this reads is the length alone.
-        let far = painter.region_len(axis);
-        let along = |from: Len, to: Len| match self.dir.sign {
-            Sign::Pos => UiSpan::new(from, to),
-            Sign::Neg => UiSpan::new(far - to, far - from),
-        };
-        // Across itself the child sits where its own alignment says, in the
-        // whole of the row: a span is what contains its children there, and
-        // nothing divides that axis.
-        let across = PlaceDescAxis::WHOLE;
+        // The row this span lays its children out along, as a length of the
+        // rel base they are laid out against. Where it starts is nothing's
+        // business -- a slot is a length from there -- so what this reads is
+        // the length alone.
+        let row = painter.region_len(axis);
         // A length for every child before their final slots are chosen: from
         // a hint where one says, and from drawing otherwise. The rel base passes
         // through unchanged, so `rel(0.5)` is half the area this span was
         // given whatever else is in it and wherever this child sits among
         // them; what a drawn child is asked in is the room left from the
         // cursor, because a text has to wrap at the width actually there.
-        let mut cursor = Len::rel_min();
+        let mut cursor = Len::ZERO;
         let mut lens = Vec::with_capacity(self.children.len());
         for child in &self.children {
             let len = match painter.size_hint(child, axis) {
                 Some(len) => len,
                 None => {
-                    let room = PlaceDescAxis::shifted(along(cursor, far));
-                    painter
-                        .widget_at(child, PlaceDesc::from_axis(axis, room, across))
-                        .len(axis)
+                    // Across itself the child sits where its own alignment
+                    // says, in the whole of the row: a span is what contains
+                    // its children there, and nothing divides that axis.
+                    let room = self.slot(row, cursor, row).shifted_desc().on_axis(axis);
+                    painter.widget_at(child, room).len(axis)
                 }
             };
-            cursor.px += len.px + self.gap;
-            cursor.rel += len.rel;
+            cursor += len.without_leftover();
+            cursor.px += self.gap;
             lens.push(len);
         }
 
@@ -58,25 +52,15 @@ impl Widget for Span {
 
         // What is left for the shares to divide: the row less everything
         // fixed, as a length of the rel base rather than a number of pixels.
-        let room = far - Len::from_parts(total.rel, total.px);
-        // Whether anything is left over is a question in pixels: `rel(0.5)`
-        // beside 300 px is full at 600 and overfull at 400. Asked of `room`
-        // itself, and answered back through the same expression, so the
-        // boundary is the drawing's own and not a second way of finding it:
-        // the three cases a rounded division needed -- the fixed parts
+        let all_fixed = total.without_leftover();
+        let room = row - all_fixed;
+        // The three cases a rounded division needed -- the fixed parts
         // growing slower than the box, faster, or exactly with it -- are the
-        // sign of `room.rel`, which `through` already reads. What the
-        // generated oracle checks is the consequence, since which children
-        // exist at all turns on this.
-        let mut shares = false;
-        if total.leftover > Weight::ZERO {
-            shares = painter.to_px(room, axis) > Px::ZERO;
-            let holds = match shares {
-                true => Holds::from(Px::STEP..=Px::MAX),
-                false => Holds::from(Px::MIN..=Px::ZERO),
-            };
-            painter.window_holds(axis, holds.through(room));
-        }
+        // sign of `room.rel`, which the range `longer_than` keeps already
+        // reads. What the generated oracle checks is the consequence, since
+        // which children exist at all turns on this.
+        let any_leftover = total.leftover > Weight::ZERO;
+        let has_room = any_leftover && painter.longer_than(row, all_fixed, axis);
 
         // Across itself a span is as long as its longest child -- unless a
         // rule beside it gives that length outright, and then reading them
@@ -86,32 +70,34 @@ impl Widget for Span {
         let shrinks = !painter.has_exact_size(!axis);
         // What the fixed parts and the gaps before here take, which is a sum
         // of lengths and exact, and how much of the leftover weight is
-        // spoken for. A position is one from the other rather than a step
-        // from the last child: the share of the room is rounded, and taking
-        // each from the one before it would carry every rounding along the
-        // row.
-        let mut fixed = Len::rel_min();
+        // spoken for. Both ends of a slot are read from those two rather
+        // than stepped from the last child: the share of the room is
+        // rounded, and taking each end from the one before it would carry
+        // every rounding along the row.
+        let mut fixed = Len::ZERO;
         let mut taken = Weight::ZERO;
-        let mut start = Len::rel_min();
         let mut ortho = LayoutLen::ZERO;
+        // Nothing divides the room where no child asked for any of it, and a
+        // ratio of a whole of nothing has no answer.
+        let reached = |fixed: Len, taken: Weight| match any_leftover {
+            false => fixed,
+            true => fixed + room.scale(Rel::ratio(taken, total.leftover)),
+        };
         for (child, &len) in self.children.iter().zip(&lens) {
             // A child asking for nothing but a part of what is left over,
             // when nothing is, is not drawn at all. One that also asked for
             // pixels or a fraction keeps those and overflows.
-            if len.leftover > Weight::ZERO && len.px == Px::ZERO && len.rel == Rel::ZERO && !shares
-            {
+            if len.is_only_leftover() && !has_room {
                 painter.undraw(child);
                 fixed.px += self.gap;
-                start = shared(fixed, taken, total.leftover, room);
                 continue;
             }
-            let from = start;
-            if len.leftover > Weight::ZERO && shares {
+            let from = reached(fixed, taken);
+            if len.leftover > Weight::ZERO && has_room {
                 taken += len.leftover;
             }
-            fixed.px += len.px;
-            fixed.rel += len.rel;
-            start = shared(fixed, taken, total.leftover, room);
+            fixed += len.without_leftover();
+            let to = reached(fixed, taken);
             // Along the row the span says where the child goes, and that slot
             // is the child's box outright rather than something to place an
             // answer inside again. A share is decided here and nowhere
@@ -119,26 +105,24 @@ impl Widget for Span {
             // it, since a text wraps at the width it is actually given. A
             // fixed child's slot is its own answer, so a drawing made in the
             // room is put there as it is, and one not made yet is made here.
-            let slot = along(from, start);
-            let slot_place = PlaceDescAxis::shifted(slot).fills();
-            let mut place = PlaceDesc::from_axis(axis, slot_place, across);
-            if len.leftover > Weight::ZERO && shares {
+            let slot = self.slot(row, from, to);
+            let mut place = slot.shifted_desc().fills().on_axis(axis);
+            if len.leftover > Weight::ZERO && has_room {
                 place = place.rel_base(axis, slot.len());
             }
             let used = painter.place_at(child, place).len(!axis);
             if shrinks {
                 // Choosing between a fixed and a relative length from the
                 // span's own eventual width admits multiple fixed points.
-                // A scalable child therefore makes Children scalable too;
+                // A scalable child therefore makes the span scalable too;
                 // only fixed children are compared with one another.
-                if used.rel != Rel::ZERO || used.leftover != Weight::ZERO {
+                if !used.is_px() {
                     ortho = LayoutLen::LEFTOVER;
                 } else if ortho.leftover == Weight::ZERO {
                     ortho.px = ortho.px.max(used.px);
                 }
             }
             fixed.px += self.gap;
-            start = shared(fixed, taken, total.leftover, room);
         }
 
         // Carried whole rather than collapsed to one share: a span that sizes
@@ -148,26 +132,26 @@ impl Widget for Span {
         // get a quarter each, which collapsing to `leftover(1)` per level does
         // not give. Resolution happens at the nearest ancestor with a length,
         // and the root always has one.
-        let along = total;
         let ortho = match shrinks {
             true => ortho,
             false => LayoutLen::rel(1.0),
         };
-        Size::from_axis(axis, along, ortho)
+        Size::from_axis(axis, total, ortho)
     }
 }
 
