Experiment: say the room from the cursor as an inset, and read the row's length only where a slot depends on it

Measured identical to its parent at every phase of the cost rig; kept as
evidence, not proposed for landing.

core/src/fixed.rs

@@ -109,6 +109,18 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
         })
     }
 
+    /// The first step at or above `v`, where [`Self::from_f32`] takes the
+    /// nearest one and is below it half the time. For a bound that has to
+    /// admit the value it came from: a measurement rounded down is a bound
+    /// that leaves out the thing it was measured from.
+    pub const fn ceil_from_f32(v: f32) -> Self {
+        let nearest = Self::from_f32(v);
+        match nearest.to_f32() < v {
+            true => nearest.next_up(),
+            false => nearest,
+        }
+    }
+
     /// From a number as it is written in source -- `16`, `1.5` -- which is
     /// the other place a value enters the grid.
     pub fn from_num(v: impl UiNum) -> Self {
@@ -372,6 +384,12 @@ impl<const SHIFT: u32> FixedVec2<SHIFT> {
         Self::new(Fixed::from_f32(v.x), Fixed::from_f32(v.y))
     }
 
+    /// The first step at or above each part, for a measurement reported as a
+    /// box: what it occupies is not less than what was measured.
+    pub fn ceil_from_f32(v: Vec2) -> Self {
+        Self::new(Fixed::ceil_from_f32(v.x), Fixed::ceil_from_f32(v.y))
+    }
+
     pub fn to_f32(self) -> Vec2 {
         Vec2::new(self.x.to_f32(), self.y.to_f32())
     }
@@ -488,6 +506,25 @@ mod tests {
         assert_eq!(Px::from_int(100) / Rel::from_f32(0.5), Px::from_int(200));
     }
 
+    /// The bound a greedy line break needs: the width it was measured at is
+    /// not on the grid, and the narrowest box the break still holds for is
+    /// the step at or above it, never the one below.
+    #[test]
+    fn a_ceiling_never_lands_below_the_number_it_came_from() {
+        let step = 1.0 / (1 << PX_SHIFT) as f32;
+        for n in 0..64 {
+            let v = 189.0 + n as f32 * step / 3.0;
+            let up = Px::ceil_from_f32(v);
+            assert!(up.to_f32() >= v, "{up:?} is below {v}");
+            assert!(
+                up.to_f32() - v < step,
+                "{up:?} is more than a step above {v}"
+            );
+        }
+        // An exact step is its own ceiling.
+        assert_eq!(Px::ceil_from_f32(189.5), Px::from_f32(189.5));
+    }
+
     #[test]
     fn a_number_from_outside_is_clamped_to_the_grid() {
         assert_eq!(Px::from_f32(1e12), Px::MAX);

core/src/layout_diagnostics.rs

@@ -32,7 +32,6 @@ pub(crate) enum Counter {
     SizeReads,
     HintHits,
     HintMisses,
-    RetainedSizeHits,
     ReuseAttempts,
     ReuseExact,
     ReuseMoved,
@@ -42,7 +41,6 @@ pub(crate) enum Counter {
     ReuseOutside,
     ReuseWrongLayer,
     ReuseWrongNode,
-    PlaceRedraws,
     QueuePops,
     DepthReads,
     LocalRedraws,
@@ -54,10 +52,13 @@ pub(crate) enum Counter {
     TextShapes,
     TextBreaks,
     GlyphPlacements,
+    OutsidePinnedLen,
+    OutsideFrame,
+    OutsideExtent,
 }
 
 impl Counter {
-    const COUNT: usize = Self::GlyphPlacements as usize + 1;
+    const COUNT: usize = Self::OutsideExtent as usize + 1;
 
     const NAMES: [&'static str; Self::COUNT] = [
         "updates",
@@ -67,7 +68,6 @@ impl Counter {
         "draw-result size reads",
         "hint hits",
         "hint misses",
-        "retained size hits",
         "reuse attempts",
         "reuse exact",
         "reuse moved",
@@ -77,7 +77,6 @@ impl Counter {
         "reuse: outside what it holds for",
         "reuse: another layer",
         "reuse: region-node choice changed",
-        "placed by redrawing",
         "redraw queue pops",
         "depth reads",
         "local redraws",
@@ -89,6 +88,9 @@ impl Counter {
         "text shapes",
         "text line breaks",
         "glyph placements",
+        "reuse outside: the length it was pinned to",
+        "reuse outside: a frame length",
+        "reuse outside: an extent length",
     ];
 }
 

core/src/orientation/align.rs

@@ -84,8 +84,7 @@ pub struct RegionAlign {
 }
 
 impl RegionAlign {
-    /// Both axes at the near edge. What a container passes as an override for
-    /// a child it is going to position itself.
+    /// 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,

core/src/orientation/axis.rs

@@ -7,6 +7,17 @@ pub enum Axis {
     Y,
 }
 
+impl Axis {
+    /// A per-axis pair with `aligned` on this axis and `ortho` on the other,
+    /// which is what `from_axis` does for a vector.
+    pub fn pair<T>(self, aligned: T, ortho: T) -> [T; 2] {
+        match self {
+            Self::X => [aligned, ortho],
+            Self::Y => [ortho, aligned],
+        }
+    }
+}
+
 impl std::ops::Not for Axis {
     type Output = Self;
 

core/src/orientation/len.rs

@@ -125,6 +125,13 @@ impl Size {
             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 {
@@ -151,6 +158,18 @@ impl LayoutLen {
         Len::from_parts(self.rel.add(share), 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);
+        Self {
+            px: part.px,
+            rel: part.rel,
+            leftover: self.leftover,
+        }
+    }
+
     pub fn px(px: impl UiNum) -> Self {
         Self {
             px: Px::from_num(px),

core/src/orientation/pos.rs

@@ -219,7 +219,7 @@ impl Len {
         }
     }
 
-    pub fn within_len(&self, len: Len) -> Self {
+    pub const fn within_len(&self, len: Len) -> Self {
         self.within(&UiSpan {
             start: Len::ZERO,
             end: len,

core/src/primitive/text.rs

@@ -107,13 +107,6 @@ impl Default for TextAttrs {
     }
 }
 
-/// How far below the longest line a width may fall and still be answered by
-/// the break in hand. A parent that offers a child the length it reported
-/// composes that length back through the box chain, so the two differ in the
-/// last bits -- and at exactly the longest line, that decides whether a line
-/// fits. Sub-pixel, so no break it admits is one a reader could see.
-const BREAK_EPSILON_PX: f32 = 0.05;
-
 /// Keeps text and its corresponding layout from getting out of sync.
 pub struct TextBuffer {
     text: String,
@@ -183,6 +176,23 @@ impl TextBuffer {
         self.layout_key.as_ref()?.max_width
     }
 
+    /// Widths covered by the current line breaks, including a wider shaping
+    /// retained when a later draw requested a narrower box.
+    pub fn width_holds(&self) -> crate::Holds {
+        let Some(width) = self.wrap_width() else {
+            return crate::Holds::ANY;
+        };
+        let width = Px::from_f32(width);
+        let soft_wrapped = self.layout.lines().any(|line| {
+            matches!(
+                line.break_reason(),
+                parley::layout::BreakReason::Regular | parley::layout::BreakReason::Emergency
+            )
+        });
+        let upper = if soft_wrapped { width } else { Px::MAX };
+        crate::Holds::from(Px::ceil_from_f32(self.layout.width()).min(width)..=upper)
+    }
+
     pub fn size(&self) -> Vec2 {
         Vec2::new(self.layout.width(), self.layout.height())
     }
@@ -200,15 +210,19 @@ impl TextBuffer {
         // A greedy break at one width is the same break at every width down
         // to the longest line it produced: each line still fits, and none can
         // take a word that would not fit in the wider box. So the layout in
-        // hand already answers, and re-breaking would only be a chance to
-        // disagree with itself -- which is what happens when a parent offers
-        // a child the length that child just reported, and the two land
-        // either side of a float.
+        // hand already answers, and re-breaking would only be work.
+        //
+        // At the longest line exactly, with no margin below it. A narrower
+        // width really does break differently, so answering one from the
+        // break in hand is how a warm tree keeps lines a cold tree would
+        // never produce. The margin was here because a text reports the
+        // width it used and a parent hands that back; the report is the step
+        // at or above its longest line now, so what comes back fits.
         if let Some(key) = &self.layout_key
             && key.attrs == *attrs
             && let (Some(broke_at), Some(want)) = (key.max_width, width)
             && want <= broke_at
-            && want + BREAK_EPSILON_PX >= self.layout.width()
+            && want >= self.layout.width()
         {
             #[cfg(feature = "layout-diagnostics")]
             diag::bump(Counter::TextShapeHits);

core/src/render/mod.rs

@@ -106,7 +106,8 @@ impl UiRenderNode {
             self.active.push(i);
             for change in draws.apply_free() {
                 if let Some(inst) = ui_render.active.get_mut(&change.id) {
-                    for h in &mut inst.primitives {
+                    for primitive in &mut inst.primitives {
+                        let h = &mut primitive.handle;
                         if h.layer == i && h.kind == change.kind && h.inst_idx == change.old {
                             h.inst_idx = change.new;
                             break;

core/src/render/shader/prelude.wgsl

@@ -35,6 +35,10 @@ struct MoveOffset {
 // belongs to the pixel above it. Flooring the product instead drops a pixel
 // wherever a fraction divides a window exactly: a fifth of 1920 comes out of
 // `REL_STEP` as 383.99998, and five tabs each lose their last column.
+//
+// Taken over the whole coordinate, fraction and pixels summed, since a floor
+// does not distribute over a sum: floored apart, a half of one and a half of
+// the other lose the pixel the two together make.
 fn snap_floor(v: vec2<f32>) -> vec2<f32> {
     return floor(v + PX_STEP * 0.5);
 }
@@ -147,8 +151,8 @@ fn vs_main(
     let bot_right_rel = vec2(r.x.end.rel, r.y.end.rel);
     let bot_right_px = vec2(r.x.end.px, r.y.end.px);
 
-    let top_left = snap_floor(top_left_rel * window.dim) + snap_floor(top_left_px);
-    let bot_right = snap_floor(bot_right_rel * window.dim) + snap_floor(bot_right_px);
+    let top_left = snap_floor(top_left_rel * window.dim + top_left_px);
+    let bot_right = snap_floor(bot_right_rel * window.dim + bot_right_px);
     let size = bot_right - top_left;
 
     let uv = vec2<f32>(
@@ -179,8 +183,8 @@ fn masked(in: VertexOutput, color: vec4<f32>) -> vec4<f32> {
     let br = vec2(m.x.end.rel, m.y.end.rel);
     let br_px = vec2(m.x.end.px, m.y.end.px);
 
-    let top_left = snap_floor(tl * window.dim) + snap_floor(tl_px);
-    let bot_right = snap_floor(br * window.dim) + snap_floor(br_px);
+    let top_left = snap_floor(tl * window.dim + tl_px);
+    let bot_right = snap_floor(br * window.dim + br_px);
     let pos = in.clip_position.xy;
     if pos.x < top_left.x || pos.x > bot_right.x || pos.y < top_left.y || pos.y > bot_right.y {
         return color * 0.0;

core/src/ui/active.rs

@@ -1,6 +1,6 @@
 use crate::{
-    Holds, LayerId, LayoutLen, MaskIdx, MoveIdx, PrimitiveHandle, RegionAlign, Size, TextureHandle,
-    UiRegion, WidgetId,
+    LayerId, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx, Place, RegionAlign, RetainedPrimitive,
+    Size, TextureHandle, UiRegion, UiVec2, WidgetId,
 };
 
 /// What is kept of a widget its parent has asked about. `drawn` says whether
@@ -9,20 +9,40 @@ use crate::{
 #[derive(Debug)]
 pub struct ActiveData {
     pub id: WidgetId,
-    /// The box its drawing is in, in `parent_move`'s coordinates.
-    pub region: UiRegion,
-    /// The box its parent first asked about it in, as a part of the box the
-    /// parent was itself asked in. Any later box it was given was decided
-    /// knowing its answer, so this is where a question about it is asked
-    /// again -- and it is kept relative so that it follows the parent's.
-    pub offer: UiRegion,
-    /// What it answered there: the size and what that held for.
-    pub answer: (Size, [Holds; 2]),
-    /// What the widget said it used of its box, the last time it drew.
+    /// Where its drawing goes, in its region node's coordinates.
+    pub extent: UiRegion,
+    /// What a fraction declared or reported under this widget is a fraction
+    /// of, as a length of the window.
+    pub frame: UiVec2,
+    /// A frame its parent decided for it on each axis -- a row's slot, or
+    /// padding's frame less its pixels -- as a length of the window. `None`
+    /// forwards the parent's frame. What it declared is kept separately in
+    /// `declared` and is a fraction of whichever of the two reached it.
+    pub narrow: [Option<Len>; 2],
+    /// Where its drawing was put, and where it was asked, each as a part of
+    /// its parent's box. The two differ where a container asks in one place
+    /// and puts the answer in another -- a row measures from its cursor and
+    /// puts the child in its slot. A part is a length from the box's start,
+    /// so a box that moved re-places every child by re-adding that start.
+    pub placed: [Place; 2],
+    pub asked: [Place; 2],
+    /// The box it was asked in, in the parent's region-node coordinates: the
+    /// box its drawing was made in and the one its contract is about. Its
+    /// drawing is placed elsewhere by re-expression, never by asking again.
+    pub 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)>,
+    /// 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 pixel lengths of `region`, per axis, that its drawing and `size`
-    /// hold for.
-    pub holds: [Holds; 2],
+    /// The window and extent reads that this drawing holds for, and the
+    /// frame and box it pinned.
+    pub holds: LayoutHolds,
     pub drawn: bool,
     pub parent: Option<WidgetId>,
     /// How far down the tree it was drawn, the root being 1. Carried down a
@@ -30,35 +50,42 @@ pub struct ActiveData {
     /// widget a frame visits and cannot drift while one is being drawn.
     pub depth: usize,
     pub textures: Vec<TextureHandle>,
-    pub primitives: Vec<PrimitiveHandle>,
+    /// Its primitives, each keeping the box it was written in -- in this
+    /// widget's extent coordinates, which is what a move recomposes from.
+    pub primitives: Vec<RetainedPrimitive>,
+    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 box.
+    /// The declared lengths whoever drew this widget resolved into its frame.
     /// 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],
-    /// The alignment its parent asked it with. A local redraw repeats that
-    /// question, including an override chosen by a container.
-    pub align: RegionAlign,
-    /// Whether that alignment was the parent's override rather than the
-    /// widget's own property.
-    pub align_override: bool,
-    /// Its own alignment when it was last drawn. A change to the property is
-    /// found against this even when its parent overrode the alignment.
+    /// 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 `region` uses.
+    /// The movable region whose coordinates `extent` uses 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.
     pub mask: MaskIdx,
+    /// That inherited one. The two differ exactly where the widget set a
+    /// mask of its own, which is the one it owns and the one a move rewrites
+    /// -- and the one a redraw of it must not be handed back, since setting
+    /// a mask asserts there is none.
+    pub parent_mask: MaskIdx,
     pub layer: LayerId,
 }
 
 impl ActiveData {
-    /// Whether its drawing and size hold for a box of these pixel lengths.
-    pub fn holds_at(&self, px: crate::PxVec2) -> bool {
-        self.holds[0].contains(px.x) && self.holds[1].contains(px.y)
+    /// What it answered when its parent asked, where it has been asked at
+    /// all. Not `size`, which is what its last drawing reported: a drawing
+    /// re-expressed in the box that answer chose is not a second answer.
+    pub fn measured(&self) -> Option<Size> {
+        self.answer.map(|(size, _)| size)
     }
 }

core/src/ui/holds.rs

@@ -1,4 +1,4 @@
-use crate::{Len, Px, REL_SHIFT, Rel, fixed::div_toward, fixed::narrow};
+use crate::{Len, Px, REL_SHIFT, fixed::div_toward, fixed::narrow};
 use std::ops::RangeInclusive;
 
 /// The lengths of a box, in pixels, that one drawing of a widget holds for:
@@ -10,9 +10,9 @@ use std::ops::RangeInclusive;
 ///
 /// The ends are lengths on the grid rather than floats with a tolerance
 /// around them: a box offered back at the length a widget reported comes back
-/// as the same number, so a range means what it says. What widening there is
-/// belongs to [`Self::through`], which has a rounding to undo, and is derived
-/// from that rounding rather than chosen.
+/// as the same number, so a range means what it says. The one place a range
+/// is wider than the length it came from is [`Self::through`], and what it is
+/// wider by is the floor that inverting a fraction undoes.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct Holds {
     pub lo: Px,
@@ -41,57 +41,53 @@ impl Holds {
     }
 
