Drop three things nothing reads, and say what a span makes scalable

`Axis::pair` and `RegionAlign::NEAR` arrived on this branch with no caller
and never got one. `Holds::contains` took `&self` where its five siblings on
the same `Copy` pair of pixels take `self`.

The comment beside a span's cross-axis accumulator said a scalable child
"makes Children scalable too"; `Children` names nothing here, and what it
makes scalable is the span.

TODO

@@ -14,3 +14,27 @@ WidgetRef<W> or smth instead of Id
 vecs for each widget type?
 
 POTENTIAL BUG: closures that store IDs will not decrement the id!!! need to not increment id if moved into closure somehow??? wait no, need to decrement ID every time an event fn is added...... only if the id is used in it..??
+
+transforms on a move entry (scale + rotation)
+    an entry is a translation today; composing through one scales the rel
+    part and passes px through untouched, so fixed-size content and glyphs
+    do not follow a shortened entry
+    want a real transform per entry, resolved in resolve_move the way the
+    translation already is, so a whole subtree transforms with one buffer
+    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/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},
@@ -23,104 +23,71 @@ use std::{
     time::Instant,
 };
 
-#[derive(Clone, Copy)]
+/// Declares a counter or timer kind beside the name its report prints. Two
-pub(crate) enum Counter {
+/// lists in the same order was one list too many: a variant inserted without
-    Updates,
+/// its label moving with it renames every total after it, and nothing says
-    DrawRequests,
+/// so.
-    WidgetDraws,
+macro_rules! labelled {
-    RegionNodeDraws,
+    ($(#[$meta:meta])* $vis:vis enum $Name:ident { $($variant:ident = $label:literal,)* }) => {
-    SizeReads,
+        $(#[$meta])*
-    HintHits,
+        #[derive(Clone, Copy)]
-    HintMisses,
+        $vis enum $Name { $($variant,)* }
-    RetainedSizeHits,
+
-    ReuseAttempts,
+        impl $Name {
-    ReuseExact,
+            const COUNT: usize = [$($label,)*].len();
-    ReuseMoved,
+            const NAMES: [&'static str; Self::COUNT] = [$($label,)*];
-    ReuseDirty,
+        }
-    ReuseWrongParent,
+    };
-    ReuseRemapped,
-    ReuseOutside,
-    ReuseWrongLayer,
-    ReuseWrongNode,
-    PlaceRedraws,
-    QueuePops,
-    DepthReads,
-    LocalRedraws,
-    SizeChanges,
-    ReaderEdges,
-    PrimitiveWrites,
-    TextRenders,
-    TextShapeHits,
-    TextShapes,
-    TextBreaks,
-    GlyphPlacements,
-    OutsidePinnedLen,
-    OutsideFrame,
-    OutsideExtent,
 }
 
-impl Counter {
+labelled! {
-    const COUNT: usize = Self::OutsideExtent as usize + 1;
+    pub(crate) enum Counter {
-
+        Updates = "updates",
-    const NAMES: [&'static str; Self::COUNT] = [
+        DrawRequests = "draw requests",
-        "updates",
+        WidgetDraws = "widget draws",
-        "draw requests",
+        RegionNodeDraws = "region-node draws",
-        "widget draws",
+        SizeReads = "draw-result size reads",
-        "region-node draws",
+        HintHits = "hint hits",
-        "draw-result size reads",
+        HintMisses = "hint misses",
-        "hint hits",
+        ReuseAttempts = "reuse attempts",
-        "hint misses",
+        ReuseExact = "reuse exact",
-        "retained size hits",
+        ReuseMoved = "reuse moved",
-        "reuse attempts",
+        ReuseDirty = "reuse: dirty",
-        "reuse exact",
+        ReuseUndrawn = "reuse: nothing drawn to keep",
-        "reuse moved",
+        ReuseWrongParent = "reuse: wrong parent",
-        "reuse: dirty",
+        ReuseRemapped = "reuse remapped",
-        "reuse: wrong parent",
+        ReuseOutside = "reuse: outside what it holds for",
-        "reuse remapped",
+        ReuseWrongLayer = "reuse: another layer",
-        "reuse: outside what it holds for",
+        ReuseWrongNode = "reuse: region-node choice changed",
-        "reuse: another layer",
+        ReuseWrongMask = "reuse: a different inherited mask",
-        "reuse: region-node choice changed",
+        QueuePops = "redraw queue pops",
-        "placed by redrawing",
+        DepthReads = "depth reads",
-        "redraw queue pops",
+        LocalRedraws = "local redraws",
-        "depth reads",
+        SizeChanges = "size changes",
-        "local redraws",
+        ReaderEdges = "reader edges",
-        "size changes",
+        PrimitiveWrites = "primitive writes",
-        "reader edges",
+        TextRenders = "text renders",
-        "primitive writes",
+        TextShapeHits = "text shape hits",
-        "text renders",
+        TextShapes = "text shapes",
-        "text shape hits",
+        TextBreaks = "text line breaks",
-        "text shapes",
+        GlyphPlacements = "glyph placements",
-        "text line breaks",
+        OutsidePinnedLen = "reuse outside: the length it was pinned to",
-        "glyph placements",
+        OutsideWindow = "reuse outside: this window",
-        "reuse outside: the length it was pinned to",
+        OutsideRelBase = "reuse outside: a rel base",
-        "reuse outside: a frame length",
+        OutsideRegion = "reuse outside: a region length",
-        "reuse outside: an extent length",
+    }
-    ];
 }
 
-#[derive(Clone, Copy)]
+labelled! {
-pub(crate) enum TimerKind {
+    pub(crate) enum TimerKind {
-    Update,
+        Update = "update total",
-    FullLayout,
+        FullLayout = "full layout",
-    IncrementalLayout,
+        IncrementalLayout = "incremental layout",
-    TextRender,
+        TextRender = "text render",
-    TextShape,
+        TextShape = "text shape",
-    TextBreak,
+        TextBreak = "text line break",
-    GlyphPlacement,
+        GlyphPlacement = "glyph placement",
-}
+    }
-
-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",
-    ];
 }
 
 #[derive(Clone)]
@@ -255,6 +222,8 @@ pub enum ReuseOutcome {
     Dirty,
     WrongParent,
     WrongLayer,
+    WrongMask,
+    WrongNode,
     Remapped,
     Outside,
     Undrawn,
@@ -268,7 +237,7 @@ pub enum TraceEvent {
         id: WidgetId,
         parent: Option<WidgetId>,
         region: UiRegion,
-        pixel_size: PxVec2,
+        region_px: PxVec2,
         region_node: bool,
     },
     Reuse {
@@ -364,7 +333,7 @@ pub(crate) fn draw_request(
     id: WidgetId,
     parent: Option<WidgetId>,
     region: UiRegion,
-    pixel_size: PxVec2,
+    region_px: PxVec2,
     region_node: bool,
 ) {
     trace(
@@ -373,7 +342,7 @@ pub(crate) fn draw_request(
             id,
             parent,
             region,
-            pixel_size,
+            region_px,
             region_node,
         },
     );
@@ -383,6 +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

@@ -1,3 +1,4 @@
+use crate::util::impl_axis_index;
 use crate::{Px, Rel};
 
 use super::*;
@@ -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);
@@ -231,3 +210,5 @@ impl RegionAlign {
         UiVec2::from(self)
     }
 }
+
+impl_axis_index!(RegionAlign => 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,
+    /// Both of them, for the layout code that asks the same question of each.
-    /// which is what `from_axis` does for a vector.
+    pub const BOTH: [Self; 2] = [Self::X, Self::Y];
-    pub fn pair<T>(self, aligned: T, ortho: T) -> [T; 2] {
-        match self {
-            Self::X => [aligned, ortho],
-            Self::Y => [ortho, aligned],
-        }
-    }
 }
 
 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 {
@@ -148,3 +115,6 @@ impl<T> BothAxis<T> {
         }
     }
 }
+
+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();
@@ -294,6 +269,15 @@ impl UiSpan {
         }
     }
 
+    /// 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)
+    }
+
     pub const fn len(&self) -> Len {
         self.end - self.start
     }
@@ -348,20 +332,6 @@ 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 +432,6 @@ impl Display for PixelRegion {
         write!(f, "{} -> {}", self.top_left, self.bot_right)
     }
 }
+
+impl_axis_index!(UiVec2 => Len);
+impl_axis_index!(UiRegion => UiSpan);

core/src/ui/active.rs

@@ -1,6 +1,6 @@
 use crate::{
-    LayerId, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, Place, RegionAlign, RetainedPrimitive, Size,
+    Declared, LayerId, LayoutHolds, MaskIdx, MoveIdx, PlaceDesc, RegionAlign, RetainedPrimitive,
-    TextureHandle, UiRegion, WidgetId,
+    Size, TextureHandle, UiRegion, UiVec2, WidgetId,
 };
 
 /// What is kept of a widget its parent has asked about. `drawn` says whether
@@ -9,35 +9,34 @@ use crate::{
 #[derive(Debug)]
 pub struct ActiveData {
     pub id: WidgetId,
-    /// Its frame in `parent_move`'s coordinates: what a fraction it declares
+    /// Where its drawing goes, in its region node's coordinates.
-    /// or reports is a fraction of, composed. Everything it draws sits inside
+    pub placement: UiRegion,
-    /// this by way of `extent`.
+    /// What a fraction declared or reported under this widget is a fraction
-    pub frame_abs: UiRegion,
+    /// of, as a length of the window.
-    /// Where its drawing goes, in the frame's own coordinates.
+    pub rel_base: UiVec2,
-    pub extent: UiRegion,
+    /// Where its drawing was put, and where it was asked. The two differ
-    /// That frame in its parent's frame coordinates, before composition:
+    /// where a container asks in one place and puts the answer in another --
-    /// forwarded whole by a transparent container, narrowed by a declared
+    /// a row measures from its cursor and puts the child in its slot. Each
-    /// length. Its length is the same on every ask, which is what
+    /// carries the rel base that ask stated, so asking again from either is
-    /// a local redraw relies on to ask its parent's own question again.
+    /// the same question it was.
-    pub frame: UiRegion,
+    pub placed: PlaceDesc,
-    /// What of its parent's extent the drawing was given, and what it was
+    pub asked: PlaceDesc,
-    /// given at the parent's first ask of it -- the question a cold layout
+    /// The box it was asked in, in the parent's region-node coordinates: the
-    /// asks. A part is a length from the extent's start, so an extent that
+    /// box its drawing was made in and the one its contract is about. Its
-    /// moved re-places every child by re-adding that start.
+    /// drawing is placed elsewhere by re-expression, never by asking again.
-    pub place: [Place; 2],
+    pub region: UiRegion,
-    pub offer_place: [Place; 2],
-    /// The box that ask gave it, in its frame's coordinates. Kept rather
-    /// than worked out again from where its parent's own box is now: a
-    /// parent drawn again in the box its own answer chose gives its children
-    /// boxes it never measured anything in, and the measurement this widget
-    /// answered is the one its parent's layout was built on.
-    pub offer_part: 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>,
-    /// What the widget said it used of its frame, the last time it drew.
+    /// 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
+    /// parent can ask either again.
+    pub re_asked: bool,
+    /// What the widget reported, in window-unit lengths.
     pub size: Size,
-    /// The frame and extent reads that this drawing holds for.
+    /// The window and region reads that this drawing holds for, and the
+    /// rel base and region it pinned.
     pub holds: LayoutHolds,
     pub drawn: bool,
     pub parent: Option<WidgetId>,
@@ -47,23 +46,23 @@ pub struct ActiveData {
     pub depth: usize,
     pub textures: Vec<TextureHandle>,
     /// Its primitives, each keeping the box it was written in -- in this
-    /// widget's extent coordinates, which is what a move recomposes from.
+    /// widget's placement coordinates, which is what a move recomposes from.
     pub primitives: Vec<RetainedPrimitive>,
+    /// 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 frame.
+    /// 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,
-    /// The movable region whose coordinates `frame_abs` uses.
+    /// The movable region whose coordinates its placement is in when this
+    /// widget does not own a region node.
     pub parent_move: MoveIdx,
     /// The mask its drawing is clipped to: one it set itself, or the one it
     /// inherited from whoever drew it.
@@ -77,20 +76,24 @@ pub struct ActiveData {
 }
 
 impl ActiveData {
-    /// What it answered when its parent measured it, where it has been
+    /// What it answered when its parent asked, where it has been asked at
-    /// measured at all. Not `size`, which is what its last drawing reported:
+    /// all. Not `size`, which is what its last drawing reported: a drawing
-    /// a drawing made in the box that answer chose is answering a different
+    /// re-expressed in the box that answer chose is not a second answer.
-    /// question.
     pub fn measured(&self) -> Option<Size> {
-        self.answer.map(|(size, _)| size)
+        self.answer.map(|answer| answer.size)
     }
 
-    /// Whether what it answered still stands for a frame of these pixel
+    /// Whether it owns a region node rather than sharing the one it was drawn
-    /// lengths. The answer was given in the box its parent first asked
+    /// under, which is what its two move indices being different says.
-    /// about, which is what it is checked against -- `holds` on the record
+    pub fn is_region_node(&self) -> bool {
-    /// is about the box the answer then chose.
+        self.move_idx != self.parent_move
-    pub fn answers_at(&self, px: crate::PxVec2, part: UiRegion) -> bool {
-        self.answer
-            .is_some_and(|(_, holds)| holds.contains(px, part))
     }
 }
+
+/// 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

@@ -29,10 +29,16 @@ impl Holds {
         Self { lo: len, hi: len }
     }
 
-    pub const fn contains(&self, len: Px) -> bool {
+    pub const fn contains(self, len: Px) -> bool {
         len.raw() >= self.lo.raw() && len.raw() <= self.hi.raw()
     }
 
+    /// 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),

core/src/ui/layout_holds.rs

@@ -1,63 +1,108 @@
-use crate::{Axis, Holds, Len, PxVec2, UiRegion};
+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 along one axis: the window
-
+/// lengths its reads hold for, the pixel lengths of its own box, and the
-/// What one evaluation of a widget depends on: the pixel lengths of its
+/// symbolic lengths of that box and of its rel base where either one is what
-/// frame and of its own box that its drawing and its answer hold for, and
+/// it was expressed in.
-/// the symbolic length of its own box where it read one.
 ///
-/// The symbolic length is a pin rather than a range: a container places its
+/// The symbolic lengths are pins rather than ranges: a container places its
-/// children as lengths of its frame measured from where its own box starts,
+/// children as lengths of its rel base measured from where its own box starts,
 /// so what it draws turns on that box's length and on nothing about where it
-/// is. It reaches the parent only where the box it pinned is the parent's
+/// is. A box pin reaches the parent only where the box it pinned is the
-/// own; anywhere else the parent chose that length itself, and a widget
+/// parent's own; anywhere else the parent chose that length itself, and a
-/// pinned this way is checked when it is re-placed.
+/// widget pinned this way is checked when it is re-placed.
+///
+/// A rel base pin says the answer or the drawing is a fraction of the rel base,
+/// which is a different length wherever the rel base is a different one -- at
+/// the same window size, so no range of window pixels can say it. A length
+/// 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 frame: [Holds; 2],
+    pub x: AxisHolds,
-    pub extent: [Holds; 2],
+    pub y: AxisHolds,
-    pub extent_len: [Option<Len>; 2],
 }
 
 impl LayoutHolds {
     pub const ANY: Self = Self {
-        frame: [Holds::ANY; 2],
+        x: AxisHolds::ANY,
-        extent: [Holds::ANY; 2],
+        y: AxisHolds::ANY,
-        extent_len: [None; 2],
     };
 
     pub fn and(self, other: Self) -> Self {
-        let mut result = Self::ANY;
+        Self {
-        for n in 0..2 {
+            x: self.x.and(other.x),
-            result.frame[n] = self.frame[n].and(other.frame[n]);
+            y: self.y.and(other.y),
-            result.extent[n] = self.extent[n].and(other.extent[n]);
-            debug_assert!(
-                self.extent_len[n].is_none()
-                    || other.extent_len[n].is_none()
-                    || self.extent_len[n] == other.extent_len[n]
-            );
-            result.extent_len[n] = self.extent_len[n].or(other.extent_len[n]);
         }
-        result
     }
 
     pub fn covers(self, other: Self) -> bool {
-        (0..2).all(|n| {
+        self.x.covers(other.x) && self.y.covers(other.y)
-            self.frame[n].lo <= other.frame[n].lo
-                && self.frame[n].hi >= other.frame[n].hi
-                && self.extent[n].lo <= other.extent[n].lo
-                && self.extent[n].hi >= other.extent[n].hi
-                && self.extent_len[n].is_none_or(|len| other.extent_len[n] == Some(len))
-        })
     }
 