-/// Where a row has reached: everything fixed before this point, which is a
-/// sum and exact, plus the share of the room the weights so far are worth,
-/// which is one rounding wherever it is asked for.
-fn shared(fixed: Len, taken: Weight, weight: Weight, room: Len) -> Len {
-    if taken == Weight::ZERO {
-        return fixed;
-    }
-    fixed + room.scale(Rel::ratio(taken, weight))
-}
-
 impl Span {
+    /// The stretch of the row between two distances from where this span
+    /// starts laying children out, as a span of its own box. A negative
+    /// direction lays out from the far end, so the same two distances mirror
+    /// in a row `row` long.
+    fn slot(&self, row: Len, from: Len, to: Len) -> UiSpan {
+        match self.dir.sign {
+            Sign::Pos => from.to(to),
+            Sign::Neg => (row - to).to(row - from),
+        }
+    }
+
     pub fn empty(dir: Dir) -> Self {
         Self {
             children: Vec::new(),

src/widget/position/stack.rs

@@ -31,14 +31,13 @@ impl Widget for Stack {
         // fraction under them is a fraction of it. A share leaves the axis
         // to whoever gave the stack its box. Where a child sits in a box
         // bigger than itself is its own business.
-        let on = |axis| {
-            let len = size.axis(axis);
+        let place = PlaceDesc::from_axes(|axis| {
+            let len = size[axis];
             match len.leftover == Weight::ZERO {
-                true => PlaceDescAxis::sized(Len::from_parts(len.rel, len.px)).fills(),
+                true => len.without_leftover().as_desc().fills(),
                 false => PlaceDescAxis::WHOLE,
             }
-        };
-        let place = PlaceDesc::new(on(Axis::X), on(Axis::Y));
+        });
         for (i, child) in self.children.iter().enumerate() {
             if sizing == Some(i) {
                 continue;

src/widget/text/edit.rs

@@ -321,12 +321,10 @@ impl<'a> TextEditCtx<'a> {
         let old = (self.text.view.buf.text().to_string(), self.text.selection);
         let mut undo = false;
         let res = self.apply_event_inner(event, modifiers, &mut undo);
-        if undo {
-            if let Some((old, selection)) = self.text.history.pop() {
+        if undo && let Some((old, selection)) = self.text.history.pop() {
             self.set(&old);
             self.text.selection = selection;
             self.clamp_selection_to_layout();
-            }
         } else if self.text.view.buf.text() != old.0 {
             self.text.history.push(old);
         }

src/widget/trait_fns.rs

@@ -19,8 +19,8 @@ widget_trait! {
         move |state| {
             let id = self.add(state);
             let widgets = &mut state.ui_mut().widgets;
-            for (axis, align) in [(Axis::X, align.x), (Axis::Y, align.y)] {
-                if let Some(align) = align {
+            for axis in Axis::BOTH {
+                if let Some(align) = align[axis] {
                     widgets.set_alignment(id, axis, align);
                 }
             }

src/widget/wrapper.rs

@@ -9,6 +9,7 @@ use std::marker::Unsize;
 ///
 /// Its child is optional so it can also be the swappable slot a tab bar
 /// needs, which is what it was written for.
+#[derive(Default)]
 pub struct Wrapper {
     pub inner: Option<StrongWidget>,
 }
@@ -26,11 +27,7 @@ impl Wrapper {
     pub fn new() -> Self {
         Self::default()
     }
-    pub fn empty() -> Self {
-        Self {
-            inner: Default::default(),
-        }
-    }
+
     pub fn set<W: ?Sized + Unsize<dyn Widget>>(&mut self, to: StrongWidget<W>) {
         self.inner = Some(to)
     }
@@ -42,9 +39,3 @@ impl Wrapper {
         self.inner.replace(to)
     }
 }
-
-impl Default for Wrapper {
-    fn default() -> Self {
-        Self::empty()
-    }
-}

tests/cases/determinism.rs

@@ -22,14 +22,14 @@ struct BranchesOnMeasurement {
 impl Widget for BranchesOnMeasurement {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
-        let top = PlaceDescAxis::shifted(UiSpan::new(Len::ZERO, cut));
+        let top = UiSpan::new(Len::ZERO, cut).shifted_desc();
         let measured = painter
-            .widget_at(&self.probe, PlaceDesc::new(PlaceDescAxis::WHOLE, top))
+            .widget_at(&self.probe, top.on_axis(Axis::Y))
             .len(Axis::X);
         let px = painter.to_px(measured.apply_leftover(), Axis::X);
 
-        let below = PlaceDescAxis::shifted(UiSpan::new(cut, painter.region_len(Axis::Y)));
-        let place = PlaceDesc::new(PlaceDescAxis::WHOLE, below);
+        let below = UiSpan::new(cut, painter.region_len(Axis::Y)).shifted_desc();
+        let place = below.on_axis(Axis::Y);
         match px > Px::from_f32(self.threshold) {
             true => painter.widget_at(&self.wide, place),
             false => painter.widget_at(&self.narrow, place),
@@ -69,8 +69,8 @@ fn a_branch_taken_on_a_measurement_holds_across_repaints() {
         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.rsc.widgets_mut().mark_for_redraw(wide);
+            h.rsc.widgets_mut().mark_for_redraw(narrow);
             h.frame();
             assert_eq!(
                 taken(&h, wide, narrow),
@@ -88,7 +88,7 @@ fn a_branch_taken_on_a_measurement_is_the_one_a_cold_start_takes() {
         let (wide, narrow) = plant(&mut warm, threshold);
         warm.resize((640, 480));
         warm.frame();
-        warm.rsc.widgets_mut().get_dyn_mut(wide);
+        warm.rsc.widgets_mut().mark_for_redraw(wide);
         warm.frame();
 
         let mut cold = Harness::new((640, 480));

tests/cases/idempotence.rs

@@ -18,7 +18,7 @@ fn a_wrapping_text_in_a_span_settles_on_one_width() {
         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.rsc.widgets_mut().mark_for_redraw(t.id());
         h.frame();
     }
     println!("widths over six frames: {widths:?}");

tests/cases/layout.rs

@@ -1,5 +1,7 @@
 //! Where a frame puts things, with no window to put them in.
 
+use std::{cell::Cell, rc::Rc};
+
 use iris::harness::{Harness, assert_corners};
 use iris::prelude::*;
 
@@ -219,6 +221,105 @@ fn an_empty_widget_takes_a_share_of_a_span() {
     assert_corners!(h, right, (300, 0), (400, 200));
 }
 