     /// What a box has to be for a part of it, `len` of the box long, to stay
-    /// in this range. A part with no relative extent is a fixed length: it
-    /// was drawn at that length and any box keeps it there.
+    /// in this range: the exact preimage of `px + floor(rel * box)`, which is
+    /// the one way a box in pixels is reached. A part with no relative extent
+    /// is a fixed length -- it was drawn at that length and any box keeps it
+    /// there.
     ///
-    /// The way in is `px + rel * box` taken to the nearest step, so a part
-    /// of exactly `lo` came from anything within half a step of it and the
-    /// answer is an interval even where this range is one length. Inverting
-    /// the length alone instead gives a point that need not even contain the
-    /// box the part was drawn in, which is a range excluding the drawing it
-    /// was made for.
+    /// The answer is an interval even where this range is a single length,
+    /// because the multiply on the way in drops to the step below and many
+    /// boxes therefore give one length. That is a floor rather than an
+    /// allowance: inverting it is two divisions and nothing else, and the
+    /// whole of a box maps back to itself.
     pub const fn through(self, len: Len) -> Self {
+        if self.lo.raw() == Px::MIN.raw() && self.hi.raw() == Px::MAX.raw() {
+            return Self::ANY;
+        }
         let rel = len.rel.raw() as i64;
         if rel == 0 {
             return Self::ANY;
         }
-        // In half steps, and no more than the arithmetic needs: too wide a
-        // range admits reusing a drawing where it does not hold, and too
-        // narrow a one leaves out the box a drawing was made in, which the
-        // `Holds` assertion in `draw_at` catches. `ROUTES` is at that floor
-        // -- two half steps fires it -- and tightening both ends moved not
-        // one of the rig's work counters, so the slack is not buying reuse.
-        //
-        // `ROUTES` covers a box composed down the chain against the same box
-        // measured against the window: two routes to one number, each
-        // rounding where the other does not. `way_in` covers the multiply
-        // this inverts, which drops a step and only ever downward, so it
-        // belongs at the top of the range alone -- and the whole of a box has
-        // no multiply in it, however many pixels were added to it, since
-        // multiplying by one is exact and taking the pixels off again is too.
-        // Allowing for it there anyway compounded, a step a level down a
-        // chain of widgets each taking the whole of its parent.
-        //
-        // Shifted by half of what a `Rel` counts in, to divide by the
-        // fraction: exact until the division takes it back to the grid.
-        const ROUTES: i64 = 3;
         let px = len.px.raw() as i64;
-        let half_rel = REL_SHIFT - 1;
-        let way_in = match rel == Rel::ONE.raw() as i64 {
-            true => 0,
-            false => 2,
-        };
-        let lo = ((self.lo.raw() as i64 - px) * 2 - ROUTES) << half_rel;
-        let hi = ((self.hi.raw() as i64 - px) * 2 + ROUTES + way_in) << half_rel;
-        // Dividing by a negative turns the ends around, so which end each
-        // bound comes from is decided before dividing rather than by taking
-        // the min and max of four divisions.
+        // `floor(rel * box) >= lo - px` is `rel * box >= (lo - px) << REL`, and
+        // `floor(rel * box) <= hi - px` is `rel * box < (hi - px + 1) << REL`.
+        let lo = (self.lo.raw() as i64 - px) << REL_SHIFT;
+        let hi = (((self.hi.raw() as i64 - px) + 1) << REL_SHIFT) - 1;
+        // Dividing by a negative fraction turns the ends around, so which
+        // bound each comes from is decided before dividing rather than by
+        // taking the min and max of four divisions.
         match rel > 0 {
             true => Self::raws(div_toward(lo, rel, true), div_toward(hi, rel, false)),
             false => Self::raws(div_toward(hi, rel, true), div_toward(lo, rel, false)),
         }
     }
 
+    /// What a box has to be for a part of it, this many pixels shorter, to
+    /// stay in this range: the range moved by that much, an end that was
+    /// unbounded staying so.
+    pub const fn longer_by(self, px: Px) -> Self {
+        let lo = match self.lo.raw() == Px::MIN.raw() {
+            true => self.lo,
+            false => self.lo.add(px),
+        };
+        let hi = match self.hi.raw() == Px::MAX.raw() {
+            true => self.hi,
+            false => self.hi.add(px),
+        };
+        Self { lo, hi }
+    }
+
     const fn raws(lo: i64, hi: i64) -> Self {
         Self {
             lo: Px::from_raw(narrow(lo)),
@@ -114,6 +110,16 @@ mod tests {
     use super::*;
     use crate::Rel;
 
+    #[test]
+    fn an_unrestricted_range_stays_unrestricted_through_any_length() {
+        for rel in [-2.0, -0.5, 0.0, 0.5, 1.0, 2.0] {
+            for px in [-8, 0, 8] {
+                let len = Len::from_parts(Rel::from_f32(rel), Px::from_int(px));
+                assert_eq!(Holds::ANY.through(len), Holds::ANY);
+            }
+        }
+    }
+
     #[test]
     fn through_reverses_a_range_for_a_negative_fraction() {
         // `10 - box / 2` is between 20 and 40 for boxes from -60 to -20.
@@ -136,26 +142,37 @@ mod tests {
     }
 
     /// A widget handed the whole of its parent's box, with or without pixels
-    /// taken off it, brings no multiply of its own: only the two routes to
-    /// the same length are left to allow for, and not a rounding that did
-    /// not happen. Widening for it as well grew the interval a level at a
-    /// time down a chain of them. Three half steps come back as one whole
-    /// one, since dividing by a whole box is dividing by one.
+    /// taken off it, has no fraction to invert: multiplying by one is exact
+    /// and taking the pixels off again is too, so the box maps back to
+    /// itself. Allowing for anything here compounded a step a level down a
+    /// chain of widgets each taking the whole of its parent.
     #[test]
-    fn the_whole_of_a_box_widens_by_the_routes_alone() {
+    fn the_whole_of_a_box_maps_back_to_itself() {
         let at = Px::from_int(956);
-        let one_step = |len: Px| Holds {
-            lo: len - Px::STEP,
-            hi: len + Px::STEP,
-        };
-        assert_eq!(Holds::at(at).through(Len::FULL), one_step(at));
+        assert_eq!(Holds::at(at).through(Len::FULL), Holds::at(at));
         let less_eight = Len::from_parts(Rel::ONE, Px::from_int(-8));
         assert_eq!(
             Holds::at(at).through(less_eight),
-            one_step(at + Px::from_int(8))
+            Holds::at(at + Px::from_int(8))
         );
     }
 
+    /// The range is the exact preimage at both ends, so a box one step
+    /// outside it really does give a length outside this range. What a wider
+    /// range costs is a drawing reused where it does not hold.
+    #[test]
+    fn a_box_one_step_outside_the_range_is_outside_it() {
+        let part = Len::from_parts(Rel::from_f32(1.0 / 3.0), Px::from_int(-146));
+        let at = Px::from_int(300);
+        let holds = Holds::at(at).through(part);
+        for inside in [holds.lo, holds.hi] {
+            assert_eq!(part.to_px(inside), at, "{inside:?} left out of {holds:?}");
+        }
+        for outside in [holds.lo.next_down(), holds.hi.next_up()] {
+            assert_ne!(part.to_px(outside), at, "{outside:?} admitted by {holds:?}");
+        }
+    }
+
     /// A truncating multiply only ever drops, so the step it needs allowing
     /// for on the way in belongs at the top of the range and not the bottom.
     #[test]

core/src/ui/layout_holds.rs

@@ -0,0 +1,79 @@
+use crate::{Axis, Holds, Len, PxVec2, UiRegion, UiVec2};
+
+const AXES: [Axis; 2] = [Axis::X, Axis::Y];
+
+/// What one evaluation of a widget depends on: the window lengths its reads
+/// hold for, the pixel lengths of its own box, and the symbolic lengths of
+/// that box and of its frame where either one is what it was expressed in.
+///
+/// The symbolic lengths are pins rather than ranges: a container places its
+/// children as lengths of its frame measured from where its own box starts,
+/// so what it draws turns on that box's length and on nothing about where it
+/// is. A box pin reaches the parent only where the box it pinned is the
+/// parent's own; anywhere else the parent chose that length itself, and a
+/// widget pinned this way is checked when it is re-placed.
+///
+/// A frame pin says the answer or the drawing is a fraction of the frame,
+/// which is a different length wherever the frame is a different one -- at
+/// the same window size, so no range of window pixels can say it. A length
+/// of the frame that is only pixels is not one: it is that many pixels
+/// whatever the frame turns out to be.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct LayoutHolds {
+    pub window: [Holds; 2],
+    pub frame_len: [Option<Len>; 2],
+    pub extent: [Holds; 2],
+    pub extent_len: [Option<Len>; 2],
+}
+
+impl LayoutHolds {
+    pub const ANY: Self = Self {
+        window: [Holds::ANY; 2],
+        frame_len: [None; 2],
+        extent: [Holds::ANY; 2],
+        extent_len: [None; 2],
+    };
+
+    pub fn and(self, other: Self) -> Self {
+        let mut result = Self::ANY;
+        for n in 0..2 {
+            result.window[n] = self.window[n].and(other.window[n]);
+            result.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]
+            );
+            debug_assert!(
+                self.frame_len[n].is_none()
+                    || other.frame_len[n].is_none()
+                    || self.frame_len[n] == other.frame_len[n]
+            );
+            result.extent_len[n] = self.extent_len[n].or(other.extent_len[n]);
+            result.frame_len[n] = self.frame_len[n].or(other.frame_len[n]);
+        }
+        result
+    }
+
+    pub fn covers(self, other: Self) -> bool {
+        (0..2).all(|n| {
+            self.window[n].lo <= other.window[n].lo
+                && self.window[n].hi >= other.window[n].hi
+                && self.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))
+                && self.frame_len[n].is_none_or(|len| other.frame_len[n] == Some(len))
+        })
+    }
+
+    pub fn contains(self, window: PxVec2, frame: UiVec2, extent: UiRegion) -> bool {
+        AXES.into_iter().all(|axis| {
+            let n = axis as usize;
+            let len = extent.axis(axis).len();
+            self.window[n].contains(window.axis(axis))
+                && self.frame_len[n].is_none_or(|pinned| pinned == frame.axis(axis))
+                && self.extent[n].contains(len.to_px(window.axis(axis)))
+                && self.extent_len[n].is_none_or(|pinned| pinned == len)
+        })
+    }
+}

core/src/ui/mod.rs

@@ -11,12 +11,16 @@ pub const CHAIN_LIMIT: u32 = 64;
 
 mod active;
 mod holds;
+mod layout_holds;
 mod painter;
+mod place;
 mod render_state;
 
 pub use active::*;
 pub use holds::*;
+pub use layout_holds::*;
 pub use painter::{Painter, PrimitiveLike};
+pub use place::*;
 pub use render_state::*;
 
 #[derive(Default)]
@@ -68,17 +72,24 @@ impl Moves {
         }
     }
 
-    /// Composes a region held in `idx`'s coordinates down the chain, which is
-    /// the same walk the vertex shader does.
+    /// The same walk the vertex shader does, in the same `Len` the shader is
+    /// handed, for asking where a drawing will actually land -- hit testing,
+    /// and nothing layout decides on. Layout threads its lengths down the
+    /// draw instead, so no box it compares is composed back up this chain.
     pub fn resolve(&self, idx: MoveIdx, local: UiRegion) -> UiRegion {
         let mut region = local;
+        self.walk(idx, |entry| region = region.within(entry));
+        region
+    }
+
+    fn walk(&self, idx: MoveIdx, mut step: impl FnMut(&UiRegion)) {
         let mut at = idx;
         for _ in 0..CHAIN_LIMIT {
             if at == MoveIdx::NONE {
-                return region;
+                return;
             }
-            let entry = self.arena[at.idx()];
-            region = region.within(&entry.region);
+            let entry = &self.arena[at.idx()];
+            step(&entry.region);
             at = entry.parent;
         }
         debug_assert!(
@@ -86,7 +97,6 @@ impl Moves {
             "a move chain longer than {CHAIN_LIMIT} resolves to the wrong place, \
              and the shader stops at the same depth"
         );
-        region
     }
 
     /// How many slots a region in `idx` is composed through, which is what

core/src/ui/painter.rs

@@ -1,14 +1,14 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
-    Axis, Holds, LayoutLen, Len, Px, PxVec2, RegionAlign, Rel, RenderedText, Size, StrongWidget,
-    TextAttrs, TextBuffer, TextData, TextureHandle, UiRegion, UiRenderState, UiRsc, UiVec2, Weight,
-    WidgetId, Widgets,
+    Axis, Holds, LayoutHolds, LayoutLen, Len, Part, Place, Px, PxVec2, RegionAlign, Rel,
+    RenderedText, RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData,
+    TextureHandle, UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
     render::{
-        GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveHandle, PrimitiveInst,
-        PrimitiveKind, TexturePrimitive,
+        GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveInst, PrimitiveKind,
+        TexturePrimitive,
     },
-    ui::render_state::DrawInfo,
+    ui::render_state::{DrawInfo, Placing},
 };
 const AXES: [Axis; 2] = [Axis::X, Axis::Y];
 
@@ -17,27 +17,40 @@ pub struct Painter<'a> {
     pub(super) state: &'a mut UiRenderState,
     pub(super) rsc: &'a mut dyn UiRsc,
 
-    /// This widget's box, in the coordinates of `move_idx`.
-    pub(super) region: UiRegion,
+    /// This widget's frame, per axis: a length of the window, and what a
+    /// fraction it or anything under it declares or reports is a fraction
+    /// of. A length rather than a box, so padding can take from both the
+    /// frame and the box without either becoming the other.
+    pub(super) frame: UiVec2,
+    /// Where this widget's drawing goes, in its region node's coordinates.
+    pub(super) extent: UiRegion,
+    /// 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],
+    /// The window in pixels. Frames and boxes become pixels against this one
+    /// unit, regardless of region-node boundaries.
+    pub(super) window: PxVec2,
     pub(super) mask: MaskIdx,
     pub(super) textures: Vec<TextureHandle>,
-    pub(super) primitives: Vec<PrimitiveHandle>,
+    pub(super) primitives: Vec<RetainedPrimitive>,
+    pub(super) mask_region: Option<UiRegion>,
+    /// 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 box each was asked in
-    /// is the one recorded as its offer.
-    pub(super) offered: Vec<WidgetId>,
-    /// The box this widget was first asked about in, in pixels.
-    pub(super) offered_px: PxVec2,
-    /// Whether this draw is in that box, 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 box in pixels, per axis: every
-    /// length until it reads one, then that one, unless it says otherwise.
-    pub(super) own: [Holds; 2],
-    /// What the children it asked about and drew keep it to.
-    pub(super) under: [Holds; 2],
+    /// What this draw itself read of the window in pixels, per axis: every
+    /// window until it reads one, then that one, unless it says otherwise.
+    pub(super) window_own: [Holds; 2],
+    /// Its frame's symbolic length where this draw read it, which makes the
+    /// drawing one that holds for that frame alone.
+    pub(super) frame_own_len: [Option<Len>; 2],
+    /// The window reads' equivalent for its own box.
+    pub(super) extent_own: [Holds; 2],
+    /// What each child's drawing depends on. Asking a child again replaces
+    /// its drawing, so it replaces this too rather than narrowing it.
+    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.
     pub(super) move_idx: MoveIdx,
@@ -57,6 +70,22 @@ impl<'a> Painter<'a> {
 
     /// Takes the kind, for a caller writing many of one primitive.
     fn write<P: Primitive>(&mut self, kind: PrimitiveKind<P>, primitive: P, region: UiRegion) {
+        self.write_resolved(kind, primitive, region, self.resolve(region));
+    }
+
+    /// A box in this widget's extent coordinates, composed into its region
+    /// node's coordinates.
+    fn resolve(&self, region: UiRegion) -> UiRegion {
+        region.within(&self.extent)
+    }
+
+    fn write_resolved<P: Primitive>(
+        &mut self,
+        kind: PrimitiveKind<P>,
+        primitive: P,
+        region: UiRegion,
+        resolved: UiRegion,
+    ) {
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::PrimitiveWrites);
         let h = self.state.layers.write(
@@ -65,15 +94,15 @@ impl<'a> Painter<'a> {
                 kind,
                 id: self.id,
                 primitive,
-                region,
+                region: resolved,
                 mask_idx: self.mask,
                 move_idx: self.move_idx,
             },
         );
-        self.push_primitive(h);
+        self.push_primitive(RetainedPrimitive { handle: h, region });
     }
 
-    fn push_primitive(&mut self, h: PrimitiveHandle) {
+    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);
@@ -81,112 +110,79 @@ impl<'a> Painter<'a> {
         self.primitives.push(h);
     }
 