-    pub fn contains(self, px: PxVec2, extent: UiRegion) -> bool {
+    pub fn contains(self, window: PxVec2, rel_base: UiVec2, region: UiRegion) -> bool {
-        AXES.into_iter().all(|axis| {
+        Axis::BOTH
-            let n = axis as usize;
+            .into_iter()
-            let len = extent.axis(axis).len();
+            .all(|axis| self[axis].contains(window[axis], rel_base[axis], region[axis].len()))
-            self.frame[n].contains(px.axis(axis))
-                && self.extent[n].contains(len.to_px(px.axis(axis)))
-                && self.extent_len[n].is_none_or(|pinned| pinned == 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

@@ -1,72 +1,54 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
-    Axis, Holds, LayoutHolds, LayoutLen, Len, Part, Place, Px, PxVec2, RegionAlign, RenderedText,
+    Axis, Declared, Holds, LayoutHolds, LayoutLen, Len, PlaceDesc, Px, PxVec2, RegionAlign, Rel,
-    RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData, TextureHandle,
+    RenderedText, RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData,
-    UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
+    TextureHandle, UiRegion, UiRenderState, UiRsc, UiVec2, Weight, WidgetId, Widgets,
     render::{
         GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveInst, PrimitiveKind,
         TexturePrimitive,
     },
-    ui::render_state::DrawInfo,
+    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> {
     pub(super) state: &'a mut UiRenderState,
     pub(super) rsc: &'a mut dyn UiRsc,
 
-    /// What a fraction this widget declares or reports is a fraction of, in
+    /// This widget's rel base, per axis: a length of the window, and what a
-    /// the coordinates of `move_idx`: forwarded from its parent unchanged
+    /// fraction it or anything under it declares or reports is a fraction
-    /// through a span, a stack or a scroll, and narrowed only by what was
+    /// of. A length rather than a box, so padding can take from both the
-    /// decided above it -- a declared length, or the root. Its length
+    /// rel base and the box without either becoming the other.
-    /// is the same on every ask of the widget, which is what keeps a fraction
+    pub(super) rel_base: UiVec2,
-    /// under it from being resolved twice.
+    /// The box this widget was asked in, in its region node's coordinates:
-    pub(super) frame: UiRegion,
+    /// what it draws in, and what its children's places are parts of.
-    /// Where this widget's drawing goes, in the frame's own coordinates.
+    pub(super) region: UiRegion,
-    /// Everything it writes is in these coordinates, and its children are
+    /// The window in pixels. Frames and boxes become pixels against this one
-    /// placed as parts of it.
+    /// unit, regardless of region-node boundaries.
-    pub(super) extent: UiRegion,
+    pub(super) window: PxVec2,
-    /// The extent's symbolic length where this draw read it, which makes the
-    /// drawing one that holds for that length alone -- the way reading a
-    /// length in pixels makes it hold for that number of pixels.
-    pub(super) extent_len: [Option<Len>; 2],
-    /// Symbolic box lengths read only to compute the answer. A container can
-    /// replace the provisional drawings used for that answer with drawings
-    /// in decided boxes, so this contract is independent of the final one.
-    pub(super) answer_extent_len: [Option<Len>; 2],
-    /// The frame in pixels, which its children's frames are a length of:
-    /// threaded down rather than composed back up the chain, so every length
-    /// in layout is one multiply from its parent's and [`Holds::through`]
-    /// inverts exactly that.
-    pub(super) px: PxVec2,
     pub(super) mask: MaskIdx,
     pub(super) textures: Vec<TextureHandle>,
     pub(super) primitives: Vec<RetainedPrimitive>,
     pub(super) mask_region: Option<UiRegion>,
+    /// The previous drawing's owned mask, available for this draw to reclaim.
+    pub(super) mask_slot: Option<MaskIdx>,
     /// Only children whose answers were read constrain this widget's answer.
     pub(super) answer_under: LayoutHolds,
     pub(super) children: Vec<WidgetId>,
-    /// The children asked about so far, so the first place each was asked in
-    /// is the one recorded as its offer.
-    pub(super) offered: Vec<WidgetId>,
-    /// Whether this draw is at the place its parent first asked about, which
-    /// makes the questions it asks the ones a cold layout asks and their
-    /// answers the ones to keep.
-    pub(super) at_offer: bool,
     /// The children whose size this widget read while drawing.
     pub(super) size_deps: Vec<WidgetId>,
-    /// What this draw itself read of its frame in pixels, per axis: every
+    /// What this draw itself reads, as against what its children's drawings
-    /// length until it reads one, then that one, unless it says otherwise.
+    /// hold for: every window and every length of its own region until it
-    pub(super) frame_own: [Holds; 2],
+    /// reads one, then that one unless it says otherwise, and the rel base or
-    /// The same for its own box.
+    /// region length it read symbolically, each of which makes the drawing
-    pub(super) extent_own: [Holds; 2],
+    /// hold for that length alone.
-    /// Pixel-box dependencies used only to compute the answer. These do not
+    pub(super) own: LayoutHolds,
-    /// constrain a retained drawing placed inside that answer.
+    /// What each child's drawing depends on. Asking a child again replaces
-    pub(super) answer_extent_own: [Holds; 2],
+    /// its drawing, so it replaces this too rather than narrowing it.
-    /// The final drawing kept for each child. Asking one child again replaces
-    /// its provisional drawing and therefore replaces this contract too.
     pub(super) under: Vec<(WidgetId, LayoutHolds)>,
     /// The movable region this widget's primitives are positioned through:
     /// its own when opted in, otherwise the nearest ancestor's.
@@ -90,12 +72,10 @@ impl<'a> Painter<'a> {
         self.write_resolved(kind, primitive, region, self.resolve(region));
     }
 
-    /// A box in this widget's extent coordinates, composed into the
+    /// A box in this widget's region, composed into its region node's
-    /// coordinates its move slot is in: through the extent, then through the
+    /// coordinates.
-    /// frame the extent is a part of. The same two steps a recomposition
-    /// replays, so a moved drawing lands where a cold one does.
     fn resolve(&self, region: UiRegion) -> UiRegion {
-        region.within(&self.extent).within(&self.frame)
+        region.within(&self.region)
     }
 
     fn write_resolved<P: Primitive>(
@@ -123,7 +103,6 @@ impl<'a> Painter<'a> {
 
     fn push_primitive(&mut self, h: RetainedPrimitive) {
         if self.mask != MaskIdx::NONE {
-            // TODO: I have no clue if this works at all :joy:
             self.rsc.ui_mut().masks.push_ref(self.mask);
         }
         self.primitives.push(h);
@@ -148,75 +127,83 @@ impl<'a> Painter<'a> {
         assert!(self.mask == MaskIdx::NONE);
         let resolved = self.resolve(region);
         let move_idx = self.move_idx;
-        self.mask = self.rsc.ui_mut().masks.push(Mask {
+        let mask = Mask {
             region: resolved,
             move_idx,
-        });
+        };
+        let masks = &mut self.rsc.ui_mut().masks;
+        self.mask = match self.mask_slot.take() {
+            Some(idx) => {
+                *masks.get_mut(idx) = mask;
+                idx
+            }
+            None => {
+                let idx = masks.push(mask);
+                // The owner keeps the slot alive even with no primitives.
+                masks.push_ref(idx);
+                idx
+            }
+        };
     }
 
-    /// Draws a widget in the whole of this widget's own box, with the frame
+    /// Draws a widget in the whole of this widget's own box, with the rel
-    /// forwarded unchanged: what a container that is only a wrapper around
+    /// base forwarded unchanged: what a container that is only a wrapper
-    /// one child wants, and what every transparent container passes for the
+    /// around one child wants.
-    /// frame.
     pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
-        self.widget_at(id, UiRegion::FULL, [Place::Within(Part::All); 2])
+        self.widget_at(id, UiRegion::FULL)
     }
 
-    /// Draws a child, saying what its fractions are of and where its drawing
+    /// Resolves what the place says about the child's rel base into a length,
-    /// goes.
+    /// 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 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();
+            at.rel_base = match len == Len::FULL {
+                true => RelBase::Inherit,
+                false => RelBase::Len(len.within_len(self.rel_base(axis))),
+            };
+        }
+        place
+    }
+
+    /// Asks a child, saying what its fractions are of and where it is asked.
     ///
-    /// `frame` is that reference, in this widget's own frame coordinates:
+    /// `place` says where the child goes and what its fractions are of:
-    /// [`UiRegion::FULL`] forwards this widget's frame, which is what a
+    /// see [`PlaceDesc`]. A `UiRegion` converts into the common case, which
-    /// container that only divides room passes, so a fraction under it means
+    /// is a box of this widget's own with the answer placed inside it.
-    /// the same wherever it sits and however deeply it is nested. Narrowing
-    /// it is for what is decided from above, and a declared length narrows
-    /// it here.
     ///
-    /// `place` is where the drawing goes, per axis, as a part of this
+    /// The child draws once, in the region that comes of it, and its answer
-    /// widget's extent: see [`Place`]. A narrowed frame is its own extent,
+    /// is placed inside that region by re-expressing the drawing. Nothing is
-    /// since the narrowing is what said where the drawing goes.
+    /// drawn again in a box an answer chose; a container that puts the
+    /// answer somewhere else says so with [`Self::place_at`].
     pub fn widget_at<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
-        frame: UiRegion,
+        place: impl Into<PlaceDesc>,
-        place: [Place; 2],
     ) -> DrawResult<'s, 'a, W> {
+        let 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());
-        let (local, extent) = frame_and_extent(
+        let (rel_base, region) =
-            frame,
+            place.rel_base_and_region(self.region, self.rel_base, declared, align);
-            part_of(self.extent, place),
-            narrowed_by(declared, frame),
-            align,
-        );
-        let within = match local == UiRegion::FULL {
-            true => self.frame,
-            false => local.within(&self.frame),
-        };
         #[cfg(feature = "layout-diagnostics")]
         if region_node {
             diag::bump(Counter::RegionNodeDraws);
-            diag::region_node(id.id(), self.id, within);
+            diag::region_node(id.id(), self.id, region);
         }
         // A child listed twice would be moved twice.
-        if !self.children.contains(&id.id()) {
+        let re_asked = self.children.contains(&id.id());
+        if !re_asked {
             self.children.push(id.id());
         }
-        let first_ask = self.offer(id.id());
+        let drawn = self.state.draw_inner(
-        let offer_place = if first_ask {
-            place
-        } else {
-            self.state
-                .active
-                .get(&id.id())
-                .map_or(place, |a| a.offer_place)
-        };
-        let px = local.size().to_px(self.px);
-        // The answer and what it holds for, both about the place asked in.
-        // The child's record may say something else once its drawing has been
-        // placed: a drawing made again in its placed box holds for that box.
-        let (size, answer_holds, holds) = self.state.draw_inner(
             id.id(),
             DrawInfo {
                 layer: self.layer,
@@ -225,18 +212,17 @@ impl<'a> Painter<'a> {
                 parent_move: self.move_idx,
                 region_node,
                 mask: self.mask,
-                frame: local,
+                rel_base,
-                frame_abs: within,
+                region,
-                part: extent,
+                placed: place,
-                place,
+                asked: place,
-                offer_place,
+                re_asked,
-                px,
             },
             None,
             self.rsc,
         );
-        let compose = |holds| in_parent(holds, local, extent, place, declared);
+        let holds = self.in_parent(drawn.drawing_holds, region, place, declared);
-        let holds = compose(holds);
+        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)),
@@ -244,8 +230,8 @@ impl<'a> Painter<'a> {
         DrawResult {
             child: id,
             painter: self,
-            size: in_parent_frame(size, local.size(), declared),
+            size: drawn.answer.size,
-            answer_holds: compose(answer_holds),
+            answer_holds,
         }
     }
 
@@ -258,53 +244,95 @@ impl<'a> Painter<'a> {
         self.state.undraw_rec(id.id(), self.rsc);
     }
 
-    /// What a widget's rules declare its lengths to be, which whoever draws
+    /// Puts a child in `place` of this widget's box, where that box is the
-    /// it resolves into its frame. Reading them depends on nothing -- the box
+    /// answer the child already gave: the drawing is re-expressed there
-    /// that comes of them is kept on the child, and `redraw` compares it
+    /// rather than made again -- what a row does once it knows every slot,
-    /// there.
+    /// having measured each child from its cursor.
-    fn declared_lens<W: ?Sized>(&self, id: &StrongWidget<W>) -> [Option<LayoutLen>; 2] {
+    ///
-        declared_lens(self.rsc.widgets(), id.id())
+    /// A child this draw has not asked about, and one whose rel base this
+    /// narrows, is asked here instead: there is no answer to re-express, or
+    /// the question has changed. So a container that places every child the
+    /// same way says it once, and which of the two happens is this widget's
+    /// business rather than the caller's.
+    pub fn place_at<'s, W: ?Sized>(
+        &'s mut self,
+        id: &'s StrongWidget<W>,
+        place: impl Into<PlaceDesc>,
+    ) -> DrawResult<'s, 'a, W> {
+        let place = self.resolve_rel_base(place.into());
+        let states_rel_base = Axis::BOTH
+            .iter()
+            .any(|&axis| matches!(place[axis].rel_base, RelBase::Len(_)));
+        if states_rel_base || !self.children.contains(&id.id()) {
+            return self.widget_at(id, place);
+        }
+        let at = self.placing();
+        self.state.place_in(id.id(), &at, place, self.rsc);
+        let active = &self.state.active[&id.id()];
+        let size = active.measured().unwrap_or(active.size);
+        DrawResult {
+            child: id,
+            painter: self,
+            size,
+            // Read where it was asked; moving it is not a second answer.
+            answer_holds: LayoutHolds::ANY,
+        }
     }
 