+/// A widget with a natural pixel size, like an image, which records the box
+/// it was asked in so a test can see which length decided it.
+struct NaturalSize {
+    len: f32,
+    asked: Rc<Cell<f32>>,
+}
+
+impl Widget for NaturalSize {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.asked.set(painter.px_len(Axis::X).to_f32());
+        Size::px(Vec2::new(self.len, self.len))
+    }
+
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::px(self.len))
+    }
+}
+
+/// A rule wins over what the widget says about itself, and a share is a rule:
+/// it is a length only to whoever divides one, and nobody here does, so the
+/// widget is asked in the whole box rather than in the size it asked for.
+#[test]
+fn a_share_rule_beats_the_widgets_own_pixel_size() {
+    let mut h = Harness::new((400, 200));
+    let asked = Rc::new(Cell::new(0.0));
+    let natural = NaturalSize {
+        len: 50.0,
+        asked: asked.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root(natural.wrapper());
+    assert_eq!(asked.get(), 50.0, "its hint gives it its own size");
+
+    h.set_len(natural, Axis::X, LayoutLen::LEFTOVER);
+    h.frame();
+
+    assert_eq!(asked.get(), 400.0, "the share is all of the box");
+}
+
+/// A share with pixels or a fraction beside it is the longer of the two: it
+/// fills what they leave of the box and overflows the box where they are
+/// longer than it. A parent that divides nothing gives the same length as a
+/// span with one child, because in both there is nobody else to divide with.
+#[test]
+fn a_share_is_a_minimum_wherever_nothing_divides_it() {
+    let asked = |rule: LayoutLen, in_a_span: bool| {
+        let mut h = Harness::new((400, 200));
+        let probe = rect(Color::RED).add(&mut h.rsc);
+        h.set_len(probe, Axis::X, rule);
+        match in_a_span {
+            true => h.set_root((probe,).span(Dir::RIGHT)),
+            false => h.set_root(probe.wrapper()),
+        }
+        h.region(&probe).unwrap().size().x
+    };
+    for (rule, want) in [
+        (LayoutLen::LEFTOVER, 400),
+        (LayoutLen::px(50) + LayoutLen::LEFTOVER, 400),
+        (LayoutLen::px(500) + LayoutLen::LEFTOVER, 500),
+        (LayoutLen::rel(0.5) + LayoutLen::LEFTOVER, 400),
+        (LayoutLen::px(500), 500),
+    ] {
+        let want = Px::from_int(want);
+        assert_eq!(asked(rule, false), want, "{rule:?} where nothing divides");
+        assert_eq!(asked(rule, true), want, "{rule:?} in a span");
+    }
+}
+
+/// Which of the two is longer is a question in pixels, so the box is decided
+/// again wherever the answer can change: a window that crosses the length the
+/// pixels ask for, and the rule itself crossing it while the window holds
+/// still. The first is a range the drawing holds for; the second cannot be
+/// seen in what the widget declares, since a share declares nothing either
+/// way, so it reaches the parent as a length only the parent can resolve.
+#[test]
+fn a_share_past_the_box_is_decided_again_on_either_side_of_the_crossing() {
+    let mut h = Harness::new((400, 200));
+    let probe = rect(Color::RED).add(&mut h.rsc);
+    h.set_len(probe, Axis::X, LayoutLen::px(500) + LayoutLen::LEFTOVER);
+    h.set_root(probe.wrapper());
+    assert_eq!(h.region(&probe).unwrap().size().x, Px::from_int(500));
+
+    h.resize((900, 200));
+    h.frame();
+    assert_eq!(h.region(&probe).unwrap().size().x, Px::from_int(900));
+
+    h.resize((400, 200));
+    h.frame();
+    assert_eq!(h.region(&probe).unwrap().size().x, Px::from_int(500));
+
+    h.set_len(probe, Axis::X, LayoutLen::px(50) + LayoutLen::LEFTOVER);
+    h.frame();
+    assert_eq!(h.region(&probe).unwrap().size().x, Px::from_int(400));
+
+    h.set_len(probe, Axis::X, LayoutLen::px(500) + LayoutLen::LEFTOVER);
+    h.frame();
+    assert_eq!(h.region(&probe).unwrap().size().x, Px::from_int(500));
+}
+
 #[test]
 fn a_child_drawn_twice_moves_once() {
     let mut h = Harness::new((400, 200));
@@ -501,10 +602,10 @@ fn a_row_of_equal_shares_fills_it_exactly() {
 fn drawn_edges(h: &Harness, id: WidgetId, axis: Axis) -> (f32, f32) {
     let active = &h.render.active[&id];
     let region = h.render.moves.resolve(active.move_idx, active.placement);
-    let dim = h.size().axis(axis);
+    let dim = h.size()[axis];
     let snap = |v: f32| (v + Px::STEP.to_f32() * 0.5).floor();
     let edge = |s: Len| snap(s.rel.to_f32() * dim + s.px.to_f32());
-    let span = region.axis(axis);
+    let span = region[axis];
     (edge(span.start), edge(span.end))
 }
 
@@ -853,8 +954,8 @@ fn a_collapsed_share_keeps_the_gaps_before_the_next_slot() {
                 Sign::Pos => (400 - tail_len, 400),
                 Sign::Neg => (0, tail_len),
             };
-            assert_eq!(region.top_left.axis(dir.axis), Px::from_int(from));
-            assert_eq!(region.bot_right.axis(dir.axis), Px::from_int(to));
+            assert_eq!(region.top_left[dir.axis], Px::from_int(from));
+            assert_eq!(region.bot_right[dir.axis], Px::from_int(to));
         }
     }
 }

tests/cases/plan.rs

@@ -5,7 +5,9 @@
 //! and the oracle another. And reducing a plan has to end, or a shrinker
 //! searching for the smallest counterexample never returns.
 
-use iris::random::{Edits, Kind, Plan, Rng, SpanEdit, plan};
+use iris::harness::Harness;
+use iris::prelude::*;
+use iris::random::{Edits, Kind, Plan, Rng, SpanEdit, grow, plan};
 use std::collections::HashMap;
 
 fn some_edits(seed: u64, of: &Plan) -> Edits {
@@ -25,11 +27,27 @@ fn some_edits(seed: u64, of: &Plan) -> Edits {
     Edits {
         sizes: pick(sized, &mut rng)
             .into_iter()
-            .map(|i| (i, [Some(LayoutLen::LEFTOVER), None]))
+            .map(|i| {
+                (
+                    i,
+                    SizeRules {
+                        x: SizeRule::Exact(LayoutLen::LEFTOVER),
+                        y: SizeRule::Free,
+                    },
+                )
+            })
             .collect(),
         aligns: pick(aligned, &mut rng)
             .into_iter()
-            .map(|i| (i, [Some(AxisAlign::POS), None]))
+            .map(|i| {
+                (
+                    i,
+                    Align {
+                        x: Some(AxisAlign::POS),
+                        y: None,
+                    },
+                )
+            })
             .collect(),
         nodes: pick(nodes, &mut rng)
             .into_iter()
@@ -51,8 +69,6 @@ fn some_edits(seed: u64, of: &Plan) -> Edits {
     }
 }
 
-use iris::prelude::*;
-
 /// The two routes to an edited tree are one tree. `plan` resolves edits out
 /// of the random stream as it draws; `edited` puts them on a tree that
 /// already exists, which is the only route a shrunk plan has, since no seed
@@ -70,11 +86,13 @@ fn editing_a_plan_is_growing_one_with_those_edits() {
     }
 }
 
-/// Every simplification is strictly smaller, so taking them in turn reaches a
-/// fixed point instead of circling. A shrinker that can return to a tree it
-/// has already tried does not stop.
+/// No simplification is larger, which is the half of "the shrinker stops" a
+/// widget count can see. Most are not smaller either -- a dropped alignment
+/// and a simpler leaf both keep the count -- so what rules out circling is
+/// that those are one-way too: a `Some` becomes a `None`, and a kind steps
+/// down a ladder with no way back up.
 #[test]
-fn every_simplification_of_a_plan_is_smaller_than_it() {
+fn no_simplification_of_a_plan_is_larger_than_it() {
     for seed in 1..=60 {
         let tree = plan(seed, 4, &Edits::default());
         let mut queue = vec![tree];
@@ -119,3 +137,20 @@ fn reducing_a_plan_all_the_way_ends() {
         );
     }
 }
+
+/// Every image in a tree is the same picture, and a handle is a reference to
+/// the texture rather than a copy of it, so one upload and one slot serve all
+/// of them however many a tree grows -- and the trees are grown in hundreds.
+#[test]
+fn a_tree_of_images_uploads_one_texture() {
+    let mut images = 0;
+    let mut tree = plan(1, 4, &Edits::default());
+    tree.walk_mut(&mut |p| images += (p.kind == Kind::Image) as usize);
+    assert!(images > 1, "a tree of {images} images tests nothing");
+
+    let mut h = Harness::new((900, 1200));
+    let (root, _) = grow(&mut h.rsc, 1, 4, &Edits::default());
+    h.state.root = Some(root);
+    h.frame();
+    assert_eq!(h.rsc.ui().textures.count(), 1);
+}

tests/cases/retained.rs

@@ -45,7 +45,6 @@ fn counted(h: &mut Harness, size: Size, reads_box: bool) -> (WeakWidget<Counted>
 
 struct Layered {
     children: [StrongWidget<Rect>; 2],
-    _revision: usize,
 }
 
 impl Widget for Layered {
@@ -65,14 +64,10 @@ fn a_redrawn_layered_widget_keeps_the_layer_it_was_entered_on() {
         rect(Color::RED).add_strong(&mut h.rsc),
         rect(Color::BLUE).add_strong(&mut h.rsc),
     ];
-    let root = Layered {
-        children,
-        _revision: 0,
-    }
-    .add(&mut h.rsc);
+    let root = Layered { children }.add(&mut h.rsc);
     h.set_root(root);
 