-    /// Writes a primitive to be rendered
+    /// Writes a primitive over the whole of this widget's own box.
     pub fn primitive(&mut self, primitive: impl PrimitiveLike) {
         let primitive = primitive.into_primitive(self);
-        self.primitive_at(primitive, self.region)
+        self.primitive_at(primitive, UiRegion::FULL)
     }
 
+    /// Writes a primitive in a part of this widget's own box, in that box's
+    /// coordinates.
     pub fn primitive_within(&mut self, primitive: impl PrimitiveLike, region: UiRegion) {
         let primitive = primitive.into_primitive(self);
-        self.primitive_at(primitive, region.within(&self.region));
+        self.primitive_at(primitive, region);
     }
 
+    /// Sets a mask, in this widget's own box's coordinates.
     pub fn set_mask(&mut self, region: UiRegion) {
+        self.mask_region = Some(region);
         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 {
-            region,
-            move_idx: self.move_idx,
+            region: resolved,
+            move_idx,
         });
     }
 
-    /// Draws a widget within this widget's region.
+    /// Draws a widget in the whole of this widget's own box, with the frame
+    /// forwarded unchanged: what a container that is only a wrapper around
+    /// one child wants, and what every transparent container passes for the
+    /// frame.
     pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
-        self.widget_within(id, UiRegion::FULL)
+        self.widget_at(id, [None; 2], [Place::Within(Part::All); 2])
     }
 
-    /// What a widget's rules declare its lengths to be, which whoever draws
-    /// it resolves into its box. Reading them depends on nothing -- the box
-    /// that comes of them is kept on the child, and `redraw` compares it
-    /// there.
-    fn declared_lens<W: ?Sized>(&self, id: &StrongWidget<W>) -> [Option<LayoutLen>; 2] {
-        declared_lens(self.rsc.widgets(), id.id())
-    }
-
-    /// Takes back a child that was drawn only to find out how long it is.
-    /// Its drawing is dropped and it is not one of this widget's children
-    /// this frame; what it answered is still something this widget asked.
-    pub fn undraw<W: ?Sized>(&mut self, id: &StrongWidget<W>) {
-        self.children.retain(|child| *child != id.id());
-        self.state.undraw_rec(id.id(), self.rsc);
-    }
-
-    /// Draws a widget somewhere within this one. `region` is in this widget's
-    /// own coordinates, and the child's declared lengths are still to be
-    /// taken from it. Where the child's drawing sits inside what it is given
-    /// is the child's alignment, applied where the child is drawn, so a
-    /// container positions a child either by handing it a box of exactly its
-    /// length or by leaving it room and letting its alignment decide.
-    pub fn widget_within<'s, W: ?Sized>(
+    /// Asks a child, saying what its fractions are of and where it is asked.
+    ///
+    /// `narrow` is a length this widget decided for the child's frame, per
+    /// axis, as a length of this widget's own frame: a resolved share, or a
+    /// box a sibling's answer decided. `None` forwards this widget's frame,
+    /// which is what a container that only divides room passes, so a
+    /// fraction under it means the same wherever it sits and however deeply
+    /// it is nested. A declared length narrows the frame here whatever the
+    /// caller says. A narrowed frame is placed in the part by the child's
+    /// alignment and is the box the child is asked in.
+    ///
+    /// `place` is where the child is asked, per axis, as a part of this
+    /// widget's box: see [`Place`]. The child draws once, in that box, and
+    /// its answer is placed inside it by re-expressing the drawing. Nothing
+    /// is 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>,
-        region: UiRegion,
-    ) -> DrawResult<'s, 'a, W> {
-        self.widget_at(id, region, None)
-    }
-
-    /// Draws a widget with an alignment chosen by its container rather than
-    /// the widget's property. Containers use this when the box they hand down
-    /// already expresses the size they report around the child.
-    pub fn widget_aligned<'s, W: ?Sized>(
-        &'s mut self,
-        id: &'s StrongWidget<W>,
-        region: UiRegion,
-        align: RegionAlign,
-    ) -> DrawResult<'s, 'a, W> {
-        self.widget_at(id, region, Some(align))
-    }
-
-    fn widget_at<'s, W: ?Sized>(
-        &'s mut self,
-        id: &'s StrongWidget<W>,
-        region: UiRegion,
-        align_override: Option<RegionAlign>,
+        narrow: [Option<Len>; 2],
+        place: [Place; 2],
     ) -> DrawResult<'s, 'a, W> {
         let region_node = self.rsc.widgets().is_region_node(id.id());
         let declared = self.declared_lens(id);
-        let align = align_override.unwrap_or_else(|| self.rsc.widgets().alignment(id.id()));
-        // Composing `FULL` through a box is not quite the identity in f32,
-        // so a child with nothing declared keeps the box it would have had.
-        let local = match declared.iter().any(Option::is_some) {
-            true => declared_box(region, declared, align),
-            false => region,
-        };
-        let within = match local == UiRegion::FULL {
-            true => self.region,
-            false => local.within(&self.region),
-        };
+        let align = self.rsc.widgets().alignment(id.id());
+        let (frame, extent) =
+            frame_and_extent(self.extent, self.frame, place, narrow, declared, align);
         #[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, extent);
         }
         // 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 offer = match first_ask {
-            true => local,
-            false => self.state.active.get(&id.id()).map_or(local, |a| a.offer),
-        };
-        let answers_offer = self.at_offer && local == offer;
-        // The answer and what it holds for, both about the box 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, holds) = self.state.draw_inner(
+        let px = frame.to_px(self.window);
+        let (size, answer_holds, holds) = self.state.draw_inner(
             id.id(),
-            within,
             DrawInfo {
                 layer: self.layer,
                 parent: Some(self.id),
@@ -194,39 +190,93 @@ impl<'a> Painter<'a> {
                 parent_move: self.move_idx,
                 region_node,
                 mask: self.mask,
-                offer,
-                offered_px: self.px_within_offer(offer),
-                align: align_override,
+                frame,
+                part: extent,
+                placed: place,
+                asked: place,
+                narrow,
+                re_asked,
+                px,
             },
             None,
             self.rsc,
         );
-        if answers_offer {
-            self.state.active.get_mut(&id.id()).unwrap().answer = (size, holds);
-        }
-        // Whatever the child's answer holds for keeps this one to the boxes
-        // that give the child a length inside it.
-        for (axis, under) in AXES.into_iter().zip(self.under.iter_mut()) {
-            *under = under.and(holds[axis as usize].through(local.axis(axis).len()));
+        let holds = self.in_parent(holds, extent, place, narrow, declared);
+        let answer_holds = self.in_parent(answer_holds, extent, place, narrow, declared);
+        match self.under.iter_mut().find(|(child, _)| *child == id.id()) {
+            Some((_, kept)) => *kept = holds,
+            None => self.under.push((id.id(), holds)),
         }
         DrawResult {
             child: id,
             painter: self,
             size,
+            answer_holds,
         }
     }
 
-    /// What a child says its length is without being drawn, if it can say.
-    /// Asking counts as reading its size.
+    /// Takes back a child that was drawn only to find out how long it is.
+    /// Its drawing is dropped and it is not one of this widget's children
+    /// this frame; what it answered is still something this widget asked.
+    pub fn undraw<W: ?Sized>(&mut self, id: &StrongWidget<W>) {
+        self.children.retain(|child| *child != id.id());
+        self.under.retain(|(child, _)| *child != id.id());
+        self.state.undraw_rec(id.id(), self.rsc);
+    }
+
+    /// Puts a child asked about in this draw somewhere else in this
+    /// widget's box: its answer, placed in this part instead. The drawing
+    /// is re-expressed there rather than made again -- what a row does once
+    /// it knows every slot, having measured each child from its cursor.
+    pub fn place_at<W: ?Sized>(&mut self, id: &StrongWidget<W>, place: [Place; 2]) {
+        debug_assert!(
+            self.children.contains(&id.id()),
+            "'{}' placed a child it did not ask about in this draw",
+            self.label()
+        );
+        let at = self.placing();
+        self.state.place_in(id.id(), &at, place, self.rsc);
+    }
+
+    /// This widget as the thing its children are placed within.
+    fn placing(&self) -> Placing {
+        Placing {
+            id: self.id,
+            extent: self.extent,
+            frame: self.frame,
+            window: self.window,
+            depth: self.depth,
+            move_idx: self.move_idx,
+            mask: self.mask,
+        }
+    }
+
+    /// What a widget's rules declare its lengths to be, which whoever draws
+    /// it resolves into its frame. Reading them depends on nothing -- the box
+    /// that comes of them is kept on the child, and `redraw` compares it
+    /// there.
+    fn declared_lens<W: ?Sized>(&self, id: &StrongWidget<W>) -> [Option<LayoutLen>; 2] {
+        declared_lens(self.rsc.widgets(), id.id())
+    }
+
+    /// 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 frame, which is the frame a child asked with
+    /// nothing narrowed gets. Asking counts as reading its size.
     pub fn size_hint<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<LayoutLen> {
         let widgets = self.rsc.widgets();
         // A rule is the answer where there is one: it wins over whatever the
         // widget would draw, so it has to win over what the widget says too.
-        let hint = widgets.size_rules(id.id()).axis(axis).exact().or_else(|| {
-            widgets
-                .get_dyn(id.id())
-                .and_then(|widget| widget.size_hint(axis))
-        });
+        let hint = widgets
+            .size_rules(id.id())
+            .axis(axis)
+            .exact()
+            .or_else(|| {
+                widgets
+                    .get_dyn(id.id())
+                    .and_then(|widget| widget.size_hint(axis))
+            })
+            .map(|hint| hint.within_len(self.frame.axis(axis)));
         #[cfg(feature = "layout-diagnostics")]
         diag::hint_read(id.id(), self.id, axis, hint);
         match hint {
@@ -234,6 +284,11 @@ impl<'a> Painter<'a> {
                 #[cfg(feature = "layout-diagnostics")]
                 diag::bump(Counter::HintHits);
                 self.depend_on(id);
+                // A fraction was just resolved against this frame, so what
+                // this draw does with it is a function of the frame's length.
+                if hint.rel != Rel::ZERO {
+                    self.frame_own_len[axis as usize] = Some(self.frame.axis(axis));
+                }
                 Some(hint)
             }
             None => {
@@ -244,63 +299,6 @@ impl<'a> Painter<'a> {
         }
     }
 
-    /// A child's length in the box it is about to be offered, if it can be
-    /// had without drawing it: from its hint, or from a drawing it already
-    /// has that holds for that box.
-    pub fn known_len<W: ?Sized>(
-        &mut self,
-        child: &StrongWidget<W>,
-        axis: Axis,
-        region: UiRegion,
-    ) -> Option<LayoutLen> {
-        let declared = self.declared_lens(child);
-        let align = self.rsc.widgets().alignment(child.id());
-        let local = declared_box(region, declared, align);
-        let within = local.within(&self.region);
-        let first_ask = self.offer(child.id());
-        if first_ask && let Some(active) = self.state.active.get_mut(&child.id()) {
-            active.offer = local;
-        }
-        if let Some(hint) = self.size_hint(child, axis) {
-            return Some(hint);
-        }
-        let px = self.state.px_of(self.move_idx, within);
-        let (size, holds) =
-            self.state
-                .retained_size(child.id(), px, self.move_idx, self.rsc.widgets())?;
-        #[cfg(feature = "layout-diagnostics")]
-        diag::bump(Counter::RetainedSizeHits);
-        self.depend_on(child);
-        if first_ask {
-            let active = self.state.active.get_mut(&child.id()).unwrap();
-            active.answer = (size, holds);
-        }
-        for (axis, under) in AXES.into_iter().zip(self.under.iter_mut()) {
-            *under = under.and(holds[axis as usize].through(local.axis(axis).len()));
-        }
-        Some(size.axis(axis))
-    }
-
-    /// Whether this is the first box a child is asked about in during a draw
-    /// that is itself in the box it was asked in -- the question a cold
-    /// layout asks, whose answer is the one to keep.
-    fn offer(&mut self, child: WidgetId) -> bool {
-        if !self.at_offer || self.offered.contains(&child) {
-            return false;
-        }
-        self.offered.push(child);
-        true
-    }
-
-    /// The pixel size of a part of the box this widget was asked in.
-    fn px_within_offer(&self, local: UiRegion) -> PxVec2 {
-        let size = local.size();
-        PxVec2::new(
-            size.x.to_px(self.offered_px.x),
-            size.y.to_px(self.offered_px.y),
-        )
-    }
-
     fn depend_on<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
         if !self.size_deps.contains(&child.id()) {
             self.size_deps.push(child.id());
@@ -319,21 +317,27 @@ impl<'a> Painter<'a> {
         ui.text.render(buffer, attrs, width)
     }
 
+    /// Writes glyphs in the selected frame or extent 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.
+        let resolved = self.resolve(origin);
         let kind = self.rsc.ui_mut().primitives.kind::<GlyphPrimitive>();
         for glyph in text.glyphs.iter() {
-            let mut region = origin;
-            region.x.end = region.x.start;
-            region.y.end = region.y.start;
-            let mut region = region.offset(UiVec2::from_px(glyph.offset));
-            let size = PxVec2::new(
-                Px::from_int(glyph.entry.width as i32),
-                Px::from_int(glyph.entry.height as i32),
-            );
-            region.x.end = region.x.start.offset(size.x);
-            region.y.end = region.y.start.offset(size.y);
-            self.write(
+            let place = |mut region: UiRegion| {
+                region.x.end = region.x.start;
+                region.y.end = region.y.start;
+                let mut region = region.offset(UiVec2::from_px(glyph.offset));
+                let size = PxVec2::new(
+                    Px::from_int(glyph.entry.width as i32),
+                    Px::from_int(glyph.entry.height as i32),
+                );
+                region.x.end = region.x.start.offset(size.x);
+                region.y.end = region.y.start.offset(size.y);
+                region
+            };
+            self.write_resolved(
                 kind,
                 GlyphPrimitive {
                     uv_min: glyph.entry.uv_min,
@@ -342,15 +346,33 @@ impl<'a> Painter<'a> {
                     color: text.color,
                     flags: glyph.entry.flags(),
                 },
-                region,
+                place(origin),
+                place(resolved),
             );
         }
     }
 
-    /// This widget's box, in the coordinates its own primitives are written
-    /// in -- so a region composed `within` it may be drawn directly.
-    pub fn region(&self) -> UiRegion {
-        self.region
+    /// 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
+    /// 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 {
+        let len = self.extent.axis(axis).len();
+        self.extent_len[axis as usize] = Some(len);
+        len
+    }
+
+    /// The symbolic length of this widget's frame along one axis: what a
+    /// fraction it or anything under it declares is a fraction of. A
+    /// container reads it to hand a length of it down -- padding, which
+    /// takes its pixels off. Reading it pins the drawing to that frame, the
+    /// way [`Self::extent_len`] pins it to the box.
+    pub fn frame_len(&mut self, axis: Axis) -> Len {
+        let len = self.frame.axis(axis);
+        self.frame_own_len[axis as usize] = Some(len);
+        len
     }
 
     /// Where this widget sits in a box longer than the length it takes. A
@@ -378,49 +400,68 @@ impl<'a> Painter<'a> {
             .is_some()
     }
 
-    /// The part of this widget's box that something of `size` takes, at the
-    /// near edge. A container that reports one child's size gives every child
-    /// this, so what it draws is inside what it says it occupies.
-    pub fn box_of(&self, size: Size) -> UiRegion {
-        placed_box(UiRegion::FULL, size, RegionAlign::NEAR, [None; 2])
-    }
-
-    /// This widget's box in pixels. Reading it makes the drawing one that
-    /// holds for this box only, until `holds` says how far it goes.
+    /// This widget's own box in pixels. Reading it makes the drawing one
+    /// that holds for this box only, until `holds` says how far it goes.
     pub fn px_size(&mut self) -> PxVec2 {
-        let px = self.state.px_of(self.move_idx, self.region);
-        for (own, len) in self.own.iter_mut().zip([px.x, px.y]) {
-            if *own == Holds::ANY {
-                *own = Holds::at(len);
-            }
-        }
-        px
+        PxVec2::new(self.px_len(Axis::X), self.px_len(Axis::Y))
     }
 
-    /// One axis of this widget's box in pixels. Prefer this to
+    /// One axis of this widget's own box in pixels. Prefer this to
     /// [`Self::px_size`] when the other axis cannot affect the drawing.
     pub fn px_len(&mut self, axis: Axis) -> Px {
-        let len = self.state.px_of(self.move_idx, self.region).axis(axis);
-        let own = &mut self.own[axis as usize];
+        let part = self.extent.axis(axis).len();
+        let len = part.to_px(self.window.axis(axis));
+        let own = &mut self.extent_own[axis as usize];
         if *own == Holds::ANY {
             *own = Holds::at(len);
         }
         len
     }
 