-    /// What a child says its length is without being drawn, if it can say.
+    /// This widget as the thing its children are placed within.
-    /// Asking counts as reading its size.
+    fn placing(&self) -> Placing {
+        Placing {
+            id: self.id,
+            region: self.region,
+            rel_base: self.rel_base,
+            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
+    /// that comes of them is kept on the child, and `redraw` compares it
+    /// there.
+    fn declared_lens<W: ?Sized>(&self, id: &StrongWidget<W>) -> Declared {
+        self.rsc.widgets().declared_lens(id.id())
+    }
+
+    /// What a child says its length is without being drawn, if it can say,
+    /// as the length its draw would report: a fraction in it is resolved
+    /// 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(|| {
+        let hint = widgets.size_rules(id.id())[axis].exact().or_else(|| {
             widgets
                 .get_dyn(id.id())
                 .and_then(|widget| widget.size_hint(axis))
         });
+        let rel_base = self.rel_base[axis];
+        let resolved = hint.map(|hint| hint.within_len(rel_base));
         #[cfg(feature = "layout-diagnostics")]
-        diag::hint_read(id.id(), self.id, axis, hint);
+        {
-        match hint {
+            diag::hint_read(id.id(), self.id, axis, resolved);
-            Some(hint) => {
+            diag::bump(match resolved {
-                #[cfg(feature = "layout-diagnostics")]
+                Some(_) => Counter::HintHits,
-                diag::bump(Counter::HintHits);
+                None => Counter::HintMisses,
-                self.depend_on(id);
+            });
-                Some(hint)
+        }
-            }
+        if let Some(hint) = hint {
-            None => {
+            self.depend_on(id);
-                #[cfg(feature = "layout-diagnostics")]
+            // Resolving a fraction against this rel base makes this draw a
-                diag::bump(Counter::HintMisses);
+            // function of the rel base's length. The fraction to ask about is
-                None
+            // 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[axis].rel_base = Some(rel_base);
             }
         }
-    }
+        resolved
-
-    /// Whether this is the first box a child is asked about in during a draw
-    /// that is itself the one its parent measured -- the question a cold
-    /// layout asks, whose answer is the one to keep. A drawing made again in
-    /// a box chosen from an answer asks about that box instead, and what it
-    /// hears back is not a measurement of anything.
-    fn offer(&mut self, child: WidgetId) -> bool {
-        if !self.at_offer || self.offered.contains(&child) {
-            return false;
-        }
-        self.offered.push(child);
-        true
     }
 
     fn depend_on<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
@@ -325,11 +353,11 @@ impl<'a> Painter<'a> {
         ui.text.render(buffer, attrs, width)
     }
 
-    /// Writes glyphs in the selected frame or extent coordinates.
+    /// Writes glyphs in the selected rel base or region coordinates.
     // TODO: merge the text methods into the primitive ones.
     pub fn glyphs(&mut self, text: &RenderedText, origin: UiRegion) {
         // Glyph offsets and sizes are pixels, which compose additively.
-        // Only the shared origin needs composing through the extent.
+        // Only the shared origin needs composing through the region.
         let resolved = self.resolve(origin);
         let kind = self.rsc.ui_mut().primitives.kind::<GlyphPrimitive>();
         for glyph in text.glyphs.iter() {
@@ -361,39 +389,28 @@ impl<'a> Painter<'a> {
     }
 
     /// The symbolic length of this widget's own box along one axis, in the
-    /// lengths of its frame that it places its children in. Reading it pins
+    /// lengths of its rel base that it places its children in. Reading it pins
     /// the drawing to that length -- and to nothing about where the box
     /// starts, which is what lets a container move without being drawn
     /// again. One axis at a time, because a container that divides one axis
     /// holds for any length of the other.
-    pub fn extent_len(&mut self, axis: Axis) -> Len {
+    pub fn region_len(&mut self, axis: Axis) -> Len {
-        let len = self.extent.axis(axis).len();
+        let len = self.region[axis].len();
-        self.extent_len[axis as usize] = Some(len);
+        self.own[axis].region_len = Some(len);
-        self.answer_extent_len[axis as usize] = Some(len);
         len
     }
 
-    /// The symbolic length used to compute this widget's answer, where the
+    /// This widget's rel base along one axis: what a fraction it or anything
-    /// final drawing itself is rebuilt without depending on that length.
+    /// under it declares or reports is a fraction of. A container reads it
-    pub fn answer_extent_len(&mut self, axis: Axis) -> Len {
+    /// to hand a length of it down -- padding, which takes its pixels off.
-        let len = self.extent.axis(axis).len();
+    /// Reading it pins the drawing to that rel base, the way
-        self.answer_extent_len[axis as usize] = Some(len);
+    /// [`Self::region_len`] pins it to the box.
+    pub fn rel_base(&mut self, axis: Axis) -> Len {
+        let len = self.rel_base[axis];
+        self.own[axis].rel_base = Some(len);
         len
     }
 
-    /// Says that the final drawing uses a symbolic length already read for
-    /// the answer.
-    pub fn drawing_uses_extent_len(&mut self, axis: Axis, len: Len) {
-        debug_assert_eq!(self.extent.axis(axis).len(), len);
-        self.extent_len[axis as usize] = Some(len);
-    }
-
-    /// A part of this widget's box, expressed in its frame coordinates so it
-    /// can be used as a child frame decided here.
-    pub fn extent_part(&self, axis: Axis, part: Part) -> UiSpan {
-        part.of(*self.extent.axis(axis))
-    }
-
     /// Where this widget sits in a box longer than the length it takes. A
     /// widget that positions its own content reads it to place that content
     /// the way the box around it would have placed the widget.
@@ -411,10 +428,7 @@ 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
+        self.rsc.widgets().size_rules(self.id)[axis]
-            .widgets()
-            .size_rules(self.id)
-            .axis(axis)
             .exact()
             .is_some()
     }
@@ -425,35 +439,16 @@ impl<'a> Painter<'a> {
         PxVec2::new(self.px_len(Axis::X), self.px_len(Axis::Y))
     }
 
-    /// This widget's own box in pixels, used only to compute its answer.
-    pub fn answer_px_size(&mut self) -> PxVec2 {
-        PxVec2::new(
-            self.answer_px_len(Axis::X),
-            self.answer_px_len(Axis::Y),
-        )
-    }
-
     /// 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 part = self.extent.axis(axis).len();
+        let len = self.region[axis].len();
-        let len = part.to_px(self.px.axis(axis));
+        let px = len.to_px(self.window[axis]);
-        let own = &mut self.extent_own[axis as usize];
+        let own = &mut self.own[axis].region;
         if *own == Holds::ANY {
-            *own = Holds::at(len);
+            *own = Holds::at(px);
         }
-        len
+        px
-    }
-
-    /// One pixel length used only to compute this widget's answer. The final
-    /// drawing may be retained when that answer is placed in another box.
-    pub fn answer_px_len(&mut self, axis: Axis) -> Px {
-        let len = self.extent.axis(axis).len().to_px(self.px.axis(axis));
-        let own = &mut self.answer_extent_own[axis as usize];
-        if *own == Holds::ANY {
-            *own = Holds::at(len);
-        }
-        len
     }
 
     /// The lengths of this widget's own box on `axis` that what it is drawing
@@ -461,41 +456,45 @@ 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 part = self.extent.axis(axis).len();
+        let len = self.region[axis].len();
         let holds = holds.into();
         debug_assert!(
-            holds.contains(part.to_px(self.px.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.extent_own[axis as usize] = holds;
+        self.own[axis].region = holds;
     }
 
-    /// One axis of this widget's frame in pixels -- what a fraction of its
+    /// A window length in pixels, which is what every length in layout is
-    /// area resolves against, and so what a container divides among its
+    /// measured in. Reading one pins the drawing to this window wherever the
-    /// children. Its own box is a part of this one.
+    /// length is a fraction of it; one that is only pixels is that many
-    pub fn frame_px_len(&mut self, axis: Axis) -> Px {
+    /// pixels in any window and pins nothing.
-        let len = self.px.axis(axis);
+    pub fn to_px(&mut self, len: Len, axis: Axis) -> Px {
-        let own = &mut self.frame_own[axis as usize];
+        let window = self.window[axis];
-        if *own == Holds::ANY {
+        if len.rel != Rel::ZERO {
-            *own = Holds::at(len);
+            let own = &mut self.own[axis].window;
+            if *own == Holds::ANY {
+                *own = Holds::at(window);
+            }
         }
-        len
+        len.to_px(window)
     }
 
-    /// [`Self::holds`] stated about the frame rather than about this
+    /// The windows this drawing holds for, stated rather than taken: a
-    /// widget's own box, for a container whose drawing turns on what its
+    /// container that branched on a length in pixels says which side of the
-    /// fractions are of rather than on the part of it it took.
+    /// boundary it was on, which is wider than the one window reading that
-    pub fn frame_holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
+    /// length pins, and replaces it.
+    pub fn window_holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
         let holds = holds.into();
         debug_assert!(
-            holds.contains(self.px.axis(axis)),
+            holds.contains(self.window[axis]),
-            "'{}' ({:?}) says its drawing holds for lengths that leave out its frame",
+            "'{}' ({:?}) says its drawing holds for windows that leave out this one",
             self.label(),
             self.id
         );
-        self.frame_own[axis as usize] = holds;
+        self.own[axis].window = holds;
     }
 
     pub fn text_data(&mut self) -> &mut TextData {
@@ -554,7 +553,7 @@ impl<W: ?Sized> DrawResult<'_, '_, W> {
     }
 
     pub fn len(self, axis: Axis) -> LayoutLen {
-        self.size().axis(axis)
+        self.size()[axis]
     }
 }
 
@@ -583,187 +582,169 @@ impl PrimitiveLike for &TextureHandle {
     }
 }
 
-/// What a child depends on, said about the boxes the widget that drew it
+impl Painter<'_> {
-/// has rather than the ones the child was given.
+    /// Moves what a child depends on into this widget's own terms, taking
-///
+    /// only what the child was asked with.
-/// `frame` is the child's frame in this widget's frame coordinates and
+    ///
-/// `extent` the box it was given, in the child's own frame coordinates. Both
+    /// Window ranges are already about the one unit and combine directly.
-/// reach it as one length, so what it holds for maps back through that
+    /// A rel base pin becomes this widget's own rel base wherever a length of it
-/// length exactly -- and where the box it was given is this widget's own,
+    /// is what reached the child; where only pixels did, no length of this
-/// what it says about that box is what this widget can say about its own.
+    /// rel base can change the child's and the pin stops here.
-pub(crate) fn in_parent(
+    ///
-    holds: LayoutHolds,
+    /// A child's validity maps back through the part of this widget's box,
-    frame: UiRegion,
+    /// where the box the child was asked in is that part; a declared length
-    extent: UiRegion,
+    /// places the box inside the part instead, and then only that length
-    place: [Place; 2],
+    /// reaches the child. A narrowed rel base is not one of these: it decides
-    declared: [Option<LayoutLen>; 2],
+    /// what fractions under the child mean and leaves the box the part it
-) -> LayoutHolds {
+    /// was given.
-    let mut result = LayoutHolds::ANY;
+    fn in_parent(
-    for axis in AXES {
+        &self,
-        let n = axis as usize;
+        holds: LayoutHolds,
-        let frame_len = frame.axis(axis).len();
+        region: UiRegion,
-        result.frame[n] = holds.frame[n].through(frame_len);
+        place: PlaceDesc,
-        match (place[n].part(), declared[n]) {
+        declared: Declared,
-            // Its box is this widget's own, or a part of it in that box's
+    ) -> LayoutHolds {
-            // own lengths: so what it holds for is a range on this widget's
+        let mut result = LayoutHolds::ANY;
-            // own box, which is what lets that box move without a redraw. A
+        for axis in Axis::BOTH {
-            // length it pinned is this widget's length wherever the part is
+            let declared = declared[axis];
-            // the whole of it, and pins the same way.
+            let holds = holds[axis];
-            (Part::All, None) if *frame.axis(axis) == UiSpan::FULL => {
+            let at = place[axis];
-                result.extent[n] = holds.extent[n];
+            let result = &mut result[axis];
-                result.extent_len[n] = holds.extent_len[n];
+            // Every read became pixels against the window, so a range on
-            }
+            // it is already in this widget's terms.
-            // Its box is a part of this widget's own box, in that box's own
+            result.window = holds.window;
-            // lengths, so what it holds for maps back through that part into
+            // A length this widget named -- a resolved share, a box a sibling
-            // a range on this widget's box.
+            // decided, a box it sized outright, which is its own base -- is
-            (Part::Of(span), None) => {
+            // not a length of this widget's rel base, so a pin on it stops
-                result.extent[n] = holds.extent[n].through(span.len());
+            // here. So does a declaration in pixels: no length of either base
-            }
+            // is in it to see.
-            // Its box is a part of this widget's frame: a length of the
+            let reaches = !matches!(at.rel_base, RelBase::Len(_))
-            // frame is all that reaches it, so what it holds for is a range
+                && declared.is_none_or(|len| len.rel != Rel::ZERO);
-            // on the frame and none of it on this widget's own box.
+            result.rel_base = holds.rel_base.and(reaches.then(|| self.rel_base[axis]));
-            _ => {
+            match (at.span, declared) {
-                result.frame[n] = result.frame[n].and(
+                // Its box is a part of this widget's own box, in that box's
-                    holds.extent[n]
+                // own lengths, so what it holds for maps back through that
-                        .through(extent.axis(axis).len())
+                // part into a range on this widget's box. A length it pinned
-                        .through(frame_len),
+                // is this widget's length less the part's pixels where the
-                );
+                // 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.
+                (PlaceSpan::Within(span), None) => {
+                    let part_len = span.len();
+                    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].len(),
+                    });
+                }
+                // Its box is a length this widget decided, from its own
+                // rel base or from a sibling's answer: no length of this
+                // 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 = result.window.and(holds.region.through(region[axis].len()));
+                }
             }
         }
+        result
     }
-    result
 }
 
-/// A child's answer as lengths of the parent's own region. A widget reports
+impl Widgets {
-/// a fraction of its own region, and `of` is that region as a length of this
+    /// What a widget's box is where a rule or its own hint says so outright.
-/// one. Pixels come through untouched, being that many pixels wherever they
+    pub(super) fn declared_lens(&self, id: WidgetId) -> Declared {
-/// end up. A declared axis is already the parent's: it resolved the rule in
+        let rules = self.size_rules(id);
-/// its own region, and the rule is what the report says.
+        let widget = self.get_dyn(id);
-fn in_parent_frame(size: Size, of: UiVec2, declared: [Option<LayoutLen>; 2]) -> Size {
+        Declared::from_axes(|axis| {
-    let mut size = size;
+            rules[axis].declared().or_else(|| {
-    for (axis, declared) in AXES.into_iter().zip(declared) {
+                // A hint still narrows the box where no rule does, which is
-        if declared.is_none() {
+                // how a widget with a natural pixel size -- an image, a gap
-            *size.axis_mut(axis) = size.axis(axis).within_len(of.axis(axis));
+                // -- 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
-    size
+                // the box it was offered.
-}
+                widget
-
+                    .and_then(|widget| widget.size_hint(axis))
-/// What a widget declares a length of its box to be. `leftover` is not one: a
+                    .and_then(LayoutLen::declared)
-/// 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(|| {
-            // 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)
         })
-    })
-}
-
-/// 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
-}
-
-/// 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 frame, 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 frame it was reported of.
-pub(crate) fn placed_extent(
-    part: UiRegion,
-    size: Size,
-    declared: [Option<LayoutLen>; 2],
-    fill: [bool; 2],
-    align: RegionAlign,
-) -> UiRegion {
-    let mut placed = part;
-    for axis in AXES {
-        let n = axis as usize;
-        let reported = size.axis(axis);
-        if fills(reported, declared[n], fill[n]) {
-            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
 }
 