-    h.rsc[root]._revision += 1;
+    h.rsc.widgets_mut().mark_for_redraw(root.id());
     h.frame();
 
     let label = h.rsc.widgets().label(root.id());
@@ -125,7 +120,7 @@ fn moving_an_ordinary_subtree_remaps_its_mask() {
     let active = &h.render.active[&masked.id()];
     assert_eq!(
         h.rsc.ui().masks[active.mask.idx()].region,
-        UiRegion::new(UiSpan::new(Len::px(150.0), Len::rel_max()), UiSpan::FULL,)
+        UiRegion::new(UiSpan::new(Len::px(150.0), Len::FULL), UiSpan::FULL,)
     );
     assert_corners!(h, inner, (150, 0), (400, 200));
 }
@@ -181,9 +176,9 @@ fn a_repaint_that_keeps_its_size_does_not_relay_out() {
     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());
+    // Marked with nothing about it changed, and it reports the same size
+    // either way, so the parent has nothing to lay out.
+    h.rsc.widgets_mut().mark_for_redraw(first.id());
     h.frame();
 
     assert_eq!(draws.get(), settled + 1);
@@ -198,7 +193,7 @@ fn a_span_child_survives_the_next_frame() {
     let bottom = rect(Color::BLUE).height(120).add(&mut h.rsc);
     h.set_root((top, bottom).span(Dir::DOWN));
 
-    h.rsc.widgets_mut().get_dyn_mut(top.id());
+    h.rsc.widgets_mut().mark_for_redraw(top.id());
     h.frame();
 
     assert_corners!(h, top, (0, 0), (400, 80));
@@ -214,10 +209,7 @@ impl Widget for FromHint {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let len = painter.size_hint(&self.inner, Axis::Y).unwrap();
         let top = UiSpan::new(Len::ZERO, Len::from_parts(Rel::ZERO, len.px));
-        painter.widget_at(
-            &self.inner,
-            PlaceDesc::new(PlaceDescAxis::WHOLE, PlaceDescAxis::shifted(top)),
-        );
+        painter.widget_at(&self.inner, top.shifted_desc().on_axis(Axis::Y));
         Size::LEFTOVER
     }
 }
@@ -663,7 +655,7 @@ fn a_masked_widget_redrawn_on_its_own_sets_its_mask_again() {
     let masked = inner.masked().add(&mut h.rsc);
     let other = rect(Color::RED).width(100).add(&mut h.rsc);
     h.set_root((other, masked).span(Dir::RIGHT));
-    h.rsc.widgets_mut().get_dyn_mut(masked.id());
+    h.rsc.widgets_mut().mark_for_redraw(masked.id());
     h.frame();
     assert_corners!(h, inner, (100, 0), (400, 200));
 }
@@ -777,6 +769,17 @@ fn a_subtree_that_changed_parents_settles_at_the_depth_it_moved_to() {
     );
 }
 
+/// Where a mask slot clips, in window pixels: the region it holds, carried
+/// through whatever move entry it hangs from. Taken by slot rather than by
+/// widget, so a test can name the slot it expects a redraw to keep.
+fn mask_bounds(h: &Harness, mask: MaskIdx) -> PixelRegion {
+    let mask = &h.rsc.ui().masks[mask.idx()];
+    h.render
+        .moves
+        .resolve(mask.move_idx, mask.region)
+        .to_px(h.render.output_size())
+}
+
 fn primitive_bounds(h: &Harness, id: WidgetId) -> Vec<PixelRegion> {
     h.render.active[&id]
         .primitives
@@ -876,10 +879,7 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                PlaceDesc::new(
-                    PlaceDescAxis::shifted(self.region.x),
-                    PlaceDescAxis::shifted(self.region.y),
-                ),
+                PlaceDesc::new(self.region.x.shifted_desc(), self.region.y.shifted_desc()),
             );
             Size::LEFTOVER
         }
@@ -925,15 +925,10 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
             primitive_bounds(&warm, leaf.id()),
             primitive_bounds(&cold, other.id())
         );
-        let mask = |h: &Harness, id: WidgetId| {
-            let active = &h.render.active[&id];
-            let mask = &h.rsc.ui().masks[active.mask.idx()];
-            h.render
-                .moves
-                .resolve(mask.move_idx, mask.region)
-                .to_px(h.render.output_size())
-        };
-        assert_eq!(mask(&warm, leaf.id()), mask(&cold, other.id()));
+        assert_eq!(
+            mask_bounds(&warm, warm.render.active[&leaf.id()].mask),
+            mask_bounds(&cold, cold.render.active[&other.id()].mask)
+        );
     }
 }
 
@@ -965,8 +960,8 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
             painter.widget_at(
                 &self.child,
                 PlaceDesc::new(
-                    PlaceDescAxis::shifted(self.region.x).fills(),
-                    PlaceDescAxis::shifted(self.region.y).fills(),
+                    self.region.x.shifted_desc().fills(),
+                    self.region.y.shifted_desc().fills(),
                 )
                 .rel_base(Axis::X, self.frame.x.len()),
             );
@@ -1000,7 +995,7 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
             assert_eq!(draws.get(), before);
             let retained = primitive_bounds(&h, text.id());
             assert!(!retained.is_empty());
-            let _ = h.rsc.widgets_mut().get_dyn_mut(text.id());
+            h.rsc.widgets_mut().mark_for_redraw(text.id());
             h.frame();
             assert!(draws.get() > before);
             assert_eq!(retained, primitive_bounds(&h, text.id()));
@@ -1135,8 +1130,8 @@ fn padding_and_stack_boxes_follow_the_region_without_drawing_again() {
             painter.widget_at(
                 &self.child,
                 PlaceDesc::new(
-                    PlaceDescAxis::shifted(self.region.x).fills(),
-                    PlaceDescAxis::shifted(self.region.y).fills(),
+                    self.region.x.shifted_desc().fills(),
+                    self.region.y.shifted_desc().fills(),
                 ),
             );
             Size::LEFTOVER
@@ -1190,15 +1185,10 @@ fn padding_and_stack_boxes_follow_the_region_without_drawing_again() {
                 assert_eq!(warm.region(&a), cold.region(&b));
                 assert_eq!(primitive_bounds(&warm, a), primitive_bounds(&cold, b));
             }
-            let mask = |h: &Harness, id: WidgetId| {
-                let active = &h.render.active[&id];
-                let mask = &h.rsc.ui().masks[active.mask.idx()];
-                h.render
-                    .moves
-                    .resolve(mask.move_idx, mask.region)
-                    .to_px(h.render.output_size())
-            };
-            assert_eq!(mask(&warm, leaf.id()), mask(&cold, other.id()));
+            assert_eq!(
+                mask_bounds(&warm, warm.render.active[&leaf.id()].mask),
+                mask_bounds(&cold, cold.render.active[&other.id()].mask)
+            );
         }
     }
 }
@@ -1222,12 +1212,10 @@ fn moving_a_childs_region_preserves_the_slot_chosen_from_its_measurement() {
             painter.widget_at(
                 &self.child,
                 PlaceDesc::new(
-                    PlaceDescAxis::shifted(UiSpan::new(
-                        Len::px(self.start),
-                        Len::px(self.start + 200.0),
-                    ))
+                    UiSpan::new(Len::px(self.start), Len::px(self.start + 200.0))
+                        .shifted_desc()
                         .fills(),
-                    PlaceDescAxis::shifted(UiSpan::FULL).fills(),
+                    UiSpan::FULL.shifted_desc().fills(),
                 ),
             );
             Size::LEFTOVER
@@ -1263,10 +1251,7 @@ fn changing_regions_keep_fractional_reports_and_numeric_dependencies_valid() {
             painter
                 .widget_at(
                     &self.child,
-                    PlaceDesc::new(
-                        PlaceDescAxis::shifted(self.region.x),
-                        PlaceDescAxis::shifted(self.region.y),
-                    ),
+                    PlaceDesc::new(self.region.x.shifted_desc(), self.region.y.shifted_desc()),
                 )
                 .size()
         }
@@ -1283,8 +1268,8 @@ fn changing_regions_keep_fractional_reports_and_numeric_dependencies_valid() {
                     .widget_at(
                         &self.child,
                         PlaceDesc::new(
-                            PlaceDescAxis::shifted(self.region.x).fills(),
-                            PlaceDescAxis::shifted(self.region.y).fills(),
+                            self.region.x.shifted_desc().fills(),
+                            self.region.y.shifted_desc().fills(),
                         ),
                     )
                     .size(),
@@ -1385,20 +1370,14 @@ fn a_redrawn_mask_keeps_reused_primitives_clipped_when_it_moves() {
         h.set_root((first, masked).span(Dir::DOWN));
         let mask = h.render.active[&masked.id()].mask;
         let settled = draws.get();
-        h.rsc.widgets_mut().get_dyn_mut(masked.id());
+        h.rsc.widgets_mut().mark_for_redraw(masked.id());
         h.frame();
         assert_eq!(primitive_masks(&h, inner.id()), vec![mask]);
         assert_eq!(draws.get(), settled, "a mask repaint must reuse its child");
         assert_eq!(h.render.active[&masked.id()].mask, mask);
         h.set_len(first, Axis::Y, 10);
         h.frame();
-        let clip = h.rsc.ui().masks[mask.idx()];
-        let clip = h
-            .render
-            .moves
-            .resolve(clip.move_idx, clip.region)
-            .to_px(h.render.output_size());
-        assert_eq!(clip, h.region(&masked).unwrap());
+        assert_eq!(mask_bounds(&h, mask), h.region(&masked).unwrap());
         assert_corners!(h, inner, (0, 10), (400, 200));
     }
 }
@@ -1488,3 +1467,99 @@ fn a_redrawn_subtree_is_not_undrawn_by_the_parent_it_left() {
         assert_eq!(h.region(&leaf), before);
     }
 }
+
+/// A leaf that reports less than the box it is given and states which lengths
+/// of that box its drawing holds for, so a test can widen the contract
+/// without changing the answer. It counts its draws, since what a kept
+/// contract costs is whether the parent has to make it draw again.
+struct Contracted {
+    holds: std::ops::RangeInclusive<Px>,
+    size: Size,
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for Contracted {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        painter.holds(Axis::X, self.holds.clone());
+        self.size
+    }
+}
+
+struct CountedParent {
+    inner: StrongWidget,
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for CountedParent {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        painter.widget(&self.inner).size()
+    }
+}
+
+#[test]
+fn widening_what_a_drawing_holds_for_does_not_relay_out_the_parent() {
+    let mut h = Harness::new((400, 200));
+    let child = Contracted {
+        holds: Px::from_int(300)..=Px::from_int(500),
+        size: Size::from((100, 200)),
+        draws: Rc::new(Cell::new(0)),
+    }
+    .add(&mut h.rsc);
+    let draws = Rc::new(Cell::new(0));
+    let root = CountedParent {
+        inner: child.upgrade(&mut h.rsc),
+        draws: draws.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root(root);
+    let settled = draws.get();
+
+    // The same answer, good for more boxes than before, so the guarantee the
+    // parent kept still holds.
+    h.rsc[child].holds = Px::from_int(200)..=Px::from_int(600);
+    h.frame();
+
+    assert_eq!(
+        draws.get(),
+        settled,
+        "a wider contract for the same answer is not a change to lay out"
+    );
+}
+
+/// The other half of the rule above: a kept contract is the narrower one, so
+/// it is only worth keeping where it still holds. A window the old range is
+/// outside is not one its parent can be handed back, and keeping it there
+/// throws away the drawing the widget just made.
+#[test]
+fn a_contract_this_window_is_outside_is_not_kept() {
+    let mut h = Harness::new((400, 200));
+    let leaf_draws = Rc::new(Cell::new(0));
+    let child = Contracted {
+        holds: Px::from_int(300)..=Px::from_int(500),
+        size: Size::from((100, 200)),
+        draws: leaf_draws.clone(),
+    }
+    .add(&mut h.rsc);
+    let root = CountedParent {
+        inner: child.upgrade(&mut h.rsc),
+        draws: Rc::new(Cell::new(0)),
+    }
+    .add(&mut h.rsc);
+    h.set_root(root);
+
+    // Wide enough that the old contract leaves the new box out, and the leaf
+    // is marked in the same frame -- so it settles itself first and its
+    // parent draws afterwards, asking about what it settled.
+    h.resize((600, 200));
+    h.rsc[child].holds = Px::from_int(200)..=Px::from_int(700);
+    let settled = leaf_draws.get();
+    h.frame();
+
+    assert_eq!(
+        leaf_draws.get(),
+        settled + 1,
+        "the leaf settled once and its parent kept what it settled"
+    );
+}