-    /// The lengths of this widget's box on `axis` that what it is drawing
+    /// The lengths of this widget's own box on `axis` that what it is drawing
     /// holds for -- the same primitives, in the same fractions and offsets
-    /// 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.
+    /// 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 holds = holds.into();
         debug_assert!(
-            holds.contains(self.state.px_of(self.move_idx, self.region).axis(axis)),
+            holds.contains(part.to_px(self.window.axis(axis))),
             "'{}' ({:?}) says its drawing holds for lengths that leave out its own box",
             self.label(),
             self.id
         );
-        self.own[axis as usize] = holds;
+        self.extent_own[axis as usize] = holds;
+    }
+
+    /// A window length in pixels, which is what every length in layout is
+    /// measured in. Reading one pins the drawing to this window wherever the
+    /// length is a fraction of it; one that is only pixels is that many
+    /// pixels in any window and pins nothing.
+    pub fn to_px(&mut self, len: Len, axis: Axis) -> Px {
+        let window = self.window.axis(axis);
+        if len.rel != Rel::ZERO {
+            let own = &mut self.window_own[axis as usize];
+            if *own == Holds::ANY {
+                *own = Holds::at(window);
+            }
+        }
+        len.to_px(window)
+    }
+
+    /// The windows this drawing holds for, stated rather than taken: a
+    /// container that branched on a length in pixels says which side of the
+    /// boundary it was on, which is wider than the one window reading that
+    /// 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.window.axis(axis)),
+            "'{}' ({:?}) says its drawing holds for windows that leave out this one",
+            self.label(),
+            self.id
+        );
+        self.window_own[axis as usize] = holds;
     }
 
     pub fn text_data(&mut self) -> &mut TextData {
@@ -463,6 +504,7 @@ pub struct DrawResult<'p, 'a, W: ?Sized> {
     painter: &'p mut Painter<'a>,
     child: &'p StrongWidget<W>,
     size: Size,
+    answer_holds: LayoutHolds,
 }
 
 impl<W: ?Sized> DrawResult<'_, '_, W> {
@@ -473,6 +515,7 @@ impl<W: ?Sized> DrawResult<'_, '_, W> {
             diag::size_read(self.child.id(), self.painter.id, self.size);
         }
         self.painter.depend_on(self.child);
+        self.painter.answer_under = self.painter.answer_under.and(self.answer_holds);
         self.size
     }
 
@@ -506,6 +549,88 @@ impl PrimitiveLike for &TextureHandle {
     }
 }
 
+/// Moves what a child depends on into this widget's own terms: this
+/// method's `impl` block is where a `Painter`'s own boxes are, so it takes
+/// only what the child was asked with.
+impl Painter<'_> {
+    /// Window ranges are already about the one unit and combine directly.
+    /// A frame pin becomes this widget's own frame wherever a length of it
+    /// is what reached the child; where only pixels did, no length of this
+    /// frame can change the child's and the pin stops here.
+    ///
+    /// Extent validity maps back through the part of this widget's box,
+    /// where the box the child was asked in is that part; a declared length
+    /// places the box inside the part instead, and then only that length
+    /// reaches the child. A narrowed frame is not one of these: it decides
+    /// what fractions under the child mean and leaves the box the part it
+    /// was given.
+    fn in_parent(
+        &self,
+        holds: LayoutHolds,
+        extent: UiRegion,
+        place: [Place; 2],
+        narrow: [Option<Len>; 2],
+        declared: [Option<LayoutLen>; 2],
+    ) -> LayoutHolds {
+        let mut result = LayoutHolds::ANY;
+        for axis in AXES {
+            let n = axis as usize;
+            // Every read became pixels against the window, so a range on
+            // it is already in this widget's terms.
+            result.window[n] = holds.window[n];
+            let reaches = narrow[n].is_none()
+                && !matches!(place[n].part(), Part::Sized(_))
+                && declared[n].is_none_or(|len| len.rel != Rel::ZERO);
+            result.frame_len[n] = holds.frame_len[n].and(reaches.then(|| self.frame.axis(axis)));
+            match (place[n].part(), declared[n].is_some()) {
+                // Its box is this widget's own, or a part of it in that
+                // box's own lengths: so what it holds for is a range on this
+                // widget's own box, which is what lets that box move without
+                // a redraw. A length it pinned is this widget's length
+                // wherever the part is the whole of it, and pins the same
+                // way.
+                (Part::All, false) => {
+                    result.extent[n] = holds.extent[n];
+                    result.extent_len[n] = holds.extent_len[n];
+                }
+                // Its box is this widget's own less the inset. Where that is
+                // pixels, its box is exactly that many shorter in any window,
+                // so what it holds for is a range on this widget's box moved
+                // by them, and a length it pinned is this widget's length
+                // less them. An inset with a fraction in it is a different
+                // number of pixels in each window, and taking it off a length
+                // rounds once more than taking it off pixels does: there the
+                // child's box is a fixed expression of this one, so this
+                // widget's length is pinned and the range goes on the window
+                // through the child's box, the way a slot's does.
+                (Part::Inset { lead, trail }, false) => {
+                    let inset = lead + trail;
+                    match inset.rel == Rel::ZERO {
+                        true => {
+                            result.extent[n] = holds.extent[n].longer_by(inset.px);
+                            result.extent_len[n] = holds.extent_len[n].map(|pinned| pinned + inset);
+                        }
+                        false => {
+                            result.window[n] = result.window[n]
+                                .and(holds.extent[n].through(extent.axis(axis).len()));
+                            result.extent_len[n] = Some(self.extent.axis(axis).len());
+                        }
+                    }
+                }
+                // Its box is a length this widget decided, from its own
+                // frame 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[n] =
+                        result.window[n].and(holds.extent[n].through(extent.axis(axis).len()));
+                }
+            }
+        }
+        result
+    }
+}
+
 /// What a widget declares a length of its box to be. `leftover` is not one: a
 /// share of what is left over is only a length to the widget dividing one,
 /// so it passes up in the size instead.
@@ -526,52 +651,100 @@ pub(crate) fn declared_lens(widgets: &Widgets, id: WidgetId) -> [Option<LayoutLe
     })
 }
 
-/// The box a drawing occupies: the size the widget reported, on the side of
-/// the box it was asked in that its alignment says. An axis reported as 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 is
-/// left alone too: `declared_box` already placed it, in the parent's box,
-/// and the rule's length is what the widget reports there.
+/// 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.
 ///
-/// A reported fraction is a fraction of the box the widget drew in, where a
-/// declared one is a fraction of the box its parent handed down -- a span
-/// reporting `rel(1.0)` means all of what it was given, whatever that was a
-/// fraction of. So this scales by the box rather than composing into it.
-pub(crate) fn placed_box(
-    region: UiRegion,
+/// 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,
-    align: RegionAlign,
     declared: [Option<LayoutLen>; 2],
+    fill: [bool; 2],
+    align: RegionAlign,
 ) -> UiRegion {
-    let mut placed = region;
-    for (axis, declared) in AXES.into_iter().zip(declared) {
+    let mut placed = part;
+    for axis in AXES {
+        let n = axis as usize;
         let reported = size.axis(axis);
-        if reported.leftover != Weight::ZERO || declared.is_some() {
+        if fills(reported, declared[n], fill[n]) {
             continue;
         }
+        let len = Len::from_parts(reported.rel, reported.px);
         let span = placed.axis_mut(axis);
-        let len = span.len().scale(reported.rel) + Len::from_parts(Rel::ZERO, reported.px);
         span.start += (span.len() - len).scale(align.axis(axis).rel());
         span.end = span.start + len;
     }
     placed
 }
 
-/// Takes a widget's declared lengths in the box `region` is given in, since a
-/// fraction of a length means a fraction of that one, and puts what is left
-/// over on the side its alignment says. A caller that already reserved the
-/// space hands back the same length, so this is the identity for it.
-pub(crate) fn declared_box(
-    mut region: UiRegion,
+/// The frame 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. `narrow` is a frame the container decided for the child -- a row's
+/// slot, or padding's frame less its pixels -- and [`Part::Sized`] one a
+/// sibling's answer decided; both are window lengths, 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 of the three that also places the box: a box the
+/// caller decided is what `place` names.
+pub(crate) fn frame_and_extent(
+    own: UiRegion,
+    parent_frame: UiVec2,
+    place: [Place; 2],
+    narrow: [Option<Len>; 2],
     declared: [Option<LayoutLen>; 2],
     align: RegionAlign,
-) -> UiRegion {
-    for (axis, len) in AXES.into_iter().zip(declared) {
-        let Some(len) = len else { continue };
-        let span = region.axis_mut(axis);
-        let len = Len::from_parts(len.rel, len.px);
-        span.start += (span.len() - len).scale(align.axis(axis).rel());
-        span.end = span.start + len;
+) -> (UiVec2, UiRegion) {
+    let part = part_of(own, place, align);
+    let mut frame = parent_frame;
+    let mut extent = part;
+    for axis in AXES {
+        let n = axis as usize;
+        let sized = match place[n].part() {
+            Part::Sized(len) => Some(len),
+            _ => None,
+        };
+        let base = sized
+            .or(narrow[n])
+            .unwrap_or_else(|| parent_frame.axis(axis));
+        let len = declared[n]
+            .map(|len| Len::from_parts(len.rel, len.px).within_len(base))
+            .unwrap_or(base);
+        *frame.axis_mut(axis) = len;
+        if declared[n].is_some() {
+            let slot = part.axis(axis);
+            let start = slot.start + (slot.len() - len).scale(align.axis(axis).rel());
+            *extent.axis_mut(axis) = UiSpan::new(start, start + len);
+        }
     }
-    region
+    (frame, extent)
+}
+
+/// The part of a widget's own box a `place` names, in the coordinates that
+/// box is in.
+fn part_of(extent: UiRegion, place: [Place; 2], align: RegionAlign) -> UiRegion {
+    let mut part = extent;
+    for axis in AXES {
+        *part.axis_mut(axis) = place[axis as usize]
+            .part()
+            .of(*extent.axis(axis), align.axis(axis));
+    }
+    part
 }

core/src/ui/place.rs

@@ -0,0 +1,74 @@
+use crate::{AxisAlign, Len, PrimitiveHandle, UiRegion, UiSpan};
+
+/// What of a widget's own box a child is given, along one axis.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum Part {
+    /// The whole of it.
+    All,
+    /// Window lengths from where the box starts, 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. A moved box
+    /// re-places every child by re-adding its start, exactly. A fraction
+    /// here is a fraction of the window and not of the box -- the whole of a
+    /// box is [`Self::All`], not a `rel(1.0)` span.
+    From(UiSpan),
+    /// The box less a window length at each end, which is what a container
+    /// that insets one speaks -- padding, or a row asking a child in the
+    /// room left from its cursor. Neither end names the box's length, so a
+    /// container can say "from here to my end" without reading how long it
+    /// is, and a box chosen from its own answer does not feed back into the
+    /// answer.
+    Inset { lead: Len, trail: Len },
+    /// A box of this length, wherever in the parent's box the child's own
+    /// alignment puts it, and that same length as its frame. Unlike `From`,
+    /// it is a length decided from above rather than a place along a
+    /// container's cursor -- what a stack's sizing child decides for the
+    /// rest.
+    Sized(Len),
+}
+
+impl Part {
+    /// Where it lands in the coordinates `extent` is in.
+    pub(crate) fn of(self, extent: UiSpan, align: AxisAlign) -> UiSpan {
+        match self {
+            Self::All => extent,
+            Self::From(span) => UiSpan::new(extent.start + span.start, extent.start + span.end),
+            Self::Inset { lead, trail } => UiSpan::new(extent.start + lead, extent.end - trail),
+            Self::Sized(len) => {
+                let start = extent.start + (extent.len() - len).scale(align.rel());
+                UiSpan::new(start, start + len)
+            }
+        }
+    }
+}
+
+/// Where a child goes along one axis, as a part of this widget's box.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum Place {
+    /// The child's answer, aligned inside the part by the child's alignment.
+    Within(Part),
+    /// Exactly the part; the answer is not placed inside it again.
+    Fill(Part),
+}
+
+impl Place {
+    pub(crate) fn part(self) -> Part {
+        match self {
+            Self::Within(part) | Self::Fill(part) => part,
+        }
+    }
+
+    /// Whether the part is the drawing's box outright, rather than the box
+    /// the answer is placed inside.
+    pub(crate) fn fills(self) -> bool {
+        matches!(self, Self::Fill(_))
+    }
+}
+
+/// A primitive as it was written: its box in the widget's own box's
+/// coordinates, which is what a move of that box re-composes from.
+#[derive(Debug)]
+pub struct RetainedPrimitive {
+    pub handle: PrimitiveHandle,
+    pub region: UiRegion,
+}

core/src/util/slot.rs

@@ -1,4 +1,4 @@
-#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, PartialOrd, Ord)]
 pub struct SlotId {
     idx: u32,
     genr: u32,

examples/tabs/main.rs

@@ -20,10 +20,14 @@ impl DefaultAppState for Client {
         let pad_test = (
             rrect.color(Color::BLUE),
             (
+                // The square is one widget and the two shares of the row it
+                // sits centred in are another: a length is a property of a
+                // widget, so `.width` here would overwrite the `.sized`.
                 rrect
                     .color(Color::RED)
                     .sized((100, 100))
                     .center()
+                    .wrapper()
                     .width(leftover(2)),
                 (
                     rrect.color(Color::ORANGE),
@@ -143,7 +147,7 @@ impl DefaultAppState for Client {
             .span(Dir::DOWN)
             .add(rsc);
 
-        let main = WidgetPtr::new().add(rsc);
+        let main = Wrapper::new().add(rsc);
 
         let vals = Rc::new(RefCell::new((0, Vec::new())));
         let mut switch_button = |color, to: WeakWidget, label| {

examples/text.rs

@@ -28,10 +28,14 @@ impl DefaultAppState for State {
             .pad(16)
             .background(panel());
 
+        // Each one takes the whole width, because `text_align` puts the
+        // glyphs somewhere in the box the text is given and a text that
+        // reports the width of its own glyphs is given exactly that.
+        let label = |text: &str, align| wtext(text).size(24).text_align(align).width(rel(1.0));
         let aligned = (
-            wtext("left").size(24).text_align(Align::LEFT),
-            wtext("centred").size(24).text_align(Align::CENTER),
-            wtext("right").size(24).text_align(Align::RIGHT),
+            label("left", Align::LEFT),
+            label("centred", Align::H_CENTER),
+            label("right", Align::RIGHT),
         )
             .span(Dir::DOWN)
             .gap(8)

src/default/mod.rs

@@ -251,7 +251,7 @@ impl<State: DefaultAppState> AppState for DefaultApp<State> {
                 ui_state.renderer.draw();
             }
             WindowEvent::Resized(size) => {
-                render.resize((size.width, size.height));
+                render.resize((size.width, size.height), rsc.widgets_mut());
                 ui_state.renderer.resize(size)
             }
             WindowEvent::KeyboardInput { event, .. } => {

src/harness.rs

@@ -144,9 +144,9 @@ impl Harness {
         // bound that comes with `SyncSender` is far past anything a test
         // leaves unread.
         let (send, updates) = sync_channel(1024);
-        let rsc = DefaultRsc::init(Arc::new(Queue(send)));
+        let mut rsc = DefaultRsc::init(Arc::new(Queue(send)));
         let mut render = UiRenderState::new();
-        render.resize(size);
+        render.resize(size, rsc.widgets_mut());
         Self {
             rsc,
             render,
@@ -161,7 +161,7 @@ impl Harness {
     }
 
     pub fn resize(&mut self, size: impl Into<Vec2>) {
-        self.render.resize(size);
+        self.render.resize(size, self.rsc.widgets_mut());
     }
 
     /// Changes a length rule after the fact, the way `.width()` sets one.

src/random.rs

@@ -117,19 +117,35 @@ pub struct Branch {
 
 impl Widget for Branch {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let mut top = UiRegion::FULL;
-        top.y.end = top.y.start.offset(Px::from_int(40));
-        let measured = painter.widget_within(&self.probe, top).len(Axis::X);
-        let px = measured.apply_leftover().to_px(painter.px_len(Axis::X));
+        let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
+        let top = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
+        let measured = painter
+            .widget_at(&self.probe, [None; 2], [Place::Within(Part::All), top])
+            .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 mut below = UiRegion::FULL;
-        below.y.start = below.y.start.offset(Px::from_int(40));
-        match px > Px::from_f32(self.threshold) {
-            true => painter.widget_within(&self.wide, below),
-            false => painter.widget_within(&self.narrow, below),
+        let below = Place::Within(Part::From(UiSpan::new(cut, painter.extent_len(Axis::Y))));
+        let place = [Place::Within(Part::All), below];
+        match px > threshold {
+            true => painter.widget_at(&self.wide, [None; 2], place),
+            false => painter.widget_at(&self.narrow, [None; 2], place),
         };
         Size::LEFTOVER
     }
+
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::LEFTOVER)
+    }
 }
 
 pub struct Spanned {

src/widget/mask.rs

@@ -6,7 +6,7 @@ pub struct Masked {
 
 impl Widget for Masked {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        painter.set_mask(painter.region());
+        painter.set_mask(UiRegion::FULL);
         painter.widget(&self.inner);
         // What it occupies is its box, on both axes, for the reason `Scroll`
         // reports the same: it clips what is inside to that box, so it can
@@ -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/mod.rs