-/// The part of a widget's own box a `place` names, in the coordinates that
+impl LayoutLen {
-/// box is in.
+    /// Whether what a widget reported along an axis is the whole of the box
-pub(crate) fn part_of(extent: UiRegion, place: [Place; 2]) -> UiRegion {
+    /// it is in rather than a part to be placed inside it. A share fills,
-    let mut part = extent;
+    /// because a share is a length only to whoever divides one, and whoever
-    for axis in AXES {
+    /// did is the one that handed down this box. A declared axis does too:
-        *part.axis_mut(axis) = place[axis as usize].part().of(*extent.axis(axis));
+    /// 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
     }
-    part
 }
 
-/// The length a rule gives a child's frame, per axis: a fraction in it is a
+impl PlaceDesc {
-/// fraction of the frame the child was given, which is the one length the
+    /// Where a widget's drawing goes inside the part its parent gave it: what
-/// rule can mean.
+    /// it reported, on the side of the part its alignment says, and the whole
-pub(crate) fn narrowed_by(declared: [Option<LayoutLen>; 2], frame: UiRegion) -> [Option<Len>; 2] {
+    /// part wherever the answer fills it.
-    AXES.map(|axis| {
+    ///
-        declared[axis as usize]
+    /// The length it reported is a length of its rel base, and the part is one
-            .map(|len| Len::from_parts(len.rel, len.px).within_len(frame.axis(axis).len()))
+    /// 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.
-/// The frame a child is asked in and the box its drawing goes in, both in
+    pub(super) fn placement(
-/// the coordinates of the widget asking.
+        self,
-///
+        region: UiRegion,
-/// `frame` is what the caller said the child's fractions are of, and `part`
+        size: Size,
-/// what of the caller's own box the drawing takes. `narrow` is the length a
+        declared: Declared,
-/// declared rule gives the frame, which makes the frame the box the drawing
+        align: RegionAlign,
-/// goes in -- a rule is what decided where it goes, and there is nothing
+    ) -> UiRegion {
-/// left to place inside it. A caller that narrowed the frame itself said the
+        let mut placed = region;
-/// same thing.
+        for axis in Axis::BOTH {
-///
+            let reported = size[axis];
-/// The length is the caller's to supply so that a widget asked again gets
+            if reported.fills(declared[axis], self[axis].fills) {
-/// the frame it already has rather than a second resolution of its rule.
+                continue;
-pub(crate) fn frame_and_extent(
-    mut frame: UiRegion,
-    part: UiRegion,
-    narrow: [Option<Len>; 2],
-    align: RegionAlign,
-) -> (UiRegion, UiRegion) {
-    let mut extent = part;
-    for (axis, narrow) in AXES.into_iter().zip(narrow) {
-        let span = frame.axis_mut(axis);
-        let narrowed = match narrow {
-            Some(len) => {
-                let slot = part.axis(axis);
-                let start = slot.start + (slot.len() - len).scale(align.axis(axis).rel());
-                *span = UiSpan::new(start, start + len);
-                true
             }
-            None => *span != UiSpan::FULL,
+            placed[axis] = placed[axis].place(reported.without_leftover(), align[axis]);
-        };
-        if narrowed {
-            *extent.axis_mut(axis) = UiSpan::FULL;
         }
+        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(super) fn rel_base_and_region(
+        self,
+        own: UiRegion,
+        parent_rel_base: UiVec2,
+        declared: Declared,
+        align: RegionAlign,
+    ) -> (UiVec2, UiRegion) {
+        let given = self.of(own, align);
+        let mut rel_base = parent_rel_base;
+        let mut region = given;
+        for axis in Axis::BOTH {
+            let base = match self[axis].rel_base {
+                RelBase::Len(len) => len,
+                RelBase::Inherit | RelBase::WithRegion => parent_rel_base[axis],
+            };
+            let len = declared[axis]
+                .map(|len| len.within_len(base))
+                .unwrap_or(base);
+            rel_base[axis] = len;
+            if declared[axis].is_some() {
+                region[axis] = given[axis].place(len, align[axis]);
+            }
+        }
+        (rel_base, region)
     }
-    (frame, extent)
 }

core/src/ui/place.rs

@@ -1,54 +1,195 @@
-use crate::{PrimitiveHandle, UiRegion, UiSpan};
+use crate::util::impl_axis_index;
+use crate::{Axis, AxisAlign, Len, PrimitiveHandle, RegionAlign, UiRegion, UiSpan};
 
-/// What of a widget's own box a child is given, along one axis.
+/// How a child's region along one axis comes from the region of the widget
+/// asking, and what its fractions are of.
+///
+/// The three ways of saying a region are the three the geometry already has:
+/// a span composed into the caller's box, a span shifted to where that box
+/// starts, and a length placed in it by alignment. Which one is meant cannot
+/// be read off the numbers, since two of them take the same span and apply
+/// it differently, so it is said here.
 #[derive(Clone, Copy, Debug, PartialEq)]
-pub enum Part {
+pub struct PlaceDescAxis {
-    /// The whole of it.
+    pub span: PlaceSpan,
-    All,
+    pub fills: bool,
-    /// Frame lengths from where the box starts, which is what a container
+    pub rel_base: RelBase,
-    /// dividing room among its children speaks: a child's report is a length
-    /// of the frame, so the cursor that sums those reports is one too. A
-    /// moved box re-places every child by re-adding its start, exactly.
-    From(UiSpan),
-    /// A part of the box in its own coordinates, which is what a container
-    /// that insets one speaks: taking eleven pixels off the end needs no
-    /// length, where saying the same thing in frame lengths would make the
-    /// container read its own box -- and a box chosen from its own answer
-    /// then feeds back into the answer.
-    Of(UiSpan),
 }
 
-impl Part {
+#[derive(Clone, Copy, Debug, PartialEq)]
-    /// Where it lands in the coordinates `extent` is in.
+pub enum PlaceSpan {
-    pub(crate) fn of(self, extent: UiSpan) -> UiSpan {
+    Within(UiSpan),
-        match self {
+    Shifted(UiSpan),
-            Self::All => extent,
+    Sized(Len),
-            Self::From(span) => UiSpan::new(extent.start + span.start, extent.start + span.end),
+}
-            Self::Of(span) => span.within(&extent),
+
+/// 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)]
+pub enum RelBase {
+    /// The caller's own, unchanged.
+    Inherit,
+    /// The caller's own, narrowed the way the region is.
+    WithRegion,
+    /// This length of the window.
+    Len(Len),
+}
+
+impl PlaceDescAxis {
+    /// The whole of the caller's box.
+    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
+    /// child asked for -- a row placing a child at the length it reported.
+    pub const fn fills(mut self) -> Self {
+        self.fills = true;
+        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 {
+        self.rel_base = RelBase::Len(len);
+        self
+    }
+
+    /// Where it lands in the coordinates `own` is in.
+    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) => own.place(len, align),
         }
     }
 }
 
-/// Where a child goes along one axis, as a part of this widget's box.
+/// Where a child is asked, on both axes. A [`UiRegion`] converts into the
+/// common case: that box of the caller's own, the answer placed inside it.
 #[derive(Clone, Copy, Debug, PartialEq)]
-pub enum Place {
+pub struct PlaceDesc {
-    /// The child's answer, aligned inside the part by the child's alignment.
+    pub x: PlaceDescAxis,
-    Within(Part),
+    pub y: PlaceDescAxis,
-    /// Exactly the part; the answer is not placed inside it again.
-    Fill(Part),
 }
 
-impl Place {
+impl PlaceDesc {
-    pub(crate) fn part(self) -> Part {
+    /// The whole of the caller's box, on both axes.
-        match self {
+    pub const WHOLE: Self = Self::splat(PlaceDescAxis::WHOLE);
-            Self::Within(part) | Self::Fill(part) => part,
+
+    pub const fn new(x: PlaceDescAxis, y: PlaceDescAxis) -> Self {
+        Self { x, y }
+    }
+
+    /// The same on both axes.
+    pub const fn splat(place: PlaceDescAxis) -> Self {
+        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 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),
         }
     }
 
-    /// Whether the part is the drawing's box outright, rather than the box
+    /// Both regions are the child's placement. See [`PlaceDescAxis::fills`].
-    /// the answer is placed inside.
+    pub const fn fills(self) -> Self {
-    pub(crate) fn fills(self) -> bool {
+        Self::new(self.x.fills(), self.y.fills())
-        matches!(self, Self::Fill(_))
+    }
+
+    /// 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] = self[axis].rel_base(len);
+        self
+    }
+
+    /// The box each axis names, in the coordinates `own` is in.
+    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(region.x.within_desc(), region.y.within_desc())
+    }
+}
+
+impl From<PlaceDescAxis> for PlaceDesc {
+    fn from(place: PlaceDescAxis) -> Self {
+        Self::splat(place)
     }
 }
 
@@ -59,3 +200,5 @@ pub struct RetainedPrimitive {
     pub handle: PrimitiveHandle,
     pub region: UiRegion,
 }
+
+impl_axis_index!(PlaceDesc => PlaceDescAxis);

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
+    /// give one.
-    /// whoever divides one, so it passes up in the reported size instead and
+    pub fn declared(&self) -> Option<Len> {
-    /// is resolved there.
+        self.exact().and_then(LayoutLen::declared)
-    pub fn declared(&self) -> Option<LayoutLen> {
-        match self {
-            Self::Exact(len) if len.leftover == Weight::ZERO => Some(*len),
-            _ => None,
-        }
     }
 
     /// 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 {
+impl_axis_index!(SizeRules => SizeRule);
-    pub fn axis(&self, axis: Axis) -> SizeRule {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
 
-    pub fn axis_mut(&mut self, axis: Axis) -> &mut SizeRule {
+/// What a widget's box is on each axis where something says so outright,
-        match axis {
+/// before it is drawn: a rule beside it, or a hint it gives about itself.
-            Axis::X => &mut self.x,
+/// Whoever draws the widget resolves these against its rel base.
-            Axis::Y => &mut self.y,
+///
+/// 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

@@ -130,10 +130,10 @@ 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);
     }
 
@@ -147,10 +147,10 @@ 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);
     }
 

src/default/render.rs

@@ -22,12 +22,12 @@ impl UiRenderer {
     }
 
     pub fn draw(&mut self) {
-        let output = match self.surface.get_current_texture() {
+        let (output, suboptimal) = match self.surface.get_current_texture() {
-            CurrentSurfaceTexture::Success(texture) => texture,
+            CurrentSurfaceTexture::Success(texture) => (texture, false),
-            CurrentSurfaceTexture::Suboptimal(texture) => {
+            // Used for this frame, and the swapchain rebuilt after it has
-                self.surface.configure(&self.device, &self.config);
+            // been presented: configuring the surface while a texture it
-                texture
+            // 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

@@ -118,20 +118,34 @@ 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 = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
+        let top = UiSpan::new(Len::ZERO, cut).shifted_desc();
         let measured = painter
-            .widget_at(&self.probe, UiRegion::FULL, [Place::Within(Part::All), top])
+            .widget_at(&self.probe, top.on_axis(Axis::Y))
             .len(Axis::X);
-        let px = measured.apply_leftover().to_px(painter.px_len(Axis::X));
+        let len = measured.apply_leftover();
+        let px = painter.to_px(len, Axis::X);
+        // The range it actually branched on, said the way a container says
+        // one: pinning the window instead would redraw this widget on every
+        // resize, which is a fixture that never exercises reuse.
+        let threshold = Px::from_f32(self.threshold);
+        let holds = match px > threshold {
+            true => Holds::from(threshold + Px::STEP..=Px::MAX),
+            false => Holds::from(Px::MIN..=threshold),
+        };
+        painter.window_holds(Axis::X, holds.through(len));
 
-        let below = Place::Within(Part::Of(UiSpan::new(cut, Len::FULL)));
+        let below = UiSpan::new(cut, painter.region_len(Axis::Y)).shifted_desc();
-        let place = [Place::Within(Part::All), below];
+        let place = below.on_axis(Axis::Y);
-        match px > Px::from_f32(self.threshold) {
+        match px > threshold {
-            true => painter.widget_at(&self.wide, UiRegion::FULL, place),
+            true => painter.widget_at(&self.wide, place),
-            false => painter.widget_at(&self.narrow, UiRegion::FULL, place),
+            false => painter.widget_at(&self.narrow, place),
         };
         Size::LEFTOVER
     }
+
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::LEFTOVER)
+    }
 }
 
 pub struct Spanned {

src/widget/image.rs

@@ -12,7 +12,7 @@ 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]))
     }
 }
 

src/widget/mask.rs

@@ -15,4 +15,8 @@ impl Widget for Masked {
         // draw, and the framework would place the drawing it clipped away.
         Size::LEFTOVER
     }
+
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::LEFTOVER)
+    }
 }

src/widget/position/offset.rs

@@ -7,14 +7,8 @@ pub struct Offset {
 
 impl Widget for Offset {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        // The whole of this widget's box, moved: the frame passes through, so
+        painter
-        // what the child declares or reports means the same as it would
+            .widget_at(&self.inner, UiRegion::FULL.offset(self.amt))
-        // without the offset.
+            .size()
-        let moved = |len: Len, amt: Len| Place::Within(Part::From(UiSpan::new(amt, len + amt)));
-        let place = [
-            moved(painter.extent_len(Axis::X), self.amt.x),
-            moved(painter.extent_len(Axis::Y), self.amt.y),
-        ];
-        painter.widget_at(&self.inner, UiRegion::FULL, place).size()
     }
 }

src/widget/position/pad.rs

@@ -14,23 +14,12 @@ impl Widget for Pad {
         // widget -- the slack is the inner's to sit in, and forcing the near
         // edge pinned it to a corner it had not asked for.
         //
-        // The padding goes around what it pads: the frame passes through, so
+        // Padding is an inset of both: it comes off the rel base, so `rel(1)`
-        // the inner's fractions mean what they would without it, and only
+        // under it fills this widget rather than overflowing it by the
-        // the box it draws in is moved in by the pixels. Said as a part of
+        // padding, and it comes off the box, so what is drawn sits inside.
-        // this widget's own box in that box's own lengths, so nothing here
+        // The two stay distinct -- the box can be narrower still, where a row
-        // reads how long the box is -- and a box chosen from this widget's
+        // asked this widget in the room left, and a text wraps at that.
-        // own answer therefore does not feed back into that answer.
+        let inner = painter.widget_at(&self.inner, self.padding.region()).size();
-        let inset = |lead: Px, trail: Px| {
-            Place::Within(Part::Of(UiSpan::new(
-                Len::from_parts(Rel::ZERO, lead),
-                Len::from_parts(Rel::ONE, -trail),
-            )))
-        };
-        let place = [
-            inset(self.padding.left, self.padding.right),
-            inset(self.padding.top, self.padding.bottom),
-        ];
-        let inner = painter.widget_at(&self.inner, UiRegion::FULL, place).size();
         Size {
             x: LayoutLen {
                 px: inner.x.px + self.padding.left + self.padding.right,