tests/cases/scroll.rs

@@ -113,7 +113,7 @@ fn wrapping_content_beside_a_fixed_length_is_stable_warm_and_cold() {
 
     let mut warm = Harness::new((900, 300));
     let (text, content) = plant(&mut warm);
-    warm.rsc.widgets_mut().get_dyn_mut(text);
+    warm.rsc.widgets_mut().mark_for_redraw(text);
     warm.frame();
 
     let mut cold = Harness::new((900, 300));
@@ -142,8 +142,8 @@ fn a_clipping_widget_reporting_more_than_its_box_is_caught() {
     let mut h = Harness::new((100, 100));
     let tall = rect(Color::RED).height(400).add_strong(&mut h.rsc);
     let clipper = Clipper(tall).add(&mut h.rsc);
+    // `set_root` lays the tree out, so this is where it is caught.
     h.set_root(clipper);
-    h.frame();
 }
 
 /// Content that fits sits in the viewport, not in a box of the window's

tests/cases/unsettled.rs

@@ -9,17 +9,45 @@
 //! reached through a region node's own entry rather than through the offer
 //! that node was given. The last is a wrapping text handed back the width
 //! it measured, rounded to a step below the line it measured there.
+//!
+//! Each says which seed it was shrunk from, of the generator as it stood when
+//! it was found. Those numbers no longer grow those trees -- a seed names one
+//! only while the generator draws the same things in the same order, and the
+//! leaves have grown an image since -- so what is written out below is the
+//! record of the case, and the seed is where it came from.
+
+use std::collections::HashSet;
 
 use iris::harness::Harness;
 use iris::prelude::*;
 use iris::random::Branch;
 
+/// Every widget in the same place warm as cold, reported all at once: which
+/// of a dozen boxes moved is the whole of what a shrunk case has to say.
+///
+/// A list that names one widget twice is an error rather than a redundant
+/// check. `width`, `sized` and `align` give back the widget they were handed,
+/// so a fixture built through them can name one text three times, and then a
+/// case comparing six boxes compares four and says nothing about it. One
+/// fixture builds both lists, so checking the warm one checks both.
+#[track_caller]
 fn assert_same_regions(
     warm: &Harness,
     warm_ids: &[WidgetId],
     cold: &Harness,
     cold_ids: &[WidgetId],
 ) {
+    assert_eq!(
+        warm_ids.len(),
+        cold_ids.len(),
+        "the warm and cold fixtures list different widgets"
+    );
+    let named: HashSet<&WidgetId> = warm_ids.iter().collect();
+    assert_eq!(
+        named.len(),
+        warm_ids.len(),
+        "a widget is listed twice: {warm_ids:?}"
+    );
     let mut wrong = Vec::new();
     for (i, (&w, &c)) in warm_ids.iter().zip(cold_ids).enumerate() {
         let (got, want) = (warm.region(&w), cold.region(&c));
@@ -92,7 +120,7 @@ fn repainting_a_stack_uses_the_box_its_sizing_child_decided() {
     let mut warm = Harness::new((900, 1200));
     let ids = plant_stack_in_its_sizing_childs_box(&mut warm);
     for &id in &ids {
-        warm.rsc.widgets_mut().get_dyn_mut(id);
+        warm.rsc.widgets_mut().mark_for_redraw(id);
     }
     warm.frame();
 
@@ -188,36 +216,31 @@ fn reordering_nested_spans_keeps_the_answer_from_the_decided_box() {
     assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
 
-/// Six widgets, shrunk from a 402-widget tree the fuzzer found. Nothing about
+/// Four 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 = wrapped.width(76).add(&mut h.rsc);
-    let aligned = sized;
+    let wrapped = wtext("Wrapping shapes")
+        .size(16)
+        .wrap(true)
+        .width(76)
+        .add(&mut h.rsc);
     h.rsc
         .widgets_mut()
-        .set_alignment(sized, Axis::X, AxisAlign::POS);
+        .set_alignment(wrapped, Axis::X, AxisAlign::POS);
     h.rsc
         .widgets_mut()
-        .set_alignment(sized, Axis::Y, AxisAlign::POS);
+        .set_alignment(wrapped, Axis::Y, AxisAlign::POS);
     let stack = Stack {
-        children: vec![plain.add_strong(&mut h.rsc), aligned.add_strong(&mut h.rsc)],
+        children: vec![plain.add_strong(&mut h.rsc), wrapped.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(),
-    ]
+    vec![plain.id(), wrapped.id(), stack.id(), root.id()]
 }
 