@@ -1,15 +1,15 @@
 mod image;
 mod mask;
 mod position;
-mod ptr;
 mod rect;
 mod text;
 mod trait_fns;
+mod wrapper;
 
 pub use image::*;
 pub use mask::*;
 pub use position::*;
-pub use ptr::*;
 pub use rect::*;
 pub use text::*;
 pub use trait_fns::*;
+pub use wrapper::*;

src/widget/position/offset.rs

@@ -7,7 +7,14 @@ pub struct Offset {
 
 impl Widget for Offset {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let region = UiRegion::FULL.offset(self.amt);
-        painter.widget_within(&self.inner, region).size()
+        // The whole of this widget's box, moved: the frame passes through, so
+        // what the child declares or reports means the same as it would
+        // without the offset.
+        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, [None; 2], place).size()
     }
 }

src/widget/position/pad.rs

@@ -13,9 +13,32 @@ impl Widget for Pad {
         // it; where the box is bigger -- a share of a row, a rule over this
         // 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.
-        let inner = painter
-            .widget_within(&self.inner, self.padding.region())
-            .size();
+        //
+        // Padding is an inset of both: it comes off the frame, so `rel(1)`
+        // under it fills this widget rather than overflowing it by the
+        // padding, and it comes off the box, so what is drawn sits inside.
+        // The two stay distinct -- the box can be narrower still, where a row
+        // asked this widget in the room left, and a text wraps at that.
+        let inset = |lead: Px, trail: Px| {
+            Place::Within(Part::Inset {
+                lead: Len::from_parts(Rel::ZERO, lead),
+                trail: Len::from_parts(Rel::ZERO, trail),
+            })
+        };
+        let place = [
+            inset(self.padding.left, self.padding.right),
+            inset(self.padding.top, self.padding.bottom),
+        ];
+        // Read from this widget's own frame rather than written as a
+        // fraction of it: a frame is a length of the window like everything
+        // else here, and taking the padding off is the whole of what this
+        // widget does to it.
+        let narrow = [
+            (Axis::X, self.padding.left + self.padding.right),
+            (Axis::Y, self.padding.top + self.padding.bottom),
+        ]
+        .map(|(axis, pixels)| Some(painter.frame_len(axis) - Len::from_parts(Rel::ZERO, pixels)));
+        let inner = painter.widget_at(&self.inner, narrow, place).size();
         Size {
             x: LayoutLen {
                 px: inner.x.px + self.padding.left + self.padding.right,
@@ -53,14 +76,18 @@ impl Padding {
             bottom: amt,
         }
     }
-    pub fn region(&self) -> UiRegion {
-        let mut region = UiRegion::FULL;
+    /// `region` less this padding on each side.
+    pub fn region_of(&self, mut region: UiRegion) -> UiRegion {
         region.x.start.px += self.left;
         region.y.start.px += self.top;
         region.x.end.px -= self.right;
         region.y.end.px -= self.bottom;
         region
     }
+
+    pub fn region(&self) -> UiRegion {
+        self.region_of(UiRegion::FULL)
+    }
     pub fn x(amt: impl UiNum) -> Self {
         let amt = Px::from_num(amt);
         Self {

src/widget/position/scroll.rs

@@ -12,15 +12,13 @@ pub struct Scroll {
 impl Widget for Scroll {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let container_len = painter.px_len(self.axis);
-        // Draw in the whole container only when its scrolling-axis length is
-        // not already known, then draw it at the scrolled offset.
-        let answer_len = match painter.known_len(&self.inner, self.axis, UiRegion::FULL) {
-            Some(len) => len,
-            None => painter.widget(&self.inner).size().axis(self.axis),
-        };
-        let content = answer_len.apply_leftover();
+        // Asked in the whole viewport, then put at the scrolled offset.
+        let answer_len = painter
+            .widget_at(&self.inner, [None; 2], [Place::Fill(Part::All); 2])
+            .len(self.axis);
+        let fixed = painter.to_px(Len::from_parts(answer_len.rel, answer_len.px), self.axis);
         self.container_len = container_len;
-        self.content_len = content.to_px(container_len);
+        self.content_len = fixed.max(container_len);
 
         if self.snap_end {
             self.amt = self.content_len - self.container_len;
@@ -33,9 +31,9 @@ impl Widget for Scroll {
         // 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 = content.rel == Rel::ZERO;
+        let fixed_len = answer_len.rel == Rel::ZERO && answer_len.leftover == Weight::ZERO;
         if fixed_len && self.content_len <= self.container_len && align == AxisAlign::NEG {
-            painter.holds(self.axis, self.content_len..=Px::MAX);
+            painter.holds(self.axis, fixed..=Px::MAX);
         } else if fixed_len && !self.snap_end {
             let left = self.content_len - self.amt;
             painter.holds(self.axis, Px::MIN..=left);
@@ -46,7 +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 region = UiRegion::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
@@ -54,22 +51,31 @@ 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 offset = UiVec2::from_axis(
-                self.axis,
-                Len::from_parts(Rel::ZERO, anchor - self.amt),
-                Len::ZERO,
-            );
-            region = region.offset(offset);
-            region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
-        }
-        painter.widget_aligned(&self.inner, region, RegionAlign::NEAR);
+        let content = match moved || self.content_len != self.container_len {
+            true => {
+                let start = Len::from_parts(Rel::ZERO, anchor - self.amt);
+                Part::From(UiSpan::new(start, start.offset(self.content_len)))
+            }
+            false => Part::All,
+        };
+        // The viewport is the inner's frame, 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 goes is the content
+        // box, scrolled: its drawing moved there, not made again there.
+        painter.place_at(
+            &self.inner,
+            self.axis.pair(Place::Fill(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

@@ -10,20 +10,51 @@ pub struct Span {
 impl Widget for Span {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let axis = self.dir.axis;
-        // A length for every child before their final boxes are chosen: from
-        // a hint where one exists, and from drawing otherwise.
+        // The room left from the cursor to the row's end, said without the
+        // row's length: a child measured in it does not make this drawing
+        // depend on how long the row is.
+        let room_from = |cursor: Len| match self.dir.sign {
+            Sign::Pos => Part::Inset {
+                lead: cursor,
+                trail: Len::ZERO,
+            },
+            Sign::Neg => Part::Inset {
+                lead: Len::ZERO,
+                trail: cursor,
+            },
+        };
+        // Across itself the child sits where its own alignment says, in the
+        // whole of the row: a span is what contains its children there, and
+        // nothing divides that axis.
+        let across = Place::Within(Part::All);
+        // A length for every child before their final slots are chosen: from
+        // a hint where one says, and from drawing otherwise. 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 a drawn child is asked in is the room left from the
+        // cursor, because a text has to wrap at the width actually there.
+        // This is the one ask a drawn fixed child gets: its slot is its
+        // answer, and the drawing is moved there once the shares are known.
+        // A hinted child is asked once, in its slot.
         let mut cursor = Len::rel_min();
         let mut lens = Vec::with_capacity(self.children.len());
+        let mut measured = Vec::with_capacity(self.children.len());
         for child in &self.children {
-            let mut span = UiSpan::new(cursor, Len::rel_max());
-            if self.dir.sign == Sign::Neg {
-                span.flip();
-            }
-            let region = UiRegion::from_axis(axis, span, UiSpan::FULL);
-            let len = match painter.known_len(child, axis, region) {
-                Some(len) => len,
-                None => painter.widget_within(child, region).len(axis),
+            let size = match painter.size_hint(child, axis) {
+                Some(len) => {
+                    measured.push(None);
+                    len
+                }
+                None => {
+                    let room = Place::Within(room_from(cursor));
+                    let size = painter
+                        .widget_at(child, [None; 2], axis.pair(room, across))
+                        .size();
+                    measured.push(Some(size));
+                    size.axis(axis)
+                }
             };
+            let len = size;
             cursor.px += len.px + self.gap;
             cursor.rel += len.rel;
             lens.push(len);
@@ -40,43 +71,42 @@ impl Widget for Span {
             |sum, len| sum + *len,
         );
 
+        // The row: this span's own box as a length of the window, read only
+        // where a slot depends on it -- shares divide what is left of it,
+        // and a negative row counts from its end. Reading it pins the
+        // drawing to this length; a positive row of fixed children is not
+        // pinned and holds for any length its children do. Its start is
+        // nothing's business: a slot is a length from it.
+        let far = (total.leftover > Weight::ZERO || self.dir.sign == Sign::Neg)
+            .then(|| painter.extent_len(axis));
+        let along = |from: Len, to: Len| match self.dir.sign {
+            Sign::Pos => UiSpan::new(from, to),
+            Sign::Neg => {
+                let far = far.expect("a negative row reads its length");
+                UiSpan::new(far - to, far - from)
+            }
+        };
+        // 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.
+        // Nothing where there are no shares, and nothing reads it there.
+        let room = far.map_or(Len::ZERO, |far| far - Len::from_parts(total.rel, total.px));
         // Whether anything is left over is a question in pixels: `rel(0.5)`
-        // beside 300 px is full at 600 and overfull at 400. The room to
-        // divide is `len * fixed - total.px`, and the length where it runs
-        // out is exactly the box a parent sizing itself from this answer
-        // hands back -- which is why this used to need a margin either side
-        // of the boundary, and why it does not now: that box and this sum are
-        // whole counts of the same step, and both routes to it land on the
-        // same count. What the generated oracle checks is the consequence,
-        // since which children exist at all turns on this.
-        let fixed = Rel::ONE - total.rel;
+        // beside 300 px is full at 600 and overfull at 400. Asked of `room`
+        // itself, and answered back through the same expression, so the
+        // boundary is the drawing's own and not a second way of finding it:
+        // the three cases a rounded division needed -- the fixed parts
+        // growing slower than the box, faster, or exactly with it -- are the
+        // sign of `room.rel`, which `through` already reads. What the
+        // generated oracle checks is the consequence, since which children
+        // exist at all turns on this.
         let mut shares = false;
         if total.leftover > Weight::ZERO {
-            let current = painter.px_len(axis);
-            let holds = if fixed > Rel::ZERO {
-                // The box length the fixed parts alone fill.
-                let full = total.px.div(fixed);
-                shares = current > full;
-                match shares {
-                    true => Holds::from(full.next_up()..=Px::MAX),
-                    false => Holds::from(Px::MIN..=full),
-                }
-            } else if fixed < Rel::ZERO {
-                // The relative parts grow faster than the box does, so here
-                // a shorter box is the one that leaves room.
-                let full = total.px.div(fixed);
-                shares = current < full;
-                match shares {
-                    true => Holds::from(Px::MIN..=full.next_down()),
-                    false => Holds::from(full..=Px::MAX),
-                }
-            } else {
-                // The relative parts take exactly the box, whatever it is, so
-                // the only room is what negative pixels leave.
-                shares = total.px < Px::ZERO;
-                Holds::ANY
+            shares = painter.to_px(room, axis) > Px::ZERO;
+            let holds = match shares {
+                true => Holds::from(Px::STEP..=Px::MAX),
+                false => Holds::from(Px::MIN..=Px::ZERO),
             };
-            painter.holds(axis, holds);
+            painter.window_holds(axis, holds.through(room));
         }
 
         // Across itself a span is as long as its longest child -- unless a
@@ -93,10 +123,10 @@ impl Widget for Span {
         // row.
         let mut fixed = Len::rel_min();
         let mut taken = Weight::ZERO;
-        let room = Len::rel_max() - Len::from_parts(total.rel, total.px);
         let mut start = Len::rel_min();
         let mut ortho = LayoutLen::ZERO;
-        for (child, len) in self.children.iter().zip(&lens) {
+        for ((child, len), measured) in self.children.iter().zip(&lens).zip(&measured) {
+            let len = *len;
             // 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.
@@ -106,22 +136,34 @@ impl Widget for Span {
                 fixed.px += self.gap;
                 continue;
             }
-            let mut span = UiSpan::FULL;
-            span.start = start;
+            let from = start;
             if len.leftover > Weight::ZERO && shares {
                 taken += len.leftover;
             }
             fixed.px += len.px;
             fixed.rel += len.rel;
             start = shared(fixed, taken, total.leftover, room);
-            span.end = start;
-            let mut region = UiRegion::from_axis(axis, span, UiSpan::FULL);
-            if self.dir.sign == Sign::Neg {
-                region.flip(axis);
+            // Along the row the span says where the child goes, and that slot
+            // is the child's box outright rather than something to place an
+            // answer inside again. A share is decided here and nowhere
+            // else: its slot narrows its frame, and the child is asked in
+            // it, since a text wraps at the width it is actually given. A
+            // fixed child's slot is its own answer, so a drawing made in the
+            // room is put there as it is, and one not made yet is made here.
+            let slot = along(from, start);
+            let place = axis.pair(Place::Fill(Part::From(slot)), across);
+            let mut narrow = [None; 2];
+            if len.leftover > Weight::ZERO && shares {
+                narrow[axis as usize] = Some(slot.len());
             }
-            let placed = painter.widget_within(child, region);
+            let used = match (measured, narrow[axis as usize]) {
+                (Some(size), None) => {
+                    painter.place_at(child, place);
+                    size.axis(!axis)
+                }
+                _ => painter.widget_at(child, narrow, place).len(!axis),
+            };
             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;

src/widget/position/stack.rs

@@ -13,34 +13,51 @@ impl Widget for Stack {
             StackSize::Default => None,
             StackSize::Child(i) => Some(i),
         };
-        // Whichever child sizes the stack decides the box every child gets.
-        // The stack reports that size, so a child given a longer box would
-        // draw outside what the stack says it occupies.
+        // Whichever child sizes the stack is given the stack's whole box --
+        // the stack is the length that child asked for, so placing that
+        // answer inside the box it decided would apply it twice.
         let size = match sizing.and_then(|i| self.children.get(i).map(|c| (i, c))) {
             // On the layer that child ends up on, so the ask below is a reuse
             // rather than a second drawing of it somewhere else: a retained
             // drawing belongs to the layer it was made on.
             Some((i, child)) => {
                 painter.child_layer_at(i);
-                painter.widget(child).size()
+                painter
+                    .widget_at(child, [None; 2], [Place::Fill(Part::All); 2])
+                    .size()
             }
             None => Size::LEFTOVER,
         };
-        let region = painter.box_of(size);
+        // 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 = [Axis::X, Axis::Y].map(|axis| {
+            let len = size.axis(axis);
+            match len.leftover == Weight::ZERO {
+                true => Place::Fill(Part::Sized(Len::from_parts(len.rel, len.px))),
+                false => Place::Within(Part::All),
+            }
+        });
         for (i, child) in self.children.iter().enumerate() {
+            if sizing == Some(i) {
+                continue;
+            }
             painter.child_layer_at(i);
-            // The sizing child placed its own content in the box its answer
-            // decided, and this box was derived from that answer, so applying
-            // its alignment again here would place it twice. Every other
-            // child is handed a box that owes nothing to its own answer, and
-            // where it sits in one bigger than itself is its own business.
-            match sizing == Some(i) {
-                true => painter.widget_aligned(child, region, RegionAlign::NEAR),
-                false => painter.widget_within(child, region),
-            };
+            painter.widget_at(child, [None; 2], 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)]

src/widget/text/mod.rs

@@ -50,15 +50,11 @@ impl TextView {
         let width = self.attrs.wrap.then(|| painter.px_len(Axis::X));
         // The shaper measures in floats, which is where a glyph advance comes
         // from; what it answers goes back on the grid.
-        let text = painter.render_text(&mut self.buf, &self.attrs, width.map(Px::to_f32));
-        // A greedy break is the same break at every width from its longest
-        // line up to the one it was made at: each line still fits, and none
-        // could take a word that did not fit in the wider box. A line too
-        // long to fit at all says nothing about narrower boxes.
-        if let Some(width) = width {
-            painter.holds(Axis::X, Px::from_f32(text.size.x).min(width)..=width);
+        painter.render_text(&mut self.buf, &self.attrs, width.map(Px::to_f32));
+        if width.is_some() {
+            painter.holds(Axis::X, self.buf.width_holds());
         }
-        text
+        self.buf.rendered().expect("render_text placed the glyphs")
     }
 
     pub fn tex(&self) -> Option<&RenderedText> {
@@ -78,9 +74,13 @@ impl TextView {
 
         let tex = self.render(painter);
         let region = tex.size.align(align);
-        let size = Size::px(tex.size);
-        let within = region.within(&painter.region());
-        painter.glyphs(tex, within);
+        // The step at or above what the shaper measured, so a parent that
+        // hands back the length this reports hands back a box the longest
+        // line fits in. Rounded to the nearest step it is half the time a
+        // hair under that line, and the break made in it is not the break a
+        // cold layout makes there.
+        let size = Size::from_px(PxVec2::ceil_from_f32(tex.size));
+        painter.glyphs(tex, region);
         (region, size)
     }
 

src/widget/trait_fns.rs

@@ -134,9 +134,13 @@ widget_trait! {
         |state| self.add(state)
     }
 
-    fn set_ptr(self, ptr: WeakWidget<WidgetPtr>, state: &mut Rsc) {
-        let id = self.add_strong(state);
-        state.ui_mut().widgets[ptr].inner = Some(id);
+    // Named for the type it makes rather than as `wrapped`, which would read
+    // as the text setting. `widget_trait!` takes no attributes, so what it is
+    // for is on `Wrapper` itself.
+    fn wrapper(self) -> impl WidgetFn<Rsc, Wrapper> {
+        |state| Wrapper {
+            inner: Some(self.add_strong(state)),
+        }
     }
 }
 

src/widget/wrapper.rs

@@ -1,11 +1,19 @@
 use crate::prelude::*;
 use std::marker::Unsize;
 