src/widget/position/scroll.rs

@@ -12,30 +12,29 @@ pub struct Scroll {
 impl Widget for Scroll {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let container_len = painter.px_len(self.axis);
-        // Measured in the whole viewport, then drawn at the scrolled offset.
+        // Asked in the whole viewport, then put at the scrolled offset.
-        let whole = UiRegion::FULL;
         let answer_len = painter
-            .widget_at(&self.inner, whole, [Place::Fill(Part::All); 2])
+            .widget_at(&self.inner, PlaceDesc::WHOLE.fills())
             .len(self.axis);
-        let fixed = Len::from_parts(answer_len.rel, answer_len.px).to_px(container_len);
+        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);
+        let align = painter.alignment()[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;
+        let answer_is_px = answer_len.is_px();
-        if fixed_len && self.content_len <= self.container_len && align == AxisAlign::NEG {
+        if answer_is_px && self.content_len <= self.container_len && align == AxisAlign::NEG {
-            painter.holds(self.axis, fixed..=Px::MAX);
+            painter.holds(self.axis, answer_px..=Px::MAX);
-        } else if fixed_len && !self.snap_end {
+        } else if answer_is_px && !self.snap_end {
             let left = self.content_len - self.amt;
             painter.holds(self.axis, Px::MIN..=left);
         }
@@ -45,7 +44,6 @@ impl Widget for Scroll {
         // 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());
-        let mut content = UiSpan::FULL;
         // 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
@@ -53,26 +51,28 @@ impl Widget for Scroll {
         // 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;
-        if moved || self.content_len != self.container_len {
+        let content = match moved || self.content_len != self.container_len {
-            let start = Len::from_parts(Rel::ZERO, anchor - self.amt);
+            true => {
-            content = UiSpan::new(start, start.offset(self.content_len));
+                let start = Len::from_parts(Rel::ZERO, anchor - self.amt);
-        }
+                UiSpan::new(start, start.offset(self.content_len)).shifted_desc()
-        // The viewport is the inner's frame, so a fraction it declares or
+            }
+            false => PlaceDescAxis::WHOLE,
+        };
+        // The viewport is the inner's rel base, so a fraction it declares or
         // reports is a fraction of what is on screen rather than of the
-        // content box its own answer decided. Where it is drawn is the
+        // content box its own answer decided. Where it goes is the content
-        // content box, scrolled.
+        // box, scrolled: its drawing moved there, not made again there.
-        painter.widget_at(
+        painter.place_at(&self.inner, content.on_axis(self.axis).fills());
-            &self.inner,
-            whole,
-            self.axis
-                .pair(Place::Fill(Part::From(content)), Place::Fill(Part::All)),
-        );
         // 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
         // is.
         Size::LEFTOVER
     }
+
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::LEFTOVER)
+    }
 }
 
 impl Scroll {

src/widget/position/span.rs

@@ -10,32 +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 frame its children
+        // The row this span lays its children out along, as a length of the
-        // are laid out against. Its start is nothing's business -- a slot is
+        // rel base they are laid out against. Where it starts is nothing's
-        // a length from it -- so what this reads is the length alone.
+        // business -- a slot is a length from there -- so what this reads is
-        let far = painter.answer_extent_len(axis);
+        // the length alone.
-        let measure_along = |from: Len, to: Len| match self.dir.sign {
+        let row = painter.region_len(axis);
-            Sign::Pos => UiSpan::new(from, to),
+        // A length for every child before their final slots are chosen: from
-            Sign::Neg => UiSpan::new(far - to, far - 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
-        // Across itself the child sits where its own alignment says, in the
+        // given whatever else is in it and wherever this child sits among
-        // whole of the row: a span is what contains its children there, and
+        // them; what a drawn child is asked in is the room left from the
-        // nothing divides that axis.
+        // cursor, because a text has to wrap at the width actually there.
-        let across = Place::Within(Part::All);
+        let mut cursor = Len::ZERO;
-        // A length for every child before their final slots are chosen. The
-        // frame 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 it is drawn 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 lens = Vec::with_capacity(self.children.len());
         for child in &self.children {
-            let room = Place::Fill(Part::From(measure_along(cursor, far)));
+            let len = match painter.size_hint(child, axis) {
-            let len = painter
+                Some(len) => len,
-                .widget_at(child, UiRegion::FULL, axis.pair(room, across))
+                None => {
-                .len(axis);
+                    // Across itself the child sits where its own alignment
-            cursor.px += len.px + self.gap;
+                    // says, in the whole of the row: a span is what contains
-            cursor.rel += len.rel;
+                    // 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 += len.without_leftover();
+            cursor.px += self.gap;
             lens.push(len);
         }
 
@@ -50,10 +50,9 @@ impl Widget for Span {
             |sum, len| sum + *len,
         );
 
-        let fixed_total = Len::from_parts(total.rel, total.px);
         // What is left for the shares to divide: the row less everything
-        // fixed, as a length of the frame rather than a number of pixels.
+        // fixed, as a length of the rel base rather than a number of pixels.
-        let room = far - fixed_total;
+        let room = row - total.without_leftover();
         // 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
@@ -63,28 +62,16 @@ impl Widget for Span {
         // 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;
+        let any_leftover = total.leftover > Weight::ZERO;
-        if total.leftover > Weight::ZERO {
+        let has_room = any_leftover && painter.to_px(room, axis) > Px::ZERO;
-            shares = room.to_px(painter.frame_px_len(axis)) > Px::ZERO;
+        if any_leftover {
-            let holds = match shares {
+            let holds = match has_room {
                 true => Holds::from(Px::STEP..=Px::MAX),
                 false => Holds::from(Px::MIN..=Px::ZERO),
             };
-            painter.frame_holds(axis, holds.through(room));
+            painter.window_holds(axis, holds.through(room));
         }
 
-        if shares {
-            painter.drawing_uses_extent_len(axis, far);
-        }
-        let drawing_far = match shares {
-            true => far,
-            false => fixed_total,
-        };
-        let along = |from: Len, to: Len| match self.dir.sign {
-            Sign::Pos => UiSpan::new(from, to),
-            Sign::Neg => UiSpan::new(drawing_far - to, drawing_far - from),
-        };
-
         // Across itself a span is as long as its longest child -- unless a
         // rule beside it gives that length outright, and then reading them
         // answers nothing and makes its size depend on theirs for it. A rule
@@ -93,61 +80,59 @@ 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
+        // spoken for. Both ends of a slot are read from those two rather
-        // from the last child: the share of the room is rounded, and taking
+        // than stepped from the last child: the share of the room is
-        // each from the one before it would carry every rounding along the
+        // rounded, and taking each end from the one before it would carry
-        // row.
+        // every rounding along the row.
-        let mut fixed = Len::rel_min();
+        let mut fixed = Len::ZERO;
         let mut taken = Weight::ZERO;
-        let mut start = Len::rel_min();
         let mut ortho = LayoutLen::ZERO;
-        for (child, len) in self.children.iter().zip(&lens) {
+        // 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;
                 continue;
             }
-            let from = start;
+            let from = reached(fixed, taken);
-            if len.leftover > Weight::ZERO && shares {
+            if len.leftover > Weight::ZERO && has_room {
                 taken += len.leftover;
             }
-            fixed.px += len.px;
+            fixed += len.without_leftover();
-            fixed.rel += len.rel;
+            let to = reached(fixed, taken);
-            start = shared(fixed, taken, total.leftover, room);
             // Along the row the span says where the child goes, and that slot
-            // is the drawing's box outright rather than something to place an
+            // is the child's box outright rather than something to place an
-            // answer inside again.
+            // answer inside again. A share is decided here and nowhere
-            let span = along(from, start);
+            // else: its slot narrows its rel base, and the child is asked in
-            let (frame, slot) = match len.leftover > Weight::ZERO && shares {
+            // it, since a text wraps at the width it is actually given. A
-                true => (
+            // fixed child's slot is its own answer, so a drawing made in the
-                    UiRegion::from_axis(
+            // room is put there as it is, and one not made yet is made here.
-                        axis,
+            let slot = self.slot(row, from, to);
-                        painter.extent_part(axis, Part::From(span)),
+            let mut place = slot.shifted_desc().fills().on_axis(axis);
-                        UiSpan::FULL,
+            if len.leftover > Weight::ZERO && has_room {
-                    ),
+                place = place.rel_base(axis, slot.len());
-                    Place::Fill(Part::All),
+            }
-                ),
+            let used = painter.place_at(child, place).len(!axis);
-                false => (UiRegion::FULL, Place::Fill(Part::From(span))),
-            };
-            let placed = painter.widget_at(child, frame, axis.pair(slot, across));
             if shrinks {
-                let used = placed.len(!axis);
                 // 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
@@ -157,26 +142,26 @@ 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

@@ -22,29 +22,40 @@ impl Widget for Stack {
             // drawing belongs to the layer it was made on.
             Some((i, child)) => {
                 painter.child_layer_at(i);
-                painter
+                painter.widget_at(child, PlaceDesc::WHOLE.fills()).size()
-                    .widget_at(child, UiRegion::FULL, [Place::Fill(Part::All); 2])
-                    .size()
             }
             None => Size::LEFTOVER,
         };
+        // Every other child gets the box the sizing child decided: the
+        // stack is that length, so that is the box they are asked in, and a
+        // 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 place = PlaceDesc::from_axes(|axis| {
+            let len = size[axis];
+            match len.leftover == Weight::ZERO {
+                true => len.without_leftover().as_desc().fills(),
+                false => PlaceDescAxis::WHOLE,
+            }
+        });
         for (i, child) in self.children.iter().enumerate() {
             if sizing == Some(i) {
                 continue;
             }
             painter.child_layer_at(i);
-            let place = [Axis::X, Axis::Y].map(|axis| {
+            painter.widget_at(child, place);
-                let len = size.axis(axis);
-                let part = match len.leftover > Weight::ZERO {
-                    true => Part::All,
-                    false => Part::From(UiSpan::new(Len::ZERO, Len::from_parts(len.rel, len.px))),
-                };
-                Place::Within(part)
-            });
-            painter.widget_at(child, UiRegion::FULL, place);
         }
         size
     }
+
+    /// Without a sizing child a stack is whatever box it is given, which it
+    /// can say without drawing anything.
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        match self.size {
+            StackSize::Default => Some(LayoutLen::LEFTOVER),
+            StackSize::Child(_) => None,
+        }
+    }
 }
 
 #[derive(Default, Debug)]

tests/cases/determinism.rs

@@ -22,17 +22,17 @@ 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 = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
+        let top = UiSpan::new(Len::ZERO, cut).shifted_desc();
         let measured = painter
-            .widget_at(&self.probe, UiRegion::FULL, [Place::Within(Part::All), top])
+            .widget_at(&self.probe, top.on_axis(Axis::Y))
             .len(Axis::X);
-        let px = measured.apply_leftover().to_px(painter.px_len(Axis::X));
+        let px = painter.to_px(measured.apply_leftover(), Axis::X);
 
-        let below = Place::Within(Part::From(UiSpan::new(cut, painter.extent_len(Axis::Y))));
+        let below = UiSpan::new(cut, painter.region_len(Axis::Y)).shifted_desc();
-        let place = [Place::Within(Part::All), below];
+        let place = below.on_axis(Axis::Y);
         match px > Px::from_f32(self.threshold) {
-            true => painter.widget_at(&self.wide, UiRegion::FULL, place),
+            true => painter.widget_at(&self.wide, place),
-            false => painter.widget_at(&self.narrow, UiRegion::FULL, place),
+            false => painter.widget_at(&self.narrow, place),
         };
         Size::LEFTOVER
     }

tests/cases/layout.rs

@@ -83,10 +83,8 @@ fn a_text_in_a_span_wraps_at_the_room_left_rather_than_the_whole_row() {
     assert!(crowded > whole_row, "{crowded} against {whole_row}");
 }
 
-/// The same reading through a pad: padding goes around what it pads and
+/// Padding is an inset: it narrows the frame a fraction resolves against and
-/// does not narrow what a fraction under it is a fraction of, so half of the
+/// adds itself back to the padded widget's reported length.
-/// window plus the padding is what the pad takes and where the next child
-/// starts.
 #[test]
 fn a_pad_puts_its_padding_around_a_fraction_of_the_whole_box() {
     let mut h = Harness::new((400, 100));
@@ -97,9 +95,83 @@ fn a_pad_puts_its_padding_around_a_fraction_of_the_whole_box() {
     // placed inside it by its own alignment, which is not what is under test.
     h.set_root((padded, tail).span(Dir::RIGHT).width(rel(1.0)));
 
-    assert_corners!(h, inner, (10, 10), (210, 90));
+    assert_corners!(h, inner, (10, 10), (200, 90));
-    assert_corners!(h, padded, (0, 0), (220, 100));
+    assert_corners!(h, padded, (0, 0), (210, 100));
-    assert_corners!(h, tail, (220, 0), (320, 100));
+    assert_corners!(h, tail, (210, 0), (310, 100));
+}
+
+const PARAGRAPH: &str = "Wrapping shapes one source into as many lines as the box \
+    leaves room for, so a paragraph's height is an answer and not a setting.";
+
+/// The worked example of what padding insets: in a 900 px row after a 24 px
+/// icon, a `rel(1.0)` inside `pad(16)` is 900 - 32 and overflows the row by
+/// the icon's width, while a wrapping text beside it is asked in the room
+/// left, 900 - 24 - 32, and wraps there.
+#[test]
+fn padding_keeps_the_rel_base_distinct_from_the_room_left_in_a_row() {
+    let mut h = Harness::new((900, 200));
+    let icon = rect(Color::RED).width(24).add(&mut h.rsc);
+    let fill = rect(Color::GREEN).width(rel(1.0)).add(&mut h.rsc);
+    let padded = fill.pad(16).add(&mut h.rsc);
+    h.set_root((icon, padded).span(Dir::RIGHT).width(rel(1.0)));
+    let fill_width = h.region(&fill).unwrap().size().x;
+    assert_eq!(fill_width, Px::from_int(868));
+
+    let mut h = Harness::new((900, 200));
+    let icon = rect(Color::RED).width(24).add(&mut h.rsc);
+    let text = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let padded = text.pad(16).add(&mut h.rsc);
+    h.set_root((icon, padded).span(Dir::RIGHT).width(rel(1.0)));
+    let active = &h.render.active[&text.id()];
+    let window = h.render.output_size().x;
+    let asked = active.region.x.len().to_px(window);
+    assert_eq!(active.rel_base.x.to_px(window), Px::from_int(868));
+    assert_eq!(asked, Px::from_int(844));
+}
+
+/// The other way round: a share inside padding. A slot is a length of the
+/// row, which is already the padded width, so what the span decided reaches
+/// the child as it stands -- taking the padding off a second time would make
+/// `rel(1.0)` in the slot shorter than the slot.
+#[test]
+fn a_share_inside_padding_fills_the_slot_it_was_given() {
+    let mut h = Harness::new((900, 200));
+    let fill = rect(Color::GREEN).width(rel(1.0)).add(&mut h.rsc);
+    let first = Span {
+        children: vec![fill.add_strong(&mut h.rsc)],
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    }
+    .width(leftover(1))
+    .add(&mut h.rsc);
+    let second = rect(Color::BLUE).width(leftover(1)).add(&mut h.rsc);
+    let row = (first, second).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root(row.pad(16));
+
+    assert_eq!(h.region(&first).unwrap().size().x, Px::from_int(434));
+    assert_eq!(h.region(&fill).unwrap().size().x, Px::from_int(434));
+}
+
+/// The same padding in a share instead: the slot is 450, so both the
+/// fraction and the wrap are the slot less the padding, and the two agree.
+#[test]
+fn padding_narrows_both_rel_base_and_box_inside_a_share() {
+    let mut h = Harness::new((900, 200));
+    let fill = rect(Color::GREEN).width(rel(1.0)).add(&mut h.rsc);
+    let padded = fill.pad(16).width(leftover(1)).add(&mut h.rsc);
+    let other = rect(Color::BLUE).width(leftover(1)).add(&mut h.rsc);
+    h.set_root((padded, other).span(Dir::RIGHT).width(rel(1.0)));
+    assert_eq!(h.region(&fill).unwrap().size().x, Px::from_int(418));
+
+    let mut h = Harness::new((900, 200));
+    let text = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let padded = text.pad(16).width(leftover(1)).add(&mut h.rsc);
+    let other = rect(Color::BLUE).width(leftover(1)).add(&mut h.rsc);
+    h.set_root((padded, other).span(Dir::RIGHT).width(rel(1.0)));
+    let active = &h.render.active[&text.id()];
+    let window = h.render.output_size().x;
+    assert_eq!(active.rel_base.x.to_px(window), Px::from_int(418));
+    assert_eq!(active.region.x.len().to_px(window), Px::from_int(418));
 }
 