 /// The first frame does not reach the layout a second one does, so "cold" is
@@ -230,7 +253,7 @@ fn one_frame_is_enough() {
     let first = h.region(&ids[1]).unwrap();
     for _ in 0..3 {
         for &id in &ids {
-            h.rsc.widgets_mut().get_dyn_mut(id);
+            h.rsc.widgets_mut().mark_for_redraw(id);
         }
         h.frame();
     }
@@ -252,46 +275,30 @@ 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.rsc.widgets_mut().mark_for_redraw(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"));
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
 
-/// Six widgets, shrunk from 905. Everything inside the declared 189x176 box
+/// Four 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 = text;
     h.rsc
         .widgets_mut()
         .set_alignment(text, Axis::X, AxisAlign::NEG);
-    let inner = (aligned,).span(Dir::RIGHT).add(&mut h.rsc);
-    let sized = inner.sized((189, 176)).add(&mut h.rsc);
+    let inner = (text,).span(Dir::RIGHT).sized((189, 176)).add(&mut h.rsc);
     let filler = rect(Color::RED).add(&mut h.rsc);
-    let root = (filler, sized).span(Dir::RIGHT).add(&mut h.rsc);
+    let root = (filler, inner).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(),
-    ]
+    vec![text.id(), inner.id(), filler.id(), root.id()]
 }
 
 #[test]
@@ -306,17 +313,10 @@ fn a_resize_does_not_reach_inside_a_box_of_declared_pixels() {
     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"));
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
 
-/// Four widgets, shrunk from 486. A span's two children are swapped: warm by
+/// Three 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>) {
@@ -340,16 +340,11 @@ fn plant_pair(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, WeakWidget<Span
         gap: Px::ZERO,
     }
     .add(&mut h.rsc);
-    let span_handle = span;
-    let aligned = span;
     h.rsc
         .widgets_mut()
         .set_alignment(span, Axis::X, AxisAlign::CENTER);
-    h.state.root = Some(aligned.add_strong(&mut h.rsc));
-    (
-        vec![wrapped.id(), plain.id(), span.id(), aligned.id()],
-        span_handle,
-    )
+    h.state.root = Some(span.add_strong(&mut h.rsc));
+    (vec![wrapped.id(), plain.id(), span.id()], span)
 }
 
 #[test]
@@ -364,17 +359,10 @@ fn swapping_two_children_lands_where_growing_them_that_way_does() {
     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"));
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
 
-/// Eight widgets, shrunk from 80. The scroll decides how wide to make its
+/// Seven 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 given that box once, so nothing at its
 /// own edge says the box was its own answer.
@@ -393,8 +381,7 @@ fn plant_scrolled(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget
         gap: Px::ZERO,
     }
     .add(&mut h.rsc);
-    let block = rect(Color::RED).add(&mut h.rsc);
-    let fixed = block.width(87).add(&mut h.rsc);
+    let fixed = rect(Color::RED).width(87).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 {
@@ -416,7 +403,6 @@ fn plant_scrolled(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget
             text.id(),
             filler.id(),
             inner.id(),
-            block.id(),
             fixed.id(),
             outer.id(),
             through.id(),
@@ -440,14 +426,7 @@ fn a_span_given_the_box_its_answer_decided_matches_a_cold_layout() {
     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"));
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
 
 /// Reports a width derived from the box it is asked in. Reading through the
@@ -471,11 +450,10 @@ impl Widget for Wider {
 fn plant_wider(h: &mut Harness, extra: f32) -> (WeakWidget<Wider>, WidgetId) {
     let content = Wider { extra }.add(&mut h.rsc);
     let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X).add(&mut h.rsc);
-    let root = scroll;
     h.rsc
         .widgets_mut()
         .set_alignment(scroll, Axis::X, AxisAlign::NEG);
-    h.set_root(root);
+    h.set_root(scroll);
     (content, scroll.id())
 }
 
@@ -567,14 +545,7 @@ fn a_box_that_only_rounds_past_its_fixed_children_leaves_nothing_over() {
     let (cold_ids, _) = plant_boundary(&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"));
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
 
 /// Five widgets, shrunk by `tests/shrink.rs` from the 277 the oracle's seed
@@ -616,20 +587,13 @@ fn plant_nested_scrolls(h: &mut Harness) -> Vec<WidgetId> {
 fn redrawing_one_widget_does_not_move_what_scrolls_around_it() {
     let mut warm = Harness::new((900, 1200));
     let ids = plant_nested_scrolls(&mut warm);
-    warm.rsc.widgets_mut().get_dyn_mut(ids[0]);
+    warm.rsc.widgets_mut().mark_for_redraw(ids[0]);
     warm.frame();
 
     let mut cold = Harness::new((900, 1200));
     let cold_ids = plant_nested_scrolls(&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"));
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
 
 /// Ten widgets, of the shape `tests/shrink.rs` reduces the oracle's seed 220
@@ -720,14 +684,7 @@ fn a_widget_under_a_region_node_is_asked_in_the_box_that_node_was_offered() {
     let (cold_ids, _) = plant_under_a_node(&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"));
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
 
 const PARAGRAPH: &str = "Wrapping shapes one source into as many lines as the \
@@ -1067,3 +1024,32 @@ fn resizing_under_a_short_scroll_snaps_its_window_tall_content_again() {
 
     assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
+
+/// A scroll clamps its position against the box it is drawn in, so drawing it
+/// once at one viewport and again at another writes state the second draw then
+/// reads. That the answer is still the one a cold layout gives is a property
+/// of the clamp, not something the layout enforces.
+#[test]
+fn a_scrolled_view_resized_lands_where_a_cold_layout_puts_it() {
+    for amt in [10.0, 40.0, 90.0, 140.0] {
+        let mut warm = Harness::new((100, 100));
+        let (_, warm_scroll) = plant_wider(&mut warm, 100.0);
+        warm.move_to((50.0, 50.0));
+        warm.scroll((-amt, 0.0));
+        warm.frame();
+        warm.resize((160, 100));
+        warm.frame();
+
+        let mut cold = Harness::new((160, 100));
+        let (_, cold_scroll) = plant_wider(&mut cold, 100.0);
+        cold.move_to((50.0, 50.0));
+        cold.scroll((-amt, 0.0));
+        cold.frame();
+
+        assert_eq!(
+            warm.region(&warm_scroll),
+            cold.region(&cold_scroll),
+            "scrolled by {amt} then widened"
+        );
+    }
+}

tests/chain_cost.rs

@@ -11,8 +11,6 @@
 //! 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::{
@@ -21,6 +19,9 @@ use iris_core::{
 };
 use wgpu::{Color as GpuColor, *};
 
+#[path = "gpu/mod.rs"]
+mod gpu;
+
 const SIZE: u32 = 1024;
 const INSTANCES: usize = 200_000;
 const FRAMES: u32 = 20;
@@ -29,18 +30,7 @@ const FRAMES: u32 = 20;
 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()?;
+    let adapter = gpu::adapter()?;
     if !adapter.features().contains(Features::TIMESTAMP_QUERY) {
         println!("no timestamp queries on {:?}", adapter.get_info().name);
         return None;
@@ -55,20 +45,6 @@ fn gpu() -> Option<(Device, Queue, f32)> {
     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>();
@@ -103,7 +79,7 @@ fn fill(ui: &mut UiData, render: &mut UiRenderState, depth: usize) {
 /// 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 node = UiRenderNode::new(device, &gpu::config(format, SIZE));
     let mut ui = UiData::default();
     let mut render = UiRenderState::new();
     fill(&mut ui, &mut render, depth);

tests/draw_cost.rs

@@ -13,10 +13,6 @@
 //! That is how `PrimitiveRender` was measured against a match in the renderer:
 //! 6 instructions per list drawn, against the ~5,400 wgpu spends recording
 //! one.
-//!
-//! The instance is leaked deliberately. A Vulkan loader may unload the driver
-//! when the last one drops, which can fault as a thread that used it exits --
-//! and every test runs on a spawned thread.
 