-pub struct WidgetPtr {
+/// One widget in a box of its own, doing as little as possible on the way:
+/// it draws its child in the whole of its box and reports back what the child
+/// said. It exists because a length and an alignment are properties of one
+/// widget, so a widget cannot both be 100 wide and take two shares of a row
+/// -- the two lengths need two widgets, and this is the smaller one.
+///
+/// Its child is optional so it can also be the swappable slot a tab bar
+/// needs, which is what it was written for.
+pub struct Wrapper {
     pub inner: Option<StrongWidget>,
 }
 
-impl Widget for WidgetPtr {
+impl Widget for Wrapper {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         match &self.inner {
             Some(id) => painter.widget(id).size(),
@@ -14,7 +22,7 @@ impl Widget for WidgetPtr {
     }
 }
 
-impl WidgetPtr {
+impl Wrapper {
     pub fn new() -> Self {
         Self::default()
     }
@@ -35,7 +43,7 @@ impl WidgetPtr {
     }
 }
 
-impl Default for WidgetPtr {
+impl Default for Wrapper {
     fn default() -> Self {
         Self::empty()
     }

tests/cases/determinism.rs

@@ -21,16 +21,18 @@ struct BranchesOnMeasurement {
 
 impl Widget for BranchesOnMeasurement {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let mut top = UiRegion::FULL;
-        top.y.end = top.y.start.offset(Px::from_int(40));
-        let measured = painter.widget_within(&self.probe, top).len(Axis::X);
-        let px = measured.apply_leftover().to_px(painter.px_len(Axis::X));
+        let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
+        let top = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
+        let measured = painter
+            .widget_at(&self.probe, [None; 2], [Place::Within(Part::All), top])
+            .len(Axis::X);
+        let px = painter.to_px(measured.apply_leftover(), Axis::X);
 
-        let mut below = UiRegion::FULL;
-        below.y.start = below.y.start.offset(Px::from_int(40));
+        let below = Place::Within(Part::From(UiSpan::new(cut, painter.extent_len(Axis::Y))));
+        let place = [Place::Within(Part::All), below];
         match px > Px::from_f32(self.threshold) {
-            true => painter.widget_within(&self.wide, below),
-            false => painter.widget_within(&self.narrow, below),
+            true => painter.widget_at(&self.wide, [None; 2], place),
+            false => painter.widget_at(&self.narrow, [None; 2], place),
         };
         Size::LEFTOVER
     }

tests/cases/layout.rs

@@ -20,6 +20,160 @@ fn a_span_gives_each_child_the_width_it_asked_for() {
     assert_corners!(h, right, (100, 0), (400, 200));
 }
 
+/// A span places each child in the room left after the one before, because a
+/// text has to wrap at the width actually there, but the child's region is
+/// the whole row. So two children asking for half each take the whole row
+/// between them, however much of it was left when each was asked, and a third
+/// overflows -- and a span passes its own region on unchanged, so a child of
+/// a nested span asking for half asks for half of the same row.
+#[test]
+fn a_span_reads_a_child_report_as_a_fraction_of_the_row() {
+    let mut h = Harness::new((400, 100));
+    let half = rect(Color::RED).width(rel(0.5)).add(&mut h.rsc);
+    let inner = rect(Color::GREEN).width(rel(0.5)).add(&mut h.rsc);
+    let nested = (inner,).span(Dir::RIGHT).add(&mut h.rsc);
+    let tail = rect(Color::BLUE).width(100).add(&mut h.rsc);
+    h.set_root((half, nested, tail).span(Dir::RIGHT).width(rel(1.0)));
+
+    // The nested span is placed at the length it reported, and its own child
+    // asks for half of the row rather than half of that placement.
+    assert_corners!(h, nested, (200, 0), (400, 100));
+    assert_corners!(h, inner, (200, 0), (400, 100));
+    assert_corners!(h, tail, (400, 0), (500, 100));
+}
+
+/// The same fraction either way round: after a 100 px child in a 400 px row,
+/// `rel(0.5)` is 100 to 300 whether the child's own rule says so or the child
+/// drew half of what it was offered and reported that. Half the row, not half
+/// of the 300 px left of it.
+#[test]
+fn a_reported_fraction_is_of_the_row_like_a_declared_one() {
+    let mut declaring = Harness::new((400, 100));
+    let head = rect(Color::RED).width(100).add(&mut declaring.rsc);
+    let declared = rect(Color::GREEN).width(rel(0.5)).add(&mut declaring.rsc);
+    declaring.set_root((head, declared).span(Dir::RIGHT).width(rel(1.0)));
+    assert_corners!(declaring, declared, (100, 0), (300, 100));
+
+    let mut reporting = Harness::new((400, 100));
+    let head = rect(Color::RED).width(100).add(&mut reporting.rsc);
+    let inner = rect(Color::GREEN).width(rel(0.5)).add(&mut reporting.rsc);
+    let reported = (inner,).span(Dir::RIGHT).add(&mut reporting.rsc);
+    reporting.set_root((head, reported).span(Dir::RIGHT).width(rel(1.0)));
+    assert_corners!(reporting, reported, (100, 0), (300, 100));
+}
+
+/// What the fraction a child reports is of and what box it is offered are
+/// two different lengths, and only the first is the whole row: a text still
+/// wraps at the room actually left after its neighbour, so the same
+/// paragraph is taller where less of the row is left for it.
+#[test]
+fn a_text_in_a_span_wraps_at_the_room_left_rather_than_the_whole_row() {
+    let paragraph = "Wrapping shapes one source into as many lines as the box \
+                     leaves room for, so a paragraph's height is an answer.";
+    let height_after = |head_width: i32| {
+        let mut h = Harness::new((400, 400));
+        let head = rect(Color::RED).width(head_width).add(&mut h.rsc);
+        let text = wtext(paragraph).size(16).wrap(true).add(&mut h.rsc);
+        h.set_root((head, text).span(Dir::RIGHT).width(rel(1.0)));
+        let region = h.region(&text).unwrap();
+        (region.bot_right.y - region.top_left.y).to_f32()
+    };
+
+    let (crowded, whole_row) = (height_after(300), height_after(0));
+    assert!(crowded > whole_row, "{crowded} against {whole_row}");
+}
+
+/// Padding is an inset: it narrows the frame a fraction resolves against and
+/// adds itself back to the padded widget's reported length.
+#[test]
+fn a_pad_puts_its_padding_around_a_fraction_of_the_whole_box() {
+    let mut h = Harness::new((400, 100));
+    let inner = rect(Color::GREEN).width(rel(0.5)).add(&mut h.rsc);
+    let padded = (inner,).span(Dir::RIGHT).pad(10).add(&mut h.rsc);
+    let tail = rect(Color::BLUE).width(100).add(&mut h.rsc);
+    // Ruled to the window: a root reporting a fraction of it is otherwise
+    // 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), (200, 90));
+    assert_corners!(h, padded, (0, 0), (210, 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_frame_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.part.x.len().to_px(window);
+    assert_eq!(active.frame.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_frame_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.frame.x.to_px(window), Px::from_int(418));
+    assert_eq!(active.part.x.len().to_px(window), Px::from_int(418));
+}
+
 #[test]
 fn a_span_ruled_across_itself_does_not_measure_its_children_there() {
     let mut h = Harness::new((400, 200));
@@ -225,21 +379,21 @@ fn only_a_region_node_lengthens_the_chain_and_it_can_be_removed() {
     h.set_root((bar, buried).span(Dir::RIGHT));
 
     let move_idx = h.render.active[&leaf.id()].parent_move;
-    assert_eq!(h.render.moves.depth(move_idx), 1, "only the root region");
+    assert_eq!(h.render.moves.depth(move_idx), 0, "the window is no entry");
 
     h.rsc.widgets_mut().set_region_node(buried, true);
     h.frame();
     let move_idx = h.render.active[&leaf.id()].parent_move;
     assert_eq!(
         h.render.moves.depth(move_idx),
-        2,
-        "the opted-in widget's region and the root region"
+        1,
+        "the opted-in widget's region alone"
     );
 
     h.rsc.widgets_mut().set_region_node(buried, false);
     h.frame();
     let move_idx = h.render.active[&leaf.id()].parent_move;
-    assert_eq!(h.render.moves.depth(move_idx), 1);
+    assert_eq!(h.render.moves.depth(move_idx), 0);
 }
 
 /// A span that sizes from its children passes their `leftover` weight up
@@ -341,16 +495,15 @@ fn a_row_of_equal_shares_fills_it_exactly() {
     }
 }
 
-/// Where the shader puts an edge: the two parts of a scalar are floored
-/// apart, so a fraction and a pixel offset snap independently, and each is
-/// taken to the boundary it composes to within half a step of. Kept in step
-/// with `snap_floor` in `prelude.wgsl`.
+/// Where the shader puts an edge: the fraction resolved against the window
+/// plus the pixel offset, taken to the boundary it composes to within half
+/// 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 region = h.render.moves.resolve(active.parent_move, active.region);
+    let region = h.render.moves.resolve(active.move_idx, active.extent);
     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) + snap(s.px.to_f32());
+    let edge = |s: Len| snap(s.rel.to_f32() * dim + s.px.to_f32());
     let span = region.axis(axis);
     (edge(span.start), edge(span.end))
 }
@@ -620,3 +773,51 @@ fn equal_shares_differ_by_at_most_two_steps_and_fill_the_row() {
         }
     }
 }
+
+#[test]
+fn a_stack_sized_by_a_child_does_not_take_that_childs_fraction_twice() {
+    let mut h = Harness::new((400, 200));
+    let half = rect(Color::RED).width(rel(0.5)).add(&mut h.rsc);
+    let behind = rect(Color::BLUE).add(&mut h.rsc);
+    let stack = Stack {
+        children: vec![behind.add_strong(&mut h.rsc), half.add_strong(&mut h.rsc)],
+        size: StackSize::Child(1),
+    }
+    .add(&mut h.rsc);
+    h.set_root((stack,).span(Dir::RIGHT).width(rel(1.0)));
+
+    assert_corners!(h, stack, (0, 0), (200, 200));
+    assert_corners!(h, half, (0, 0), (200, 200));
+    assert_corners!(h, behind, (0, 0), (200, 200));
+}
+
+#[test]
+fn a_fixed_child_is_centered_in_its_wrappers_share() {
+    let mut h = Harness::new((600, 300));
+    let leaf = rect(Color::RED).sized((100, 100)).center().add(&mut h.rsc);
+    let wrapper = leaf
+        .wrapper()
+        .width(leftover(2))
+        .height(rel(1.0))
+        .add(&mut h.rsc);
+    let other = rect(Color::BLUE).width(200).add(&mut h.rsc);
+    h.set_root((other, wrapper).span(Dir::RIGHT));
+
+    assert_corners!(h, wrapper, (200, 0), (600, 300));
+    assert_corners!(h, leaf, (350, 100), (450, 200));
+
+    h.resize((900, 400));
+    h.frame();
+    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));
+}

tests/cases/retained.rs

@@ -156,16 +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);
-    // Reading its box makes its drawing hold for the measuring box alone,
-    // and it reports less than that box: so it is drawn again in the box its
-    // answer places it in, and once more in the final box the span chooses.
-    // A widget that says what it holds for, as text does, skips the middle
-    // one.
-    assert_eq!(
-        asked_draws.get(),
-        3,
-        "drawn to be measured, in its placed box, then in its final box"
-    );
+    // Asked once, from the cursor; its slot is its answer and the drawing is
+    // moved there.
+    assert_eq!(asked_draws.get(), 1);
 }
 
 #[test]
@@ -220,9 +213,12 @@ struct FromHint {
 impl Widget for FromHint {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let len = painter.size_hint(&self.inner, Axis::Y).unwrap();
-        let mut region = UiRegion::FULL;
-        region.y.end = region.y.start.offset(len.px);
-        painter.widget_within(&self.inner, region);
+        let top = UiSpan::new(Len::ZERO, Len::from_parts(Rel::ZERO, len.px));
+        painter.widget_at(
+            &self.inner,
+            [None; 2],
+            [Place::Within(Part::All), Place::Within(Part::From(top))],
+        );
         Size::LEFTOVER
     }
 }
@@ -259,10 +255,9 @@ impl Widget for ReadsBox {
 /// 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 of these report a quarter of what they read, without saying that the
-/// drawing holds there too, so each length they are asked at costs two draws:
-/// one to answer, and one in the quarter-sized box that answer places them
-/// in. The counts below are in those pairs.
+/// Both of these report a quarter of what they read. The quarter-sized box
+/// 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>>,
 }
@@ -312,6 +307,51 @@ fn a_span_ruled_across_itself_moves_its_child_without_redrawing_it() {
     assert_eq!(h.render.active[&span.id()].size.y, LayoutLen::rel(1.0));
 }
 
+/// A row places its children as lengths from where its own box starts, so a
+/// child that grew moves the ones after it and nothing else: each of them is
+/// the same box in a new place, which the retained drawing follows without
+/// being made again. Both kinds of length: one the row resolves from a rule,
+/// and one it takes from what the child reported.
+#[test]
+fn a_row_moves_what_follows_a_child_that_grew_rather_than_drawing_it() {
+    for declared in [false, true] {
+        let mut h = Harness::new((400, 200));
+        let first = rect(Color::RED).width(50).add(&mut h.rsc);
+        let ruled = Rc::new(Cell::new(0));
+        let second = Counted {
+            draws: ruled.clone(),
+            size: Size::LEFTOVER,
+            reads_box: false,
+        };
+        let second = match declared {
+            true => second.width(rel(0.25)).add(&mut h.rsc),
+            false => second.width(60).add(&mut h.rsc),
+        };
+        let (third, reported) = counted(&mut h, Size::from((70, 20)), false);
+        h.set_root((first, second, third).span(Dir::RIGHT).width(rel(1.0)));
+        let (was_ruled, was_reported) = (ruled.get(), reported.get());
+        // A quarter of the row is a quarter of the row, wherever it sits in
+        // it and whatever the first child takes.
+        let width = match declared {
+            true => 100,
+            false => 60,
+        };
+        assert_corners!(h, second, (50, 0), (50 + width, 200));
+
+        h.set_len(first, Axis::X, 80);
+        h.frame();
+
+        assert_eq!(ruled.get(), was_ruled, "the ruled child was drawn again");
+        assert_eq!(
+            reported.get(),
+            was_reported,
+            "the reported child was drawn again"
+        );
+        assert_corners!(h, second, (80, 0), (80 + width, 200));
+        assert_corners!(h, third, (80 + width, 90), (150 + width, 110));
+    }
+}
+
 /// The output is the root of the box chain, so a resize is a box that changed
 /// length like any other -- there is not a second rule for the window. A
 /// drawing that holds for one length is drawn again whichever box moved.
@@ -343,7 +383,7 @@ fn a_resize_redraws_what_read_its_box() {
     h.resize((800, 100));
     h.frame();
 
-    assert_eq!(draws.get(), settled + 2);
+    assert_eq!(draws.get(), settled + 1);
 }
 