 #[test]
@@ -428,12 +500,11 @@ fn a_row_of_equal_shares_fills_it_exactly() {
 /// a step of. Kept in step with `snap_floor` in `prelude.wgsl`.
 fn drawn_edges(h: &Harness, id: WidgetId, axis: Axis) -> (f32, f32) {
     let active = &h.render.active[&id];
-    let drawn = active.extent.within(&active.frame_abs);
+    let region = h.render.moves.resolve(active.move_idx, active.placement);
-    let region = h.render.moves.resolve(active.parent_move, drawn);
+    let dim = h.size()[axis];
-    let dim = h.size().axis(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))
 }
 
@@ -740,3 +811,50 @@ fn a_fixed_child_is_centered_in_its_wrappers_share() {
     assert_corners!(h, wrapper, (200, 0), (900, 400));
     assert_corners!(h, leaf, (500, 150), (600, 250));
 }
+
+/// The root's frame is the window and its rule is a fraction of that, which
+/// is one resolution and not two: nothing above it narrowed anything.
+#[test]
+fn a_root_with_a_fraction_rule_is_that_fraction_of_the_window() {
+    let mut h = Harness::new((900, 200));
+    let root = rect(Color::RED).width(rel(0.5)).add(&mut h.rsc);
+    h.set_root(root);
+    assert_eq!(h.region(&root).unwrap().size().x, Px::from_int(450));
+}
+
+#[test]
+fn a_collapsed_share_keeps_the_gaps_before_the_next_slot() {
+    for dir in [Dir::RIGHT, Dir::LEFT, Dir::DOWN, Dir::UP] {
+        for collapsed in [1, 2] {
+            let mut h = Harness::new((400, 400));
+            let head = rect(Color::RED).add(&mut h.rsc);
+            h.set_len(head, dir.axis, 200);
+            let tail = rect(Color::BLUE).add(&mut h.rsc);
+            let tail_len = 200 - 10 * (collapsed + 1);
+            h.set_len(tail, dir.axis, tail_len);
+            let mut children: Vec<StrongWidget> = vec![head.add_strong(&mut h.rsc)];
+            let mut shares = Vec::new();
+            for _ in 0..collapsed {
+                let share = rect(Color::GREEN).add(&mut h.rsc);
+                shares.push(share);
+                children.push(share.add_strong(&mut h.rsc));
+            }
+            children.push(tail.add_strong(&mut h.rsc));
+            h.set_root(Span {
+                children,
+                dir,
+                gap: Px::from_int(10),
+            });
+            for share in shares {
+                assert!(h.region(&share).is_none());
+            }
+            let region = h.region(&tail).unwrap();
+            let (from, to) = match dir.sign {
+                Sign::Pos => (400 - tail_len, 400),
+                Sign::Neg => (0, tail_len),
+            };
+            assert_eq!(region.top_left[dir.axis], Px::from_int(from));
+            assert_eq!(region.bot_right[dir.axis], Px::from_int(to));
+        }
+    }
+}

tests/cases/retained.rs

@@ -12,7 +12,6 @@ struct Counted {
     draws: Rc<Cell<usize>>,
     size: Size,
     reads_box: bool,
-    reads_answer_box: bool,
 }
 
 impl Widget for Counted {
@@ -20,8 +19,6 @@ impl Widget for Counted {
         self.draws.set(self.draws.get() + 1);
         if self.reads_box {
             painter.px_size();
-        } else if self.reads_answer_box {
-            painter.answer_px_size();
         }
         self.size
     }
@@ -41,18 +38,11 @@ fn counted(h: &mut Harness, size: Size, reads_box: bool) -> (WeakWidget<Counted>
         draws: draws.clone(),
         size,
         reads_box,
-        reads_answer_box: false,
     }
     .add(&mut h.rsc);
     (id, Counts(draws))
 }
 
-fn answer_counted(h: &mut Harness, size: Size) -> (WeakWidget<Counted>, Counts) {
-    let (id, draws) = counted(h, size, false);
-    h.rsc[id].reads_answer_box = true;
-    (id, draws)
-}
-
 struct Layered {
     children: [StrongWidget<Rect>; 2],
     _revision: usize,
@@ -135,7 +125,7 @@ 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));
 }
@@ -166,9 +156,9 @@ fn a_span_child_that_declares_its_length_is_drawn_once() {
     h.set_root((hinted, asked).span(Dir::RIGHT));
 
     assert_eq!(told_draws.get(), 1);
-    // Its final slot is a parent decision, so it is evaluated there after
+    // Asked once, from the cursor; its slot is its answer and the drawing is
-    // the provisional ask established its length.
+    // moved there.
-    assert_eq!(asked_draws.get(), 2);
+    assert_eq!(asked_draws.get(), 1);
 }
 
 #[test]
@@ -224,11 +214,7 @@ 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(
+        painter.widget_at(&self.inner, top.shifted_desc().on_axis(Axis::Y));
-            &self.inner,
-            UiRegion::FULL,
-            [Place::Within(Part::All), Place::Within(Part::From(top))],
-        );
         Size::LEFTOVER
     }
 }
@@ -250,7 +236,7 @@ fn a_parent_that_only_read_a_hint_relays_out_when_the_hint_changes() {
     assert_corners!(h, inner, (0, 0), (400, 120));
 }
 
-/// Reads its box's size to compute its answer.
+/// Reads its box's size, which nothing but its own draw can put right.
 struct ReadsBox {
     draws: Rc<Cell<usize>>,
 }
@@ -258,36 +244,25 @@ struct ReadsBox {
 impl Widget for ReadsBox {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
-        Size::from_px(painter.answer_px_size().div_int(4))
+        Size::from_px(painter.px_size().div_int(4))
     }
 }
 
 /// Reads its box across one axis only, so its drawing holds for a taller
 /// box on its own and only a wider one is worth a draw.
 ///
-/// Both report a quarter of what they read. Their empty drawings are
+/// Both of these report a quarter of what they read. The quarter-sized box
-/// independent of that read, so a changed question costs one draw.
+/// the answer places them in is not a question: the drawing is moved there,
+/// so each length they are asked at costs one draw.
 struct ReadsWidth {
     draws: Rc<Cell<usize>>,
 }
 
-struct ReadsDrawingWidth {
-    draws: Rc<Cell<usize>>,
-}
-
-impl Widget for ReadsDrawingWidth {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
-        self.draws.set(self.draws.get() + 1);
-        painter.px_len(Axis::X);
-        Size::LEFTOVER
-    }
-}
-
 impl Widget for ReadsWidth {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
         Size::from_px(PxVec2::new(
-            painter.answer_px_len(Axis::X).div_int(4),
+            painter.px_len(Axis::X).div_int(4),
             Px::from_int(20),
         ))
     }
@@ -343,7 +318,6 @@ fn a_row_moves_what_follows_a_child_that_grew_rather_than_drawing_it() {
             draws: ruled.clone(),
             size: Size::LEFTOVER,
             reads_box: false,
-            reads_answer_box: false,
         };
         let second = match declared {
             true => second.width(rel(0.25)).add(&mut h.rsc),
@@ -521,7 +495,7 @@ fn a_change_two_levels_under_its_reader_still_reaches_it() {
     // Every wrapper up to the outer pad read the size below it, so the outer
     // pad is what draws again -- and the span it hands the box to is the same
     // size as before, which is what lets a draw reuse its way past the leaf.
-    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), false);
+    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), true);
     let padded = leaf.pad(10).add(&mut h.rsc);
     let below = rect(Color::RED).add(&mut h.rsc);
     h.set_root((padded, below).span(Dir::DOWN).pad(12));
@@ -819,7 +793,7 @@ fn primitive_bounds(h: &Harness, id: WidgetId) -> Vec<PixelRegion> {
 }
 
 #[test]
-fn changing_an_inherited_extent_keeps_the_original_measurement_offer() {
+fn changing_an_inherited_region_keeps_the_original_measurement_offer() {
     fn build(h: &mut Harness, width: i32, text: &str) -> (WeakWidget<Text>, WeakWidget<Rect>) {
         let first = rect(Color::RED).width(width).add(&mut h.rsc);
         let words = wtext(text).size(20).wrap(true).add(&mut h.rsc);
@@ -899,11 +873,7 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                UiRegion::FULL,
+                PlaceDesc::new(self.region.x.shifted_desc(), self.region.y.shifted_desc()),
-                [
-                    Place::Within(Part::From(self.region.x)),
-                    Place::Within(Part::From(self.region.y)),
-                ],
             );
             Size::LEFTOVER
         }
@@ -981,18 +951,18 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
     }
     struct Frame {
         child: StrongWidget,
+        frame: UiRegion,
         region: UiRegion,
-        extent: UiRegion,
     }
     impl Widget for Frame {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                self.region,
+                PlaceDesc::new(
-                [
+                    self.region.x.shifted_desc().fills(),
-                    Place::Fill(Part::From(self.extent.x)),
+                    self.region.y.shifted_desc().fills(),
-                    Place::Fill(Part::From(self.extent.y)),
+                )
-                ],
+                .rel_base(Axis::X, self.frame.x.len()),
             );
             Size::LEFTOVER
         }
@@ -1008,15 +978,15 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
         h.rsc.widgets_mut().set_region_node(text, node);
         let root = Frame {
             child: text.add_strong(&mut h.rsc),
+            frame: UiRegion::FULL,
             region: UiRegion::FULL,
-            extent: UiRegion::FULL,
         }
         .add(&mut h.rsc);
         h.set_root(root);
         for (start, end) in [(0.13, 0.83), (-0.17, 1.23), (0.31, 0.67)] {
             let before = draws.get();
-            h.rsc[root].region.x = UiSpan::new(Len::px(13.125), Len::px(287.375));
+            h.rsc[root].frame.x = UiSpan::new(Len::px(13.125), Len::px(287.375));
-            h.rsc[root].extent = UiRegion::new(
+            h.rsc[root].region = UiRegion::new(
                 UiSpan::new(Len::rel(start), Len::rel(end)),
                 UiSpan::new(Len::px(7.25), Len::rel(end)),
             );
@@ -1087,7 +1057,7 @@ fn a_declared_size_change_stops_at_an_independent_parent() {
 fn an_unmeasured_child_still_invalidates_its_parents_drawing_on_resize() {
     let mut h = Harness::new((400, 200));
     let draws = Rc::new(Cell::new(0));
-    let leaf = ReadsDrawingWidth {
+    let leaf = ReadsWidth {
         draws: draws.clone(),
     }
     .add(&mut h.rsc);
@@ -1098,7 +1068,7 @@ fn an_unmeasured_child_still_invalidates_its_parents_drawing_on_resize() {
     h.frame();
 
     assert!(draws.get() > settled);
-    assert_corners!(h, leaf, (0, 0), (800, 200));
+    assert_corners!(h, leaf, (300, 90), (500, 110));
 }
 
 #[test]
@@ -1139,7 +1109,7 @@ fn widening_and_restoring_a_contract_does_not_invalidate_its_reader() {
     assert_eq!(leaf_draws.get(), settled + 1);
 }
 #[test]
-fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
+fn padding_and_stack_boxes_follow_the_region_without_drawing_again() {
     struct Observed<W> {
         widget: W,
         draws: Rc<Cell<usize>>,
@@ -1152,23 +1122,22 @@ fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
     }
     struct Frame {
         child: StrongWidget,
-        extent: UiRegion,
+        region: UiRegion,
     }
     impl Widget for Frame {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                UiRegion::FULL,
+                PlaceDesc::new(
-                [
+                    self.region.x.shifted_desc().fills(),
-                    Place::Fill(Part::From(self.extent.x)),
+                    self.region.y.shifted_desc().fills(),
-                    Place::Fill(Part::From(self.extent.y)),
+                ),
-                ],
             );
             Size::LEFTOVER
         }
     }
     for node in [false, true] {
-        let plant = |h: &mut Harness, extent| {
+        let plant = |h: &mut Harness, region| {
             let draws = Rc::new(Cell::new(0));
             let leaf = rect(Color::BLUE).masked().add(&mut h.rsc);
             h.rsc.widgets_mut().set_region_node(leaf, node);
@@ -1189,7 +1158,7 @@ fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
                 draws: draws.clone(),
             }
             .add_strong(&mut h.rsc);
-            let root = Frame { child: pad, extent }.add(&mut h.rsc);
+            let root = Frame { child: pad, region }.add(&mut h.rsc);
             h.set_root(root);
             (root, leaf, fixed, draws)
         };
@@ -1204,13 +1173,13 @@ fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
         let mut warm = Harness::new((403, 211));
         let (root, leaf, fixed, draws) = plant(&mut warm, at(0.13));
         for start in [0.13, -0.17, 0.31] {
-            let extent = at(start);
+            let region = at(start);
             let before = draws.get();
-            warm.rsc[root].extent = extent;
+            warm.rsc[root].region = region;
             warm.frame();
             assert_eq!(draws.get(), before);
             let mut cold = Harness::new((403, 211));
-            let (_, other, other_fixed, _) = plant(&mut cold, extent);
+            let (_, other, other_fixed, _) = plant(&mut cold, region);
             for (a, b) in [(leaf.id(), other.id()), (fixed.id(), other_fixed.id())] {
                 assert_eq!(warm.region(&a), cold.region(&b));
                 assert_eq!(primitive_bounds(&warm, a), primitive_bounds(&cold, b));
@@ -1229,11 +1198,11 @@ fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
 }
 