 use std::time::Instant;
 
@@ -27,6 +23,9 @@ use iris_core::{
 };
 use wgpu::{Color as GpuColor, *};
 
+#[path = "gpu/mod.rs"]
+mod gpu;
+
 const SIZE: u32 = 1024;
 const FRAMES: u32 = 200;
 /// Reported as the best of this many batches, since the mean moves by more
@@ -34,39 +33,11 @@ const FRAMES: u32 = 200;
 const BATCHES: u32 = 8;
 
 fn gpu() -> Option<(Device, Queue)> {
-    // Probed rather than assumed: there may be no Vulkan adapter, and GL is
-    // what is left when there is not.
-    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()
-        }),
-    };
-    // Leaked rather than dropped: see the note at the top of the file.
-    let instance: &'static Instance = Box::leak(Box::new(instance));
-    let adapter =
-        pollster::block_on(instance.request_adapter(&RequestAdapterOptions::default())).ok()?;
+    let adapter = gpu::adapter()?;
     println!("adapter: {:?}", adapter.get_info());
     pollster::block_on(adapter.request_device(&DeviceDescriptor::default())).ok()
 }
 
-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![],
-    }
-}
-
 /// Every layer draws all three primitives, so the renderer takes a different
 /// path for each list it walks -- which is the case a single-primitive layer
 /// would never exercise. Images are bound per instance, so there are few.
@@ -136,7 +107,7 @@ fn fill(
 
 fn frame_cost(device: &Device, queue: &Queue, layers: usize, per_layer: usize) -> f64 {
     let format = TextureFormat::Bgra8Unorm;
-    let mut node = UiRenderNode::new(device, &config(format));
+    let mut node = UiRenderNode::new(device, &gpu::config(format, SIZE));
     let mut ui = UiData::default();
     let mut render = UiRenderState::new();
     let _handles = fill(&mut ui, &mut render, layers, per_layer);

tests/generated.rs

@@ -25,10 +25,14 @@ fn depth() -> usize {
     env("IRIS_GENERATED_DEPTH", 4)
 }
 
-/// The seeds the ordinary tests take. Seven that have never failed; 86,
-/// which a `Scroll` fixed point once settled differently on; and 20, which
-/// caught a locally redrawn widget being placed twice in the box its parent
-/// had already placed it in.
+/// The seeds the ordinary tests take: a corpus rather than a set of
+/// regression cases, since a seed names a tree only for as long as the
+/// generator draws the same things in the same order. Adding images to the
+/// leaves moved every one of them, so 20 and 86 -- which once caught a widget
+/// placed twice in a box its parent had already placed it in, and a `Scroll`
+/// fixed point settling differently -- no longer grow those trees. Both
+/// defects are pinned by the shrunk fixtures in `cases/unsettled.rs`, which
+/// are trees rather than numbers.
 const SEEDS: [u64; 10] = [1, 2, 3, 5, 8, 10, 13, 20, 86, 98];
 
 fn check(seed: u64, depth: usize, case: Case) {
@@ -47,57 +51,50 @@ fn check_plan(grown: &Plan, seed: u64, depth: usize, case: Case) {
     }
 }
 
-macro_rules! case {
-    ($name:ident, $case:expr) => {
+/// A test per case, and the list of which cases have one, from the same
+/// place. A case the ordinary suite leaves out runs only in the long scan,
+/// which nobody runs by hand.
+macro_rules! cases {
+    ($($name:ident = $case:expr,)*) => {
+        $(
             #[test]
             fn $name() {
                 for seed in SEEDS {
                     check(seed, depth(), $case);
                 }
             }
+        )*
+
+        const NAMED: [Case; [$($case,)*].len()] = [$($case,)*];
     };
 }
 
-case!(
-    many_widgets_redrawing_at_once_leaves_every_box_where_it_was,
-    Case::RepaintSome
-);
-case!(
-    everything_redrawing_at_once_leaves_every_box_where_it_was,
-    Case::Repaint
-);
-case!(
-    a_resize_lands_where_starting_at_that_size_would,
-    Case::Resize
-);
-case!(
-    a_resize_and_a_repaint_land_where_starting_that_way_would,
-    Case::ResizeRepaint
-);
-case!(
-    a_size_change_after_a_resize_lands_the_same_way,
-    Case::ResizeSize
-);
-case!(
-    a_size_change_lands_where_growing_it_that_way_would,
-    Case::Size
-);
-case!(
-    every_size_changing_at_once_lands_where_growing_it_that_way_would,
-    Case::EverySize
-);
-case!(
-    an_alignment_change_lands_where_growing_it_that_way_would,
-    Case::Align
-);
-case!(
-    giving_and_taking_a_movable_region_rebuilds_what_resolves_it,
-    Case::RegionNode
-);
-case!(
-    reordering_a_span_lands_where_growing_it_that_way_would,
-    Case::Reorder
-);
+cases! {
+    many_widgets_redrawing_at_once_leaves_every_box_where_it_was = Case::RepaintSome,
+    everything_redrawing_at_once_leaves_every_box_where_it_was = Case::Repaint,
+    a_resize_lands_where_starting_at_that_size_would = Case::Resize,
+    a_resize_and_a_repaint_land_where_starting_that_way_would = Case::ResizeRepaint,
+    a_size_change_after_a_resize_lands_the_same_way = Case::ResizeSize,
+    a_resize_after_a_size_change_lands_the_same_way = Case::SizeResize,
+    a_size_change_lands_where_growing_it_that_way_would = Case::Size,
+    every_size_changing_at_once_lands_where_growing_it_that_way_would = Case::EverySize,
+    an_alignment_change_lands_where_growing_it_that_way_would = Case::Align,
+    giving_and_taking_a_movable_region_rebuilds_what_resolves_it = Case::RegionNode,
+    reordering_a_span_lands_where_growing_it_that_way_would = Case::Reorder,
+}
+
+/// The shuffles are one test between them, so they are the only cases `ALL`
+/// may hold without a test of their own.
+#[test]
+fn every_case_runs_without_the_long_scan() {
+    for case in ALL {
+        assert!(
+            NAMED.contains(&case) || matches!(case, Case::Shuffle(_)),
+            "{} runs only in the long seed scan; give it a case here",
+            case.name()
+        );
+    }
+}
 
 #[test]
 fn adding_and_removing_span_children_lands_where_growing_it_that_way_would() {
@@ -111,7 +108,7 @@ fn adding_and_removing_span_children_lands_where_growing_it_that_way_would() {
 }
 
 #[test]
-#[ignore = "as many seeds as it is asked for, rather than the nine the others check"]
+#[ignore = "as many seeds as it is asked for, rather than the ten the others check"]
 fn a_long_run_of_seeds_agrees() {
     let depth = depth();
     let seeds: Vec<u64> = match std::env::var("IRIS_GENERATED_SEED")

tests/gpu/mod.rs

@@ -0,0 +1,40 @@
+//! The adapter and the surface configuration the GPU measurement rigs share,
+//! so the two cannot probe for a device in two different ways.
+
+use wgpu::*;
+
+/// An adapter on whatever this machine has, or `None` where there is none.
+///
+/// Probed rather than assumed: there may be no Vulkan adapter, and GL is what
+/// is left when there is not.
+///
+/// The instance is leaked deliberately. A Vulkan loader may unload the driver
+/// when the last one drops, which can fault as a thread that used it exits --
+/// and every test runs on a spawned thread.
+pub fn adapter() -> Option<Adapter> {
+    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));
+    pollster::block_on(instance.request_adapter(&RequestAdapterOptions::default())).ok()
+}
+
+pub fn config(format: TextureFormat, size: u32) -> 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![],
+    }
+}

tests/layout_diagnostics.rs

@@ -22,6 +22,30 @@ use std::time::Instant;
 
 const OUTPUT: (f32, f32) = (1920.0, 1200.0);
 