 #[test]
@@ -363,7 +403,7 @@ fn a_resize_only_redraws_read_axes() {
 
     h.resize((800, 300));
     h.frame();
-    assert_eq!(draws.get(), settled + 2, "width changes its answer");
+    assert_eq!(draws.get(), settled + 1, "width changes its answer");
 }
 
 /// A window is measured onto the grid like everything else, so a resize too
@@ -390,7 +430,7 @@ fn a_resize_within_one_step_is_not_a_resize() {
 
     h.resize((400.0 + step, 200.0));
     h.frame();
-    assert_eq!(draws.get(), settled + 2);
+    assert_eq!(draws.get(), settled + 1);
 }
 
 /// The same for a box that changes because a sibling did: what is compared
@@ -613,3 +653,692 @@ fn a_stacks_sizing_child_is_drawn_once_where_it_belongs() {
     assert_ne!(layer(front.id()), layer(background.id()));
     assert_eq!(draws.get(), 1);
 }
+
+/// A widget's own mask is not the one it inherited, and a redraw of it
+/// inherits the second: handing back the first is handing it its own mask to
+/// set a second time, which `set_mask` asserts against.
+#[test]
+fn a_masked_widget_redrawn_on_its_own_sets_its_mask_again() {
+    let mut h = Harness::new((400, 200));
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let masked = inner.masked().add(&mut h.rsc);
+    let other = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((other, masked).span(Dir::RIGHT));
+    h.rsc.widgets_mut().get_dyn_mut(masked.id());
+    h.frame();
+    assert_corners!(h, inner, (100, 0), (400, 200));
+}
+
+/// The two spans a subtree changes hands between, and the branch that is not
+/// in the tree yet -- kept alive by the test until it is.
+struct Handover {
+    leaf: WidgetId,
+    first: WeakWidget<Span>,
+    second: WeakWidget<Span>,
+    root: WeakWidget<Span>,
+    spare: StrongWidget,
+}
+
+/// A subtree that changes hands while its box does not move, so nothing about
+/// reusing its drawing says it changed parents. `deeper` puts a span between
+/// the root and `second`, so it changes depth by changing hands as well.
+fn plant_handover(h: &mut Harness, moved: bool, deeper: bool, width: f32) -> Handover {
+    let leaf = rect(Color::RED).add(&mut h.rsc);
+    let sized = leaf.width(width).add(&mut h.rsc);
+    let holder = (sized,).span(Dir::RIGHT).add(&mut h.rsc);
+    let first = Span {
+        children: match moved {
+            true => Vec::new(),
+            false => vec![holder.add_strong(&mut h.rsc)],
+        },
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    }
+    .add(&mut h.rsc);
+    let second = Span {
+        children: match moved {
+            true => vec![holder.add_strong(&mut h.rsc)],
+            false => Vec::new(),
+        },
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    }
+    .add(&mut h.rsc);
+    let branch = match deeper {
+        true => (second,).span(Dir::RIGHT).add_strong(&mut h.rsc),
+        false => second.add_strong(&mut h.rsc),
+    };
+    let (in_tree, spare) = match moved {
+        true => (branch, first.add_strong(&mut h.rsc)),
+        false => (first.add_strong(&mut h.rsc), branch),
+    };
+    let root = Span {
+        children: vec![in_tree],
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    }
+    .add(&mut h.rsc);
+    h.state.root = Some(root.add_strong(&mut h.rsc));
+    Handover {
+        leaf: sized.id(),
+        first,
+        second,
+        root,
+        spare,
+    }
+}
+
+/// Moves the subtree and swaps the branch it sits in for the one it left.
+fn hand_over(h: &mut Harness, tree: Handover) -> WidgetId {
+    let holder = h.rsc[tree.first].children.remove(0);
+    h.rsc[tree.second].children.push(holder);
+    h.rsc[tree.root].children.clear();
+    h.rsc[tree.root].children.push(tree.spare);
+    h.frame();
+    tree.leaf
+}
+
+#[test]
+fn a_subtree_that_changed_parents_is_not_undrawn_by_the_one_it_left() {
+    let mut warm = Harness::new((400, 200));
+    let tree = plant_handover(&mut warm, false, false, 40.0);
+    warm.frame();
+    let leaf = hand_over(&mut warm, tree);
+
+    let mut cold = Harness::new((400, 200));
+    let grown = plant_handover(&mut cold, true, false, 40.0);
+    cold.frame();
+
+    assert_eq!(
+        warm.region(&leaf),
+        cold.region(&grown.leaf),
+        "the span it left still listed it and undrew it"
+    );
+}
+
+#[test]
+fn a_subtree_that_changed_parents_settles_at_the_depth_it_moved_to() {
+    let mut warm = Harness::new((400, 200));
+    let tree = plant_handover(&mut warm, false, true, 40.0);
+    warm.frame();
+    let leaf = hand_over(&mut warm, tree);
+    // After it has changed hands, so what has to reach the new parent is a
+    // change made under the subtree it now holds.
+    warm.set_len(leaf, Axis::X, LayoutLen::px(90.0));
+    warm.frame();
+
+    let mut cold = Harness::new((400, 200));
+    let grown = plant_handover(&mut cold, true, true, 90.0);
+    cold.frame();
+
+    assert_eq!(
+        warm.region(&leaf),
+        cold.region(&grown.leaf),
+        "the span it moved to is the one the change has to reach"
+    );
+}
+
+fn primitive_bounds(h: &Harness, id: WidgetId) -> Vec<PixelRegion> {
+    h.render.active[&id]
+        .primitives
+        .iter()
+        .map(|primitive| {
+            let handle = &primitive.handle;
+            let instance = &h.render.layers[handle.layer].primitives()[handle.kind as usize]
+                .as_ref()
+                .unwrap()
+                .instances()[handle.inst_idx];
+            h.render
+                .moves
+                .resolve(instance.move_idx, instance.region)
+                .to_px(h.render.output_size())
+        })
+        .collect()
+}
+
+#[test]
+fn changing_an_inherited_extent_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);
+        let through = Stretchy {
+            inner: words.add_strong(&mut h.rsc),
+            draws: Rc::new(Cell::new(0)),
+        }
+        .add(&mut h.rsc);
+        h.set_root((first, through).span(Dir::RIGHT));
+        (words, first)
+    }
+    let short = "one two";
+    let long = "one two three four five six seven eight nine ten eleven twelve";
+    let mut warm = Harness::new((400, 200));
+    let (words, first) = build(&mut warm, 50, short);
+    warm.set_len(first, Axis::X, 200);
+    warm.frame();
+    *warm.rsc[words].content = long.to_string();
+    warm.frame();
+    let mut cold = Harness::new((400, 200));
+    let (other, _) = build(&mut cold, 200, long);
+    assert_eq!(warm.region(&words), cold.region(&other));
+    assert_eq!(
+        primitive_bounds(&warm, words.id()),
+        primitive_bounds(&cold, other.id())
+    );
+}
+
+#[test]
+fn widening_text_without_soft_breaks_reuses_its_drawing() {
+    struct CountedText {
+        text: Text,
+        draws: Rc<Cell<usize>>,
+    }
+    impl Widget for CountedText {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.draws.set(self.draws.get() + 1);
+            self.text.draw(painter)
+        }
+    }
+    for content in ["Short text", "Two hard\nline breaks\nhere", ""] {
+        let plant = |h: &mut Harness| {
+            let mut text = Text::new(content);
+            text.wrap = true;
+            let draws = Rc::new(Cell::new(0));
+            let root = CountedText {
+                text,
+                draws: draws.clone(),
+            }
+            .add(&mut h.rsc);
+            h.set_root(root);
+            (root, draws)
+        };
+        let mut warm = Harness::new((300, 200));
+        let (root, draws) = plant(&mut warm);
+        let before = draws.get();
+        warm.resize((500, 200));
+        warm.frame();
+        assert_eq!(draws.get(), before, "{content:?}");
+        let mut cold = Harness::new((500, 200));
+        let (other, _) = plant(&mut cold);
+        assert_eq!(warm.region(&root), cold.region(&other));
+        assert_eq!(
+            primitive_bounds(&warm, root.id()),
+            primitive_bounds(&cold, other.id())
+        );
+    }
+}
+
+#[test]
+fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
+    struct Frame {
+        child: StrongWidget,
+        region: UiRegion,
+    }
+    impl Widget for Frame {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter.widget_at(
+                &self.child,
+                [None; 2],
+                [
+                    Place::Within(Part::From(self.region.x)),
+                    Place::Within(Part::From(self.region.y)),
+                ],
+            );
+            Size::LEFTOVER
+        }
+    }
+    struct Painted(Rc<Cell<usize>>);
+    impl Widget for Painted {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.0.set(self.0.get() + 1);
+            painter.set_mask(UiRegion::FULL);
+            painter.primitive(RectPrimitive::color(Color::BLUE));
+            Size::LEFTOVER
+        }
+    }
+    let fixed = |start, end| UiRegion::new(UiSpan::new(Len::px(start), Len::px(end)), UiSpan::FULL);
+    for node in [false, true] {
+        let plant = |h: &mut Harness, region| {
+            let draws = Rc::new(Cell::new(0));
+            let leaf = Painted(draws.clone()).add(&mut h.rsc);
+            h.rsc.widgets_mut().set_region_node(leaf, node);
+            let inner = Frame {
+                child: leaf.add_strong(&mut h.rsc),
+                region: UiRegion::new(UiSpan::new(Len::rel(0.23), Len::rel(0.83)), UiSpan::FULL),
+            }
+            .add_strong(&mut h.rsc);
+            let root = Frame {
+                child: inner,
+                region,
+            }
+            .add(&mut h.rsc);
+            h.set_root(root);
+            (root, leaf, draws)
+        };
+        let mut warm = Harness::new((400, 200));
+        let (root, leaf, draws) = plant(&mut warm, fixed(7.0, 104.0));
+        let before = draws.get();
+        warm.rsc[root].region = fixed(19.0, 180.0);
+        warm.frame();
+        assert_eq!(draws.get(), before);
+        let mut cold = Harness::new((400, 200));
+        let (_, other, _) = plant(&mut cold, fixed(19.0, 180.0));
+        assert_eq!(warm.region(&leaf), cold.region(&other));
+        assert_eq!(
+            primitive_bounds(&warm, leaf.id()),
+            primitive_bounds(&cold, other.id())
+        );
+        let mask = |h: &Harness, id: WidgetId| {
+            let active = &h.render.active[&id];
+            let mask = &h.rsc.ui().masks[active.mask.idx()];
+            h.render
+                .moves
+                .resolve(mask.move_idx, mask.region)
+                .to_px(h.render.output_size())
+        };
+        assert_eq!(mask(&warm, leaf.id()), mask(&cold, other.id()));
+    }
+}
+
+#[test]
+fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
+    struct Glyphs {
+        buffer: TextBuffer,
+        draws: Rc<Cell<usize>>,
+    }
+    impl Widget for Glyphs {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.draws.set(self.draws.get() + 1);
+            let text = painter.render_text(&mut self.buffer, &TextAttrs::default(), None);
+            let origin = UiRegion::new(
+                UiSpan::new(Len::rel(0.23) + Len::px(-7.125), Len::FULL),
+                UiSpan::new(Len::rel(0.37) + Len::px(3.25), Len::FULL),
+            );
+            painter.glyphs(text, origin);
+            Size::LEFTOVER
+        }
+    }
+    struct Frame {
+        child: StrongWidget,
+        region: UiRegion,
+        extent: UiRegion,
+    }
+    impl Widget for Frame {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter.widget_at(
+                &self.child,
+                [Some(self.region.x.len()), None],
+                [
+                    Place::Fill(Part::From(self.extent.x)),
+                    Place::Fill(Part::From(self.extent.y)),
+                ],
+            );
+            Size::LEFTOVER
+        }
+    }
+    for node in [false, true] {
+        let mut h = Harness::new((403, 211));
+        let draws = Rc::new(Cell::new(0));
+        let text = Glyphs {
+            buffer: TextBuffer::new("Glyphs: gj AV\nsecond line"),
+            draws: draws.clone(),
+        }
+        .add(&mut h.rsc);
+        h.rsc.widgets_mut().set_region_node(text, node);
+        let root = Frame {
+            child: text.add_strong(&mut h.rsc),
+            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].extent = UiRegion::new(
+                UiSpan::new(Len::rel(start), Len::rel(end)),
+                UiSpan::new(Len::px(7.25), Len::rel(end)),
+            );
+            h.frame();
+            assert_eq!(draws.get(), before);
+            let retained = primitive_bounds(&h, text.id());
+            assert!(!retained.is_empty());
+            let _ = h.rsc.widgets_mut().get_dyn_mut(text.id());
+            h.frame();
+            assert!(draws.get() > before);
+            assert_eq!(retained, primitive_bounds(&h, text.id()));
+        }
+    }
+}
+
+#[test]
+fn resizing_does_not_remeasure_a_fixed_stack_for_its_unmeasured_overlay() {
+    let mut h = Harness::new((400, 200));
+    let (sizing, _) = counted(&mut h, Size::from((100, 80)), false);
+    let (overlay, draws) = counted(&mut h, Size::LEFTOVER, true);
+    h.set_root((sizing, overlay).stack().size(StackSize::Child(0)));
+    let settled = draws.get();
+
+    h.resize((800, 300));
+    h.frame();
+
+    assert_eq!(draws.get(), settled);
+    assert_corners!(h, overlay, (350, 110), (450, 190));
+}
+
+struct Unmeasured {
+    child: StrongWidget,
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for Unmeasured {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        painter.widget(&self.child);
+        Size::LEFTOVER
+    }
+}
+
+#[test]
+fn a_declared_size_change_stops_at_an_independent_parent() {
+    let mut h = Harness::new((400, 200));
+    let leaf = rect(Color::RED).width(100).add(&mut h.rsc);
+    let parent = Unmeasured {
+        child: leaf.add_strong(&mut h.rsc),
+        draws: Rc::new(Cell::new(0)),
+    }
+    .add_strong(&mut h.rsc);
+    let draws = Rc::new(Cell::new(0));
+    h.set_root(Unmeasured {
+        child: parent,
+        draws: draws.clone(),
+    });
+    let settled = draws.get();
+
+    h.set_len(leaf, Axis::X, 150);
+    h.frame();
+
+    assert_corners!(h, leaf, (125, 0), (275, 200));
+    assert_eq!(draws.get(), settled);
+}
+
+#[test]
+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 = ReadsWidth {
+        draws: draws.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root((leaf,).stack());
+    let settled = draws.get();
+
+    h.resize((800, 200));
+    h.frame();
+
+    assert!(draws.get() > settled);
+    assert_corners!(h, leaf, (300, 90), (500, 110));
+}
+
+#[test]
+fn changed_drawing_dependencies_reach_ancestors_without_a_size_change() {
+    let mut h = Harness::new((400, 200));
+    let (leaf, draws) = counted(&mut h, Size::LEFTOVER, false);
+    h.set_root(((leaf,).stack(),).stack());
+
+    h.rsc[leaf].reads_box = true;
+    h.frame();
+    let settled = draws.get();
+    h.resize((800, 200));
+    h.frame();
+
+    assert_eq!(draws.get(), settled + 1);
+    assert_corners!(h, leaf, (0, 0), (800, 200));
+}
+
+#[test]
+fn widening_and_restoring_a_contract_does_not_invalidate_its_reader() {
+    let mut h = Harness::new((400, 200));
+    let (leaf, leaf_draws) = counted(&mut h, Size::LEFTOVER, true);
+    let draws = Rc::new(Cell::new(0));
+    let child = leaf.add_strong(&mut h.rsc);
+    h.set_root(Unmeasured {
+        child,
+        draws: draws.clone(),
+    });
+    let settled = draws.get();
+    for reads_box in [false, true, false, true] {
+        h.rsc[leaf].reads_box = reads_box;
+        h.frame();
+        assert_eq!(draws.get(), settled);
+    }
+    let settled = leaf_draws.get();
+    h.resize((800, 200));
+    h.frame();
+    assert_eq!(leaf_draws.get(), settled + 1);
+}
+#[test]
+fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
+    struct Observed<W> {
+        widget: W,
+        draws: Rc<Cell<usize>>,
+    }
+    impl<W: Widget> Widget for Observed<W> {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.draws.set(self.draws.get() + 1);
+            self.widget.draw(painter)
+        }
+    }
+    struct Frame {
+        child: StrongWidget,
+        extent: UiRegion,
+    }
+    impl Widget for Frame {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter.widget_at(
+                &self.child,
+                [None; 2],
+                [
+                    Place::Fill(Part::From(self.extent.x)),
+                    Place::Fill(Part::From(self.extent.y)),
+                ],
+            );
+            Size::LEFTOVER
+        }
+    }
+    for node in [false, true] {
+        let plant = |h: &mut Harness, extent| {
+            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);
+            let fixed = rect(Color::RED).width(31).height(19).add(&mut h.rsc);
+            let stack = Observed {
+                widget: Stack {
+                    children: vec![leaf.add_strong(&mut h.rsc), fixed.add_strong(&mut h.rsc)],
+                    size: StackSize::Default,
+                },
+                draws: draws.clone(),
+            }
+            .add_strong(&mut h.rsc);
+            let pad = Observed {
+                widget: Pad {
+                    inner: stack,
+                    padding: Padding::uniform(7).with_left(13),
+                },
+                draws: draws.clone(),
+            }
+            .add_strong(&mut h.rsc);
+            let root = Frame { child: pad, extent }.add(&mut h.rsc);
+            h.set_root(root);
+            (root, leaf, fixed, draws)
+        };
+        // The same box in three places. A pad places its child as lengths of
+        // its own box measured from where that box starts, so moving it is
+        // nothing to the pad -- where changing its length is a different
+        // question, and does draw it again.
+        let at = |start: f32| {
+            let span = |start: Len| UiSpan::new(start, start + Len::rel(0.4));
+            UiRegion::new(span(Len::rel(start) + Len::px(3.125)), span(Len::px(11.25)))
+        };
+        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 before = draws.get();
+            warm.rsc[root].extent = extent;
+            warm.frame();
+            assert_eq!(draws.get(), before);
+            let mut cold = Harness::new((403, 211));
+            let (_, other, other_fixed, _) = plant(&mut cold, extent);
+            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));
+            }
+            let mask = |h: &Harness, id: WidgetId| {
+                let active = &h.render.active[&id];
+                let mask = &h.rsc.ui().masks[active.mask.idx()];
+                h.render
+                    .moves
+                    .resolve(mask.move_idx, mask.region)
+                    .to_px(h.render.output_size())
+            };
+            assert_eq!(mask(&warm, leaf.id()), mask(&cold, other.id()));
+        }
+    }
+}
+
+#[test]
+fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
+    struct Measured;
+    impl Widget for Measured {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            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 }))
+        }
+    }
+    struct Frame {
+        child: StrongWidget,
+        start: f32,
+    }
+    impl Widget for Frame {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter.widget_at(
+                &self.child,
+                [None; 2],
+                [
+                    Place::Fill(Part::From(UiSpan::new(
+                        Len::px(self.start),
+                        Len::px(self.start + 200.0),
+                    ))),
+                    Place::Fill(Part::From(UiSpan::FULL)),
+                ],
+            );
+            Size::LEFTOVER
+        }
+    }
+    let mut h = Harness::new((400, 200));
+    let leaf = Measured.add(&mut h.rsc);
+    let stack = (leaf,).stack().add_strong(&mut h.rsc);
+    let root = Frame {
+        child: stack,
+        start: 0.0,
+    }
+    .add(&mut h.rsc);
+    h.set_root(root);
+    assert_corners!(h, leaf, (60, 80), (140, 120));
+    h.rsc[root].start = 30.0;
+    h.frame();
+    assert_corners!(h, leaf, (90, 80), (170, 120));
+    assert_eq!(
+        primitive_bounds(&h, leaf.id()),
+        vec![h.region(&leaf).unwrap()]
+    );
+}
+
+#[test]
+fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
+    struct Container {
+        child: StrongWidget,
+        region: UiRegion,
+    }
+    impl Widget for Container {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter
+                .widget_at(
+                    &self.child,
+                    [None; 2],
+                    [
+                        Place::Within(Part::From(self.region.x)),
+                        Place::Within(Part::From(self.region.y)),
+                    ],
+                )
+                .size()
+        }
+    }
+    struct Frame {
+        child: StrongWidget,
+        extent: UiRegion,
+        answer: Rc<Cell<Size>>,
+    }
+    impl Widget for Frame {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.answer.set(
+                painter
+                    .widget_at(
+                        &self.child,
+                        [None; 2],
+                        [
+                            Place::Fill(Part::From(self.extent.x)),
+                            Place::Fill(Part::From(self.extent.y)),
+                        ],
+                    )
+                    .size(),
+            );
+            Size::LEFTOVER
+        }
+    }
+    for fractional in [false, true] {
+        for region in [
+            UiRegion::FULL,
+            UiRegion::new(UiSpan::new(Len::rel(0.13), Len::rel(0.79)), UiSpan::FULL),
+        ] {
+            let plant = |h: &mut Harness, extent| {
+                let size = if fractional {
+                    Size {
+                        x: rel(0.5),
+                        y: LayoutLen::px(27),
+                    }
+                } else {
+                    Size::from((80, 27))
+                };
+                let (leaf, _) = counted(h, size, !fractional);
+                let child = Container {
+                    child: leaf.add_strong(&mut h.rsc),
+                    region,
+                }
+                .add_strong(&mut h.rsc);
+                let answer = Rc::new(Cell::new(Size::ZERO));
+                let root = Frame {
+                    child,
+                    extent,
+                    answer: answer.clone(),
+                }
+                .add(&mut h.rsc);
+                h.set_root(root);
+                (root, leaf, answer)
+            };
+            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 =
+                    UiRegion::new(UiSpan::new(Len::px(13.125), Len::px(width)), UiSpan::FULL);
+                warm.rsc[root].extent = extent;
+                warm.frame();
+                let mut cold = Harness::new((403, 211));
+                let (_, other, other_answer) = plant(&mut cold, extent);
+                assert_eq!(answer.get(), other_answer.get());
+                assert_eq!(warm.region(&leaf), cold.region(&other));
+            }
+        }
+    }
+}