 #[test]
-fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
+fn moving_a_childs_region_preserves_the_slot_chosen_from_its_measurement() {
     struct Measured;
     impl Widget for Measured {
         fn draw(&mut self, painter: &mut Painter) -> Size {
-            let width = painter.answer_px_len(Axis::X);
+            let width = painter.px_len(Axis::X);
             painter.primitive(RectPrimitive::color(Color::BLUE));
             Size::from((80, if width > Px::from_int(100) { 40 } else { 60 }))
         }
@@ -1246,14 +1215,12 @@ fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                UiRegion::FULL,
+                PlaceDesc::new(
-                [
+                    UiSpan::new(Len::px(self.start), Len::px(self.start + 200.0))
-                    Place::Fill(Part::From(UiSpan::new(
+                        .shifted_desc()
-                        Len::px(self.start),
+                        .fills(),
-                        Len::px(self.start + 200.0),
+                    UiSpan::FULL.shifted_desc().fills(),
-                    ))),
+                ),
-                    Place::Fill(Part::From(UiSpan::FULL)),
-                ],
             );
             Size::LEFTOVER
         }
@@ -1278,7 +1245,7 @@ fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
 }
 
 #[test]
-fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
+fn changing_regions_keep_fractional_reports_and_numeric_dependencies_valid() {
     struct Container {
         child: StrongWidget,
         region: UiRegion,
@@ -1288,18 +1255,14 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
             painter
                 .widget_at(
                     &self.child,
-                    UiRegion::FULL,
+                    PlaceDesc::new(self.region.x.shifted_desc(), self.region.y.shifted_desc()),
-                    [
-                        Place::Within(Part::From(self.region.x)),
-                        Place::Within(Part::From(self.region.y)),
-                    ],
                 )
                 .size()
         }
     }
     struct Frame {
         child: StrongWidget,
-        extent: UiRegion,
+        region: UiRegion,
         answer: Rc<Cell<Size>>,
     }
     impl Widget for Frame {
@@ -1308,11 +1271,10 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
                 painter
                     .widget_at(
                         &self.child,
-                        UiRegion::FULL,
+                        PlaceDesc::new(
-                        [
+                            self.region.x.shifted_desc().fills(),
-                            Place::Fill(Part::From(self.extent.x)),
+                            self.region.y.shifted_desc().fills(),
-                            Place::Fill(Part::From(self.extent.y)),
+                        ),
-                        ],
                     )
                     .size(),
             );
@@ -1324,7 +1286,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
             UiRegion::FULL,
             UiRegion::new(UiSpan::new(Len::rel(0.13), Len::rel(0.79)), UiSpan::FULL),
         ] {
-            let plant = |h: &mut Harness, extent| {
+            let plant = |h: &mut Harness, outer| {
                 let size = if fractional {
                     Size {
                         x: rel(0.5),
@@ -1333,10 +1295,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
                 } else {
                     Size::from((80, 27))
                 };
-                let (leaf, _) = match fractional {
+                let (leaf, _) = counted(h, size, !fractional);
-                    true => counted(h, size, false),
-                    false => answer_counted(h, size),
-                };
                 let child = Container {
                     child: leaf.add_strong(&mut h.rsc),
                     region,
@@ -1345,7 +1304,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
                 let answer = Rc::new(Cell::new(Size::ZERO));
                 let root = Frame {
                     child,
-                    extent,
+                    region: outer,
                     answer: answer.clone(),
                 }
                 .add(&mut h.rsc);
@@ -1355,15 +1314,262 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
             let mut warm = Harness::new((403, 211));
             let (root, leaf, answer) = plant(&mut warm, UiRegion::FULL);
             for width in [191.125, 297.25, 83.75] {
-                let extent =
+                let region =
                     UiRegion::new(UiSpan::new(Len::px(13.125), Len::px(width)), UiSpan::FULL);
-                warm.rsc[root].extent = extent;
+                warm.rsc[root].region = region;
                 warm.frame();
                 let mut cold = Harness::new((403, 211));
-                let (_, other, other_answer) = plant(&mut cold, extent);
+                let (_, other, other_answer) = plant(&mut cold, region);
                 assert_eq!(answer.get(), other_answer.get());
                 assert_eq!(warm.region(&leaf), cold.region(&other));
             }
         }
     }
 }
+
+struct OptionalMask {
+    inner: StrongWidget,
+    enabled: bool,
+}
+
+impl Widget for OptionalMask {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        if self.enabled {
+            painter.set_mask(UiRegion::FULL);
+        }
+        painter.widget(&self.inner);
+        Size::LEFTOVER
+    }
+}
+
+fn primitive_masks(h: &Harness, id: WidgetId) -> Vec<MaskIdx> {
+    h.render.active[&id]
+        .primitives
+        .iter()
+        .map(|primitive| {
+            let handle = &primitive.handle;
+            h.render.layers[handle.layer].primitives()[handle.kind as usize]
+                .as_ref()
+                .unwrap()
+                .instances()[handle.inst_idx]
+                .mask_idx
+        })
+        .collect()
+}
+
+#[test]
+fn a_redrawn_mask_keeps_reused_primitives_clipped_when_it_moves() {
+    for node in [false, true] {
+        let mut h = Harness::new((400, 200));
+        let first = rect(Color::RED).height(50).add(&mut h.rsc);
+        let inner = rect(Color::BLUE).add(&mut h.rsc);
+        let draws = Rc::new(Cell::new(0));
+        let child = Stretchy {
+            inner: inner.add_strong(&mut h.rsc),
+            draws: draws.clone(),
+        }
+        .add(&mut h.rsc);
+        let masked = child.masked().add(&mut h.rsc);
+        h.rsc.widgets_mut().set_region_node(masked, node);
+        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.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_corners!(h, inner, (0, 10), (400, 200));
+    }
+}
+
+#[test]
+fn adding_and_removing_a_mask_updates_existing_primitives() {
+    let mut h = Harness::new((400, 200));
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let masked = OptionalMask {
+        inner: inner.add_strong(&mut h.rsc),
+        enabled: false,
+    }
+    .add(&mut h.rsc);
+    h.set_root(masked);
+    for enabled in [true, false, true, false] {
+        h.rsc[masked].enabled = enabled;
+        h.frame();
+        let mask = h.render.active[&masked.id()].mask;
+        assert_eq!(mask == MaskIdx::NONE, !enabled);
+        assert_eq!(primitive_masks(&h, inner.id()), vec![mask]);
+    }
+    assert_eq!(h.rsc.ui().masks.len(), 1, "retired slots must be reusable");
+}
+
+#[test]
+fn an_empty_masks_slot_is_released_when_the_mask_is_removed_or_undrawn() {
+    let mut h = Harness::new((400, 200));
+    let (inner, _) = counted(&mut h, Size::LEFTOVER, false);
+    let masked = OptionalMask {
+        inner: inner.add_strong(&mut h.rsc),
+        enabled: true,
+    }
+    .add(&mut h.rsc);
+    let row = (masked,).span(Dir::DOWN).add(&mut h.rsc);
+    h.set_root(row);
+    for _ in 0..3 {
+        h.rsc[masked].enabled = false;
+        h.frame();
+        h.rsc[masked].enabled = true;
+        h.frame();
+        let child = h.rsc[row].pop().unwrap();
+        h.frame();
+        h.rsc[row].push(child);
+        h.frame();
+    }
+    assert_eq!(h.rsc.ui().masks.len(), 1);
+}
+
+struct SharedChild(Rc<StrongWidget>);
+
+impl Widget for SharedChild {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        painter.widget(self.0.as_ref()).size()
+    }
+}
+
+struct SwitchParent {
+    choices: [StrongWidget; 2],
+    choice: usize,
+}
+
+impl Widget for SwitchParent {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        painter.widget(&self.choices[self.choice]).size()
+    }
+}
+
+#[test]
+fn a_redrawn_subtree_is_not_undrawn_by_the_parent_it_left() {
+    for node in [false, true] {
+        let mut h = Harness::new((400, 200));
+        let leaf = rect(Color::RED).width(40).add(&mut h.rsc);
+        let held: StrongWidget = leaf.add_strong(&mut h.rsc);
+        let shared = Rc::new(held);
+        let first = SharedChild(shared.clone()).add_strong(&mut h.rsc);
+        let second = SharedChild(shared).add_strong(&mut h.rsc);
+        h.rsc.widgets_mut().set_region_node(&second, node);
+        let root = SwitchParent {
+            choices: [first, second],
+            choice: 0,
+        }
+        .add(&mut h.rsc);
+        h.set_root(root);
+        let before = h.region(&leaf);
+        h.rsc[root].choice = 1;
+        h.frame();
+        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

@@ -145,3 +145,19 @@ fn a_clipping_widget_reporting_more_than_its_box_is_caught() {
     h.set_root(clipper);
     h.frame();
 }
+
+/// Content that fits sits in the viewport, not in a box of the window's
+/// length anchored at the viewport's start. `Part::From` takes window
+/// lengths, so a `rel(1.0)` span in one is the window, and only a scroll
+/// filling the window would land right.
+#[test]
+fn content_that_fits_is_placed_in_the_viewport_and_not_in_the_window() {
+    let mut h = Harness::new((400, 400));
+    let head = rect(Color::RED).height(100).add(&mut h.rsc);
+    let inner = rect(Color::BLUE).height(50).add(&mut h.rsc);
+    let scroll = Scroll::new(inner.add_strong(&mut h.rsc), Axis::Y).add(&mut h.rsc);
+    h.set_root((head, scroll).span(Dir::DOWN));
+
+    assert_corners!(h, scroll, (0, 100), (400, 400));
+    assert_corners!(h, inner, (0, 225), (400, 275));
+}

tests/cases/unsettled.rs

@@ -733,6 +733,51 @@ fn a_widget_under_a_region_node_is_asked_in_the_box_that_node_was_offered() {
 const PARAGRAPH: &str = "Wrapping shapes one source into as many lines as the \
     box leaves room for, so a paragraph's height is an answer and not a setting.";
 
+fn plant_stack_resized_from_free(h: &mut Harness, fixed: bool) -> (Vec<WidgetId>, WidgetId) {
+    let sizing = rect(Color::CYAN.alpha(126)).add(&mut h.rsc);
+    h.rsc.widgets_mut().set_size_rules(sizing.id(), None, None);
+    if fixed {
+        h.rsc.widgets_mut().set_size_rules(
+            sizing.id(),
+            Some(LayoutLen::px(112)),
+            Some(LayoutLen::px(101)),
+        );
+    }
+    let text = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let pad = Pad {
+        padding: Padding::ZERO,
+        inner: text.add_strong(&mut h.rsc),
+    }
+    .add(&mut h.rsc);
+    let stack = Stack {
+        children: vec![sizing.add_strong(&mut h.rsc), pad.add_strong(&mut h.rsc)],
+        size: StackSize::Child(0),
+    }
+    .add(&mut h.rsc);
+    h.set_root(stack);
+    (
+        vec![sizing.id(), text.id(), pad.id(), stack.id()],
+        sizing.id(),
+    )
+}
+
+#[test]
+fn fixing_a_stacks_sizing_child_repositions_its_overlay() {
+    let mut warm = Harness::new((900, 1200));
+    let (ids, sizing) = plant_stack_resized_from_free(&mut warm, false);
+    warm.frame();
+    warm.rsc.widgets_mut().set_size_rules(
+        sizing,
+        Some(LayoutLen::px(112)),
+        Some(LayoutLen::px(101)),
+    );
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let (cold_ids, _) = plant_stack_resized_from_free(&mut cold, true);
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
+
 /// Eight widgets, shrunk from a 118-widget tree (seed 1121, depth 4,
 /// `shuffle-swap-for-three`). The stack takes its size from the span above,
 /// the span takes its width from the longest line of the texts in it, and
@@ -846,3 +891,208 @@ fn adding_text_to_a_reverse_row_keeps_its_shared_height() {
     let (_, other, _) = build(&mut cold, true);
     assert_eq!(warm.region(&shared), cold.region(&other));
 }
+
+/// Nine widgets, shrunk from seed 946 at depth 6. The column is a share of
+/// the row while its rect has room to draw and a fixed width once it has
+/// not, so the row asks it twice: in the room, where it answers a share,
+/// and in its slot, where it answers its text's width. Emptying the column
+/// changes only the first answer. A local redraw that asked only the second
+/// question kept the row as it was; the column has to defer to the row.
+fn plant_column_that_is_a_share_only_while_its_rect_fits(
+    h: &mut Harness,
+    emptied: bool,
+) -> (Vec<WidgetId>, WeakWidget<Span>, Vec<StrongWidget>) {
+    let first = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let filler = rect(Color::CYAN.alpha(126)).add(&mut h.rsc);
+    let second = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let mut spare: Vec<StrongWidget> =
+        vec![filler.add_strong(&mut h.rsc), second.add_strong(&mut h.rsc)];
+    let mut children: Vec<StrongWidget> = vec![first.add_strong(&mut h.rsc)];
+    if !emptied {
+        children.append(&mut spare);
+    }
+    let column = Span {
+        children,
+        dir: Dir::DOWN,
+        gap: Px::ZERO,
+    }
+    .height(159)
+    .add(&mut h.rsc);
+    let left = rect(Color::MAGENTA.alpha(189)).add(&mut h.rsc);
+    let right = rect(Color::BLUE.alpha(0)).add(&mut h.rsc);
+    let row = Span {
+        children: vec![
+            left.add_strong(&mut h.rsc),
+            column.add_strong(&mut h.rsc),
+            right.add_strong(&mut h.rsc),
+        ],
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    }
+    .add(&mut h.rsc);
+    let end = rect(Color::MAGENTA.alpha(189)).add(&mut h.rsc);
+    let root = Span {
+        children: vec![end.add_strong(&mut h.rsc), row.add_strong(&mut h.rsc)],
+        dir: Dir::LEFT,
+        gap: Px::ZERO,
+    }
+    .add(&mut h.rsc);
+    h.set_root(root);
+    (
+        vec![
+            first.id(),
+            filler.id(),
+            second.id(),
+            column.id(),
+            left.id(),
+            right.id(),
+            row.id(),
+            end.id(),
+            root.id(),
+        ],
+        column,
+        spare,
+    )
+}
+
+#[test]
+fn emptying_a_column_the_row_asked_twice_asks_the_row_again() {
+    let mut warm = Harness::new((900, 1200));
+    let (ids, column, _spare) =
+        plant_column_that_is_a_share_only_while_its_rect_fits(&mut warm, false);
+    warm.frame();
+    // Kept alive: dropping the last share of a widget frees its id.
+    let _removed: Vec<StrongWidget> = warm.rsc[column].children.drain(1..).collect();
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let (cold_ids, _, _spare) =
+        plant_column_that_is_a_share_only_while_its_rect_fits(&mut cold, true);
+    cold.frame();
+
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
+
+/// Six widgets, shrunk from seed 59 at depth 5 (`resize-size`). The column
+/// divides the box it is given between two shares, so its drawing holds for
+/// that box's length alone, and the pads above it pass that dependency up:
+/// each one's box is a part of the box it was asked in. Padding narrowing
+/// the frame it hands down does not change that, and while it was taken to,
+/// changing the rule over the pads relocated the column's drawing into the
+/// new box instead of dividing it again.
+fn plant_two_shares_under_two_pads(h: &mut Harness, height: f32) -> Vec<WidgetId> {
+    let top = rect(Color::CYAN.alpha(126)).add(&mut h.rsc);
+    let bottom = rect(Color::RED).add(&mut h.rsc);
+    let column = (top, bottom).span(Dir::DOWN).add(&mut h.rsc);
+    let inner = Pad {
+        padding: Padding::ZERO,
+        inner: column.add_strong(&mut h.rsc),
+    }
+    .add(&mut h.rsc);
+    let outer = Pad {
+        padding: Padding::ZERO,
+        inner: inner.add_strong(&mut h.rsc),
+    }
+    .height(height)
+    .add(&mut h.rsc);
+    let beside = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((outer, beside).span(Dir::RIGHT));
+    vec![
+        top.id(),
+        bottom.id(),
+        column.id(),
+        inner.id(),
+        outer.id(),
+        beside.id(),
+    ]
+}
+
+#[test]
+fn changing_a_rule_over_two_pads_divides_the_column_again() {
+    let mut warm = Harness::new((900, 1200));
+    let ids = plant_two_shares_under_two_pads(&mut warm, 88.0);
+    warm.frame();
+    warm.rsc
+        .widgets_mut()
+        .set_size_rules(ids[4], None, Some(LayoutLen::px(105)));
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let cold_ids = plant_two_shares_under_two_pads(&mut cold, 105.0);
+    cold.frame();
+
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
+
+/// Six widgets, shrunk from seed 942 at depth 6 (`resize`). A `Branch` asks
+/// its probe in the top 40 px of its box and forwards the frame, so the
+/// scroll's own box is 40 px tall whatever the window is -- but its content
+/// is as tall as the frame, which is the window, and a scroll kept to its
+/// end has to be told when that changes. Resolving a length against the
+/// window is what reads it, so that is where the dependency is taken.
+fn plant_a_window_tall_column_in_a_short_scroll(h: &mut Harness) -> Vec<WidgetId> {
+    let leaf = rect(Color::RED).add(&mut h.rsc);
+    let column = Span {
+        children: vec![leaf.add_strong(&mut h.rsc)],
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    }
+    .height(rel(1.0))
+    .add(&mut h.rsc);
+    let scroll = Scroll::new(column.add_strong(&mut h.rsc), Axis::Y).add(&mut h.rsc);
+    let wide = rect(Color::BLUE).add(&mut h.rsc);
+    let narrow = rect(Color::GREEN).add(&mut h.rsc);
+    let root = Branch {
+        probe: scroll.add_strong(&mut h.rsc),
+        wide: wide.add_strong(&mut h.rsc),
+        narrow: narrow.add_strong(&mut h.rsc),
+        threshold: 55.0,
+    }
+    .add(&mut h.rsc);
+    h.set_root(root);
+    vec![leaf.id(), column.id(), scroll.id(), root.id()]
+}
+
+#[test]
+fn resizing_under_a_short_scroll_snaps_its_window_tall_content_again() {
+    let mut warm = Harness::new((1920, 1200));
+    let ids = plant_a_window_tall_column_in_a_short_scroll(&mut warm);
+    warm.frame();
+    warm.resize((640, 900));
+    warm.frame();
+
+    let mut cold = Harness::new((640, 900));
+    let cold_ids = plant_a_window_tall_column_in_a_short_scroll(&mut cold);
+    cold.frame();
+
+    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/generated.rs