+/// A scroll whose content fits is the same drawing in every box it still
+/// fits in, so a longer or shorter one relays out nothing. Where the content
+/// sits in that box is decided by placing its answer in the whole of it,
+/// which is a fraction of the box and holds at every length -- so the
+/// contract must not turn on the alignment. It did, and at the default
+/// alignment, which is the middle, every box change redrew the scroll.
+#[cfg(feature = "layout-diagnostics")]
+#[test]
+fn a_fitting_scroll_holds_for_every_box_its_content_fits_in() {
+    use iris::core::layout_diagnostics as diag;
+
+    for align in [Align::TOP_LEFT, Align::CENTER, Align::BOT_RIGHT] {
+        let mut harness = Harness::new((400, 200));
+        let inner = rect(Color::RED).height(50).add(&mut harness.rsc);
+        harness.set_root(inner.scrollable().align(align));
+        harness.frame();
+        let _ = diag::take();
+        // Still far longer than the 50 the content needs.
+        harness.resize((400, 180));
+        harness.frame();
+        assert_eq!(diag::take().distinct_widgets(), 0, "{align:?}");
+    }
+}
+
 #[cfg(feature = "layout-diagnostics")]
 #[test]
 fn a_selected_widget_retains_its_layout_events() {
@@ -37,8 +61,8 @@ fn a_selected_widget_retains_its_layout_events() {
     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.rsc.widgets_mut().mark_for_redraw(root.id());
+    harness.rsc.widgets_mut().mark_for_redraw(leaf.id());
     harness.frame();
 
     let report = diagnostics::take();
@@ -195,7 +219,6 @@ fn layout_cost() {
         #[cfg(feature = "layout-diagnostics")]
         let _ = iris::core::layout_diagnostics::take();
         run("cold", 1, &mut harness, |_, _| {});
-        drop(tree);
     }
 
     if selected("repaint") {
@@ -203,7 +226,7 @@ fn layout_cost() {
         trace_selected(&tree);
         let leaf = tree.ids[0];
         run("repaint", frames, &mut harness, move |harness, _| {
-            let _ = harness.rsc.widgets_mut().get_dyn_mut(leaf);
+            harness.rsc.widgets_mut().mark_for_redraw(leaf);
         });
     }
 
@@ -218,7 +241,7 @@ fn layout_cost() {
         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);
+                harness.rsc.widgets_mut().mark_for_redraw(id);
             }
         });
     }
@@ -251,6 +274,5 @@ fn layout_cost() {
         run("resize", frames, &mut harness, |harness, frame| {
             harness.resize((OUTPUT.0 - ((frame + 1) % 2) as f32 * 8.0, OUTPUT.1));
         });
-        drop(tree);
     }
 }

tests/revision_cost.rs

@@ -194,7 +194,9 @@ fn text_memory() {
         h.frame();
     }
     report("after 40 resizes");
-    // Settled: the output holds still and one leaf repaints per frame.
+    // Settled: the output holds still and one leaf repaints per frame. Marked
+    // by taking it mutably because the revision at the top of this file has no
+    // `mark_for_redraw`, and the same source has to build against both.
     for _ in 0..10 {
         let _ = h.rsc.widgets_mut().get_dyn_mut(paragraphs[0]);
         h.frame();

tests/scenario/mod.rs

@@ -13,7 +13,7 @@
 
 use iris::harness::Harness;
 use iris::prelude::*;
-use iris::random::{Aligns, Edits, Kind, Lens, Plan, Rng, SpanEdit, Tree, build};
+use iris::random::{Edits, Kind, Plan, Rng, SpanEdit, Tree, build};
 use std::collections::HashMap;
 
 /// A seed per thread but one, since a seed grows, lays out and drops its tree
@@ -190,7 +190,7 @@ impl Case {
 
 fn mark(warm: &mut Harness, tree: &Tree, step: usize) {
     for &id in tree.ids.iter().step_by(step) {
-        warm.rsc.widgets_mut().get_dyn_mut(id);
+        warm.rsc.widgets_mut().mark_for_redraw(id);
     }
 }
 
@@ -198,27 +198,32 @@ fn a_len(rng: &mut Rng) -> Option<LayoutLen> {
     Some(LayoutLen::px(20.0 + rng.below(180) as f32))
 }
 
-fn resize_one(warm: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> Lens {
-    let lens = [a_len(rng), a_len(rng)];
+fn resize_one(warm: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> SizeRules {
+    let lens = SizeRules {
+        x: a_len(rng).into(),
+        y: a_len(rng).into(),
+    };
     warm.rsc
         .widgets_mut()
-        .set_size_rules(tree.sized[idx], lens[0], lens[1]);
+        .set_size_rules(tree.sized[idx], lens.x, lens.y);
     lens
 }
 
-fn realign_one(warm: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> Aligns {
+fn realign_one(warm: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> Align {
     let side = |rng: &mut Rng| match rng.below(4) {
         0 => None,
         1 => Some(AxisAlign::NEG),
         2 => Some(AxisAlign::CENTER),
         _ => Some(AxisAlign::POS),
     };
-    let align = [side(rng), side(rng)];
+    let align = Align {
+        x: side(rng),
+        y: side(rng),
+    };
     let id = tree.aligned[idx];
-    for (axis, align) in [Axis::X, Axis::Y].into_iter().zip(align) {
-        warm.rsc
-            .widgets_mut()
-            .set_alignment(id, axis, align.unwrap_or_default());
+    let taken = RegionAlign::from(align);
+    for axis in Axis::BOTH {
+        warm.rsc.widgets_mut().set_alignment(id, axis, taken[axis]);
     }
     align
 }

tests/trace_unsettled.rs

@@ -1,5 +1,5 @@
-//! 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.
+//! Traces the four-widget trees in `unsettled.rs`, to see what box their text
+//! is actually drawn in on a first frame against a settled one.
 
 #![cfg(feature = "layout-diagnostics")]
 
@@ -9,30 +9,25 @@ 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 = wrapped.width(76).add(&mut h.rsc);
-    let aligned = sized;
+    let wrapped = wtext("Wrapping shapes")
+        .size(16)
+        .wrap(true)
+        .width(76)
+        .add(&mut h.rsc);
     h.rsc
         .widgets_mut()
-        .set_alignment(sized, Axis::X, AxisAlign::POS);
+        .set_alignment(wrapped, Axis::X, AxisAlign::POS);
     h.rsc
         .widgets_mut()
-        .set_alignment(sized, Axis::Y, AxisAlign::POS);
+        .set_alignment(wrapped, Axis::Y, AxisAlign::POS);
     let stack = Stack {
-        children: vec![plain.add_strong(&mut h.rsc), aligned.add_strong(&mut h.rsc)],
+        children: vec![plain.add_strong(&mut h.rsc), wrapped.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(),
-    ]
+    vec![plain.id(), wrapped.id(), stack.id(), root.id()]
 }
 
 fn dump(label: &str, report: &diag::Report, text: WidgetId) {
@@ -42,12 +37,12 @@ fn dump(label: &str, report: &diag::Report, text: WidgetId) {
             TraceEvent::DrawRequest {
                 id,
                 region,
-                pixel_size,
+                region_px,
                 ..
             } if *id == text => {
                 println!(
                     "  draw in {:.2}x{:.2}  region {region:?}",
-                    pixel_size.x, pixel_size.y
+                    region_px.x, region_px.y
                 )
             }
             TraceEvent::SizeReported { id, size } if *id == text => {
@@ -81,7 +76,7 @@ fn what_box_the_text_is_drawn_in() {
 
     for _ in 0..2 {
         for &id in &ids {
-            h.rsc.widgets_mut().get_dyn_mut(id);
+            h.rsc.widgets_mut().mark_for_redraw(id);
         }
         let _ = diag::take();
         h.frame();
@@ -93,23 +88,14 @@ fn what_box_the_text_is_drawn_in() {
 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 = text;
     h.rsc
         .widgets_mut()
         .set_alignment(text, Axis::X, AxisAlign::NEG);
-    let inner = (aligned,).span(Dir::RIGHT).add(&mut h.rsc);
-    let sized = inner.sized((189, 176)).add(&mut h.rsc);
+    let inner = (text,).span(Dir::RIGHT).sized((189, 176)).add(&mut h.rsc);
     let filler = rect(Color::RED).add(&mut h.rsc);
-    let root = (filler, sized).span(Dir::RIGHT).add(&mut h.rsc);
+    let root = (filler, inner).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(),
-    ]
+    vec![text.id(), inner.id(), filler.id(), root.id()]
 }
 
 #[test]