@@ -14,7 +14,7 @@
 #[path = "scenario/mod.rs"]
 mod scenario;
 
-use iris::random::{Edits, plan};
+use iris::random::{Edits, Plan, plan};
 use scenario::{ALL, Case, diverges, env, over_seeds};
 
 /// How deep the generator branches. The generator widens two to four ways per
@@ -32,8 +32,11 @@ fn depth() -> usize {
 const SEEDS: [u64; 10] = [1, 2, 3, 5, 8, 10, 13, 20, 86, 98];
 
 fn check(seed: u64, depth: usize, case: Case) {
-    let grown = plan(seed, depth, &Edits::default());
+    check_plan(&plan(seed, depth, &Edits::default()), seed, depth, case);
-    if let Some(how) = diverges(&grown, case, seed) {
+}
+
+fn check_plan(grown: &Plan, seed: u64, depth: usize, case: Case) {
+    if let Some(how) = diverges(grown, case, seed) {
         panic!(
             "seed {seed} at depth {depth} differs after {}: {how}\n\
              reduce it with SHRINK_SEED={seed} SHRINK_DEPTH={depth} \
@@ -44,57 +47,50 @@ fn check(seed: u64, depth: usize, case: Case) {
     }
 }
 
-macro_rules! case {
+/// A test per case, and the list of which cases have one, from the same
-    ($name:ident, $case:expr) => {
+/// place. A case the ordinary suite leaves out runs only in the long scan,
-        #[test]
+/// which nobody runs by hand.
-        fn $name() {
+macro_rules! cases {
-            for seed in SEEDS {
+    ($($name:ident = $case:expr,)*) => {
-                check(seed, depth(), $case);
+        $(
+            #[test]
+            fn $name() {
+                for seed in SEEDS {
+                    check(seed, depth(), $case);
+                }
             }
-        }
+        )*
+
+        const NAMED: [Case; [$($case,)*].len()] = [$($case,)*];
     };
 }
 
-case!(
+cases! {
-    many_widgets_redrawing_at_once_leaves_every_box_where_it_was,
+    many_widgets_redrawing_at_once_leaves_every_box_where_it_was = Case::RepaintSome,
-    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,
-case!(
+    a_resize_and_a_repaint_land_where_starting_that_way_would = Case::ResizeRepaint,
-    everything_redrawing_at_once_leaves_every_box_where_it_was,
+    a_size_change_after_a_resize_lands_the_same_way = Case::ResizeSize,
-    Case::Repaint
+    a_resize_after_a_size_change_lands_the_same_way = Case::SizeResize,
-);
+    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,
-    a_resize_lands_where_starting_at_that_size_would,
+    an_alignment_change_lands_where_growing_it_that_way_would = Case::Align,
-    Case::Resize
+    giving_and_taking_a_movable_region_rebuilds_what_resolves_it = Case::RegionNode,
-);
+    reordering_a_span_lands_where_growing_it_that_way_would = Case::Reorder,
-case!(
+}
-    a_resize_and_a_repaint_land_where_starting_that_way_would,
+
-    Case::ResizeRepaint
+/// The shuffles are one test between them, so they are the only cases `ALL`
-);
+/// may hold without a test of their own.
-case!(
+#[test]
-    a_size_change_after_a_resize_lands_the_same_way,
+fn every_case_runs_without_the_long_scan() {
-    Case::ResizeSize
+    for case in ALL {
-);
+        assert!(
-case!(
+            NAMED.contains(&case) || matches!(case, Case::Shuffle(_)),
-    a_size_change_lands_where_growing_it_that_way_would,
+            "{} runs only in the long seed scan; give it a case here",
-    Case::Size
+            case.name()
-);
+        );
-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
-);
 
 #[test]
 fn adding_and_removing_span_children_lands_where_growing_it_that_way_would() {
@@ -119,8 +115,9 @@ fn a_long_run_of_seeds_agrees() {
         None => (1..=env("IRIS_GENERATED_SEEDS", 100_u64)).collect(),
     };
     over_seeds(seeds, |seed| {
+        let grown = plan(seed, depth, &Edits::default());
         for case in ALL {
-            check(seed, depth, case);
+            check_plan(&grown, seed, depth, case);
         }
     });
 }

tests/layout_dump.rs

@@ -0,0 +1,42 @@
+//! Prints where a cold layout puts every widget of many grown trees, so two
+//! commits can be compared on cold layout alone. The warm/cold oracle cannot
+//! see a change that moves cold layout, since both of its sides move; this
+//! can, by diffing its output across the change:
+//!
+//!     IRIS_DUMP_SEEDS=400 IRIS_DUMP_DEPTH=5 cargo test --release \
+//!       --test layout_dump -- --ignored --nocapture > /tmp/before.txt
+//!
+//! then the same after, and `diff` the two. A line is one widget: the seed,
+//! its index in creation order, and its box in window pixels, or `-` where
+//! it is not drawn.
+
+use iris::harness::Harness;
+use iris::random::{Edits, grow};
+
+fn env<T: std::str::FromStr>(name: &str, fallback: T) -> T {
+    std::env::var(name)
+        .ok()
+        .and_then(|value| value.parse().ok())
+        .unwrap_or(fallback)
+}
+
+#[test]
+#[ignore = "a dump to diff across commits, not a check"]
+fn every_cold_layout_is_printed() {
+    let seeds = env("IRIS_DUMP_SEEDS", 400_u64);
+    let depth = env("IRIS_DUMP_DEPTH", 5_usize);
+    let mut out = String::new();
+    for seed in 1..=seeds {
+        let mut harness = Harness::new((1920.0, 1200.0));
+        let (root, tree) = grow(&mut harness.rsc, seed, depth, &Edits::default());
+        harness.state.root = Some(root);
+        harness.frame();
+        for (index, id) in tree.ids.iter().enumerate() {
+            match harness.region(id) {
+                Some(region) => out.push_str(&format!("{seed} {index} {region:?}\n")),
+                None => out.push_str(&format!("{seed} {index} -\n")),
+            }
+        }
+    }
+    print!("{out}");
+}

tests/scenario/mod.rs

@@ -103,6 +103,11 @@ pub enum Case {
     /// A resize and then a size change, so a retained answer is asked to
     /// survive two different kinds of invalidation in a row.
     ResizeSize,
+    /// A size change and then a resize, which is the other order and not the
+    /// same test: a length answered as a fraction of one box and kept as a
+    /// fraction of another agrees at the size it was changed at and parts
+    /// from it at every other one.
+    SizeResize,
     /// A few declared sizes.
     Size,
     /// Every declared size at once, so every reader of a size has a changed
@@ -119,12 +124,13 @@ pub enum Case {
     Shuffle(Shuffle),
 }
 
-pub const ALL: [Case; 15] = [
+pub const ALL: [Case; 16] = [
     Case::Repaint,
     Case::RepaintSome,
     Case::Resize,
     Case::ResizeRepaint,
     Case::ResizeSize,
+    Case::SizeResize,
     Case::Size,
     Case::EverySize,
     Case::Align,
@@ -146,6 +152,7 @@ impl Case {
             Self::Resize => "resize",
             Self::ResizeRepaint => "resize-repaint",
             Self::ResizeSize => "resize-size",
+            Self::SizeResize => "size-resize",
             Self::Size => "size",
             Self::EverySize => "every-size",
             Self::Align => "align",
@@ -170,6 +177,15 @@ impl Case {
             _ => (STILL, STILL),
         }
     }
+
+    /// The window the warm tree is taken to after the change, where the case
+    /// is about what the change left behind rather than about the change.
+    fn then_resize(self) -> Option<(f32, f32)> {
+        match self {
+            Self::SizeResize => Some(INNER),
+            _ => None,
+        }
+    }
 }
 
 fn mark(warm: &mut Harness, tree: &Tree, step: usize) {
@@ -284,7 +300,7 @@ fn change(case: Case, warm: &mut Harness, tree: &mut Tree, plan: &Plan, rng: &mu
             warm.frame();
             return out;
         }
-        Case::Size | Case::ResizeSize => Edits {
+        Case::Size | Case::ResizeSize | Case::SizeResize => Edits {
             sizes: some_sizes(warm, tree, rng),
             ..Default::default()
         },
@@ -384,6 +400,17 @@ fn describe_widget(id: WidgetId, h: &Harness) -> String {
     label
 }
 
+/// One widget's layout as it stands: the frame its fractions resolved
+/// against, the box it was asked in, the box its drawing went in, and what
+/// it reported. In window units, which is what both trees are in.
+fn record(id: WidgetId, h: &Harness) -> String {
+    let active = &h.render.active[&id];
+    format!(
+        "rel_base {} region {} placement {} size {}",
+        active.rel_base, active.region, active.placement, active.size,
+    )
+}
+
 /// Runs `case` on the tree `plan` describes, warm and cold, and says where
 /// the two disagree. `seed` chooses only the values a case picks at random,
 /// so one plan under one case is one comparison however it was reached.
@@ -400,6 +427,17 @@ pub fn diverges(plan: &Plan, case: Case, seed: u64) -> Option<String> {
         warm.frame();
     }
     let cold_plan = change(case, &mut warm, &mut tree, plan, &mut Rng::new(seed));
+    // Whatever the change left, seen at another window: an answer kept as a
+    // fraction of the wrong length is the same number of pixels where it was
+    // made and a different one everywhere else.
+    let end = match case.then_resize() {
+        Some(after) => {
+            warm.resize(after);
+            warm.frame();
+            after
+        }
+        None => end,
+    };
 
     let mut cold = Harness::new(end);
     let (root, cold_tree) = build(&mut cold.rsc, &cold_plan);
@@ -413,9 +451,16 @@ pub fn diverges(plan: &Plan, case: Case, seed: u64) -> Option<String> {
         if got == want {
             continue;
         }
+        let places: HashMap<WidgetId, usize> = tree
+            .ids
+            .iter()
+            .enumerate()
+            .map(|(i, &id)| (id, i))
+            .collect();
         // Where two trees disagree is rarely where the cause is, so the
         // ancestry comes with it, marking the widgets that own a region.
         let mut chain = Vec::new();
+        let mut records = Vec::new();
         let mut at = Some(w);
         while let Some(id) = at {
             let active = &warm.render.active[&id];
@@ -424,11 +469,22 @@ pub fn diverges(plan: &Plan, case: Case, seed: u64) -> Option<String> {
                 false => "*",
             };
             chain.push(format!("{}{node}", describe(id, &warm)));
+            // What each level was asked in on both sides, since the level
+            // where the two stop agreeing is the one to look at rather than
+            // the leaf that reported the difference.
+            let cold_id = places.get(&id).and_then(|&i| cold_tree.ids.get(i));
+            records.push(format!(
+                "    {}\n      warm {}\n      cold {}",
+                describe(id, &warm),
+                record(id, &warm),
+                cold_id.map_or("-".into(), |&id| record(id, &cold)),
+            ));
             at = active.parent;
         }
         return Some(format!(
-            "widget {i}\n  warm {got:?}\n  cold {want:?}\n  {}",
+            "widget {i}\n  warm {got:?}\n  cold {want:?}\n  {}\n{}",
-            chain.join(" < ")
+            chain.join(" < "),
+            records.join("\n"),
         ));
     }
     match drawn {

tests/shrink.rs

@@ -70,10 +70,14 @@ fn no_grown_tree_lays_out_differently_warm_than_cold() {
     over_seeds(seeds, |seed| {
         let grown = plan(seed, depth, &Edits::default());
         for &case in &cases {
-            let Some(how) = diverges(&grown, case, seed) else {
+            if diverges(&grown, case, seed).is_none() {
                 continue;
-            };
+            }
             let small = shrink(grown.clone(), case, seed);
+            // Described from the shrunk tree: the grown tree's chain names
+            // widgets that are no longer there, and the ancestry of the
+            // failure is what a test is written from.
+            let how = diverges(&small, case, seed).unwrap_or_default();
             println!(
                 "seed {seed} case {}: {how}\ngrown {} widgets, shrank to {}\n{small:#?}",
                 case.name(),

tests/trace_unsettled.rs

@@ -42,12 +42,12 @@ 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 => {