tests/cases/scroll.rs

@@ -60,6 +60,69 @@ fn a_wheel_scrolls_the_content_and_stops_at_its_end() {
     assert_corners!(h, top, (0, 0), (400, 200));
 }
 
+#[test]
+fn fixed_content_and_a_share_fill_one_viewport() {
+    let mut h = Harness::new((900, 100));
+    let content = rect(Color::RED)
+        .width(LayoutLen {
+            px: Px::from_int(600),
+            rel: Rel::ZERO,
+            leftover: Weight::ONE,
+        })
+        .add(&mut h.rsc);
+    let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
+    h.set_root(scroll);
+
+    assert_corners!(h, content, (0, 0), (900, 100));
+}
+
+#[test]
+fn fixed_content_wider_than_the_viewport_still_scrolls() {
+    let mut h = Harness::new((900, 100));
+    let content = rect(Color::RED).width(1200).add(&mut h.rsc);
+    let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
+    h.set_root(scroll);
+
+    assert_corners!(h, content, (-300, 0), (900, 100));
+}
+
+#[test]
+fn a_lone_share_fills_without_scrolling() {
+    let mut h = Harness::new((900, 100));
+    let content = rect(Color::RED).width(LayoutLen::LEFTOVER).add(&mut h.rsc);
+    let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
+    h.set_root(scroll);
+
+    assert_corners!(h, content, (0, 0), (900, 100));
+}
+
+#[test]
+fn wrapping_content_beside_a_fixed_length_is_stable_warm_and_cold() {
+    fn plant(h: &mut Harness) -> (WidgetId, WidgetId) {
+        let fixed = rect(Color::RED).width(600).add(&mut h.rsc);
+        let text = wtext("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.")
+            .size(16)
+            .wrap(true)
+            .width(LayoutLen::LEFTOVER)
+            .add(&mut h.rsc);
+        let content = (fixed, text).span(Dir::RIGHT).add(&mut h.rsc);
+        let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
+        h.set_root(scroll);
+        (text.id(), content.id())
+    }
+
+    let mut warm = Harness::new((900, 300));
+    let (text, content) = plant(&mut warm);
+    warm.rsc.widgets_mut().get_dyn_mut(text);
+    warm.frame();
+
+    let mut cold = Harness::new((900, 300));
+    let (cold_text, cold_content) = plant(&mut cold);
+
+    assert_eq!(warm.region(&text), cold.region(&cold_text));
+    assert_eq!(warm.region(&content), cold.region(&cold_content));
+}
+
 /// A widget that clips to its box may not report more than the box: its
 /// parent would place the part it cut off, and the framework would put a
 /// drawing longer than its box somewhere. `Masked` is the second of these
@@ -71,7 +134,7 @@ fn a_clipping_widget_reporting_more_than_its_box_is_caught() {
 
     impl Widget for Clipper {
         fn draw(&mut self, painter: &mut Painter) -> Size {
-            painter.set_mask(painter.region());
+            painter.set_mask(UiRegion::FULL);
             painter.widget(&self.0).size()
         }
     }
@@ -82,3 +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

@@ -3,13 +3,190 @@
 //! frame that had not settled: a wrapping text shaped at a width it was
 //! measured in rather than the one it was given. The rest are a widget
 //! measured again in a box its own answer had decided, where the old answer
-//! is a fixed point whatever the content now says. The last two are neither:
-//! one box length, composed two ways, landing either side of the boundary
-//! that decided whether a child was drawn at all, and one box as long as the
-//! box a widget was offered but somewhere else.
+//! is a fixed point whatever the content now says. The last three are
+//! neither: one box length, composed two ways, landing either side of the
+//! boundary that decided whether a child was drawn at all, and two boxes
+//! reached through a region node's own entry rather than through the offer
+//! that node was given. The last is a wrapping text handed back the width
+//! it measured, rounded to a step below the line it measured there.
 
 use iris::harness::Harness;
 use iris::prelude::*;
+use iris::random::Branch;
+
+fn assert_same_regions(
+    warm: &Harness,
+    warm_ids: &[WidgetId],
+    cold: &Harness,
+    cold_ids: &[WidgetId],
+) {
+    let mut wrong = Vec::new();
+    for (i, (&w, &c)) in warm_ids.iter().zip(cold_ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        if got != want {
+            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+}
+
+/// Ten widgets, shrunk from seed 2 at depth 5. The stack is as tall as its
+/// first child, so its other children belong in that one-line box. A cold
+/// layout used to keep the span's answer from the larger measuring box while
+/// a repaint asked it in the stack's final box.
+fn plant_stack_in_its_sizing_childs_box(h: &mut Harness) -> Vec<WidgetId> {
+    let sizing = wtext("one line, overflowing whatever it is given")
+        .size(16)
+        .wrap(false)
+        .add(&mut h.rsc);
+    let filler = rect(Color::CYAN.alpha(252)).add(&mut h.rsc);
+    let plain = wtext("one line, overflowing whatever it is given")
+        .size(16)
+        .wrap(false)
+        .add(&mut h.rsc);
+    let span = (filler, plain).span(Dir::DOWN).add(&mut h.rsc);
+    let pad = Pad {
+        padding: Padding::ZERO,
+        inner: span.add_strong(&mut h.rsc),
+    }
+    .add(&mut h.rsc);
+    let probe = rect(Color::RED).add(&mut h.rsc);
+    let wide = rect(Color::YELLOW.alpha(252)).add(&mut h.rsc);
+    let narrow = rect(Color::RED).add(&mut h.rsc);
+    let branch = Branch {
+        probe: probe.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);
+    let stack = Stack {
+        children: vec![
+            sizing.add_strong(&mut h.rsc),
+            pad.add_strong(&mut h.rsc),
+            branch.add_strong(&mut h.rsc),
+        ],
+        size: StackSize::Child(0),
+    }
+    .add(&mut h.rsc);
+    h.rsc
+        .widgets_mut()
+        .set_size_rules(stack.id(), Some(LayoutLen::LEFTOVER), None);
+    h.set_root(stack);
+    vec![
+        sizing.id(),
+        filler.id(),
+        plain.id(),
+        span.id(),
+        pad.id(),
+        probe.id(),
+        wide.id(),
+        narrow.id(),
+        branch.id(),
+        stack.id(),
+    ]
+}
+
+#[test]
+fn repainting_a_stack_uses_the_box_its_sizing_child_decided() {
+    let mut warm = Harness::new((900, 1200));
+    let ids = plant_stack_in_its_sizing_childs_box(&mut warm);
+    for &id in &ids {
+        warm.rsc.widgets_mut().get_dyn_mut(id);
+    }
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let cold_ids = plant_stack_in_its_sizing_childs_box(&mut cold);
+
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
+
+/// Ten widgets, shrunk from seed 108 at depth 5. The nested reverse spans
+/// evaluate the branch in successively narrower boxes. The answer from the
+/// final, decided box must be the one retained after every span is reordered.
+fn plant_branch_in_nested_reverse_spans(
+    h: &mut Harness,
+    reordered: bool,
+) -> (Vec<WidgetId>, [WeakWidget<Span>; 3]) {
+    let pair = |first: StrongWidget, second: StrongWidget| match reordered {
+        true => vec![second, first],
+        false => vec![first, second],
+    };
+    let probe = rect(Color::RED.alpha(63)).add(&mut h.rsc);
+    let wide = rect(Color::RED).add(&mut h.rsc);
+    let narrow = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let branch = Branch {
+        probe: probe.add_strong(&mut h.rsc),
+        wide: wide.add_strong(&mut h.rsc),
+        narrow: narrow.add_strong(&mut h.rsc),
+        threshold: 483.0,
+    }
+    .add(&mut h.rsc);
+    let wrapped = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let down = Span {
+        children: pair(
+            branch.add_strong(&mut h.rsc),
+            wrapped.add_strong(&mut h.rsc),
+        ),
+        dir: Dir::DOWN,
+        gap: Px::ZERO,
+    }
+    .add(&mut h.rsc);
+    let inner_filler = rect(Color::CYAN.alpha(63)).add(&mut h.rsc);
+    let inner = Span {
+        children: pair(
+            down.add_strong(&mut h.rsc),
+            inner_filler.add_strong(&mut h.rsc),
+        ),
+        dir: Dir::LEFT,
+        gap: Px::ZERO,
+    }
+    .height(LayoutLen::rel(1.0))
+    .add(&mut h.rsc);
+    let outer_filler = rect(Color::GREEN.alpha(63)).add(&mut h.rsc);
+    let outer = Span {
+        children: pair(
+            inner.add_strong(&mut h.rsc),
+            outer_filler.add_strong(&mut h.rsc),
+        ),
+        dir: Dir::LEFT,
+        gap: Px::ZERO,
+    }
+    .height(LayoutLen::rel(1.0))
+    .add(&mut h.rsc);
+    h.set_root(outer);
+    (
+        vec![
+            probe.id(),
+            wide.id(),
+            narrow.id(),
+            branch.id(),
+            wrapped.id(),
+            down.id(),
+            inner_filler.id(),
+            inner.id(),
+            outer_filler.id(),
+            outer.id(),
+        ],
+        [down, inner, outer],
+    )
+}
+
+#[test]
+fn reordering_nested_spans_keeps_the_answer_from_the_decided_box() {
+    let mut warm = Harness::new((900, 1200));
+    let (ids, spans) = plant_branch_in_nested_reverse_spans(&mut warm, false);
+    for span in spans {
+        warm.rsc[span].children.rotate_left(1);
+    }
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let (cold_ids, _) = plant_branch_in_nested_reverse_spans(&mut cold, true);
+
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
 
 /// Six widgets, shrunk from a 402-widget tree the fuzzer found. Nothing about
 /// the tree changes -- every widget is marked for redraw and the frame is
@@ -428,12 +605,12 @@ fn plant_nested_scrolls(h: &mut Harness) -> Vec<WidgetId> {
     vec![text.id(), inner.id(), filler.id(), span.id(), root.id()]
 }
 
-/// A local redraw asks a dirty widget in the box its parent asked it in, and
-/// then again in the box its parent chose from that answer. Skipping the
-/// second ask because the two boxes are the same *length* left this inner
-/// scroll, which owns a region node, drawn at its offer. The offer is the
-/// outer scroll's whole viewport and the final box is 24px above it -- the
-/// height of the sized child the outer scroll snaps to the end of -- so the
+/// A local redraw asks a dirty widget in the box its parent gave it, and only
+/// where that box is as long as the one it was offered; anything else is a
+/// question its parent has to ask. This inner scroll's offer is the outer
+/// scroll's whole viewport and the box it was given is 24px shorter -- the
+/// height of the sized child the outer scroll snaps to the end of -- so what
+/// it must not do is settle itself. It was drawn at its offer once, and the
 /// inner scroll and its text stayed 24px too low.
 #[test]
 fn redrawing_one_widget_does_not_move_what_scrolls_around_it() {
@@ -454,3 +631,439 @@ fn redrawing_one_widget_does_not_move_what_scrolls_around_it() {
     }
     assert!(wrong.is_empty(), "{}", wrong.join("\n"));
 }
+
+/// Ten widgets, of the shape `tests/shrink.rs` reduces the oracle's seed 220
+/// to. The pad owns a movable region and is the scroll's content, so the box
+/// the scroll places it in is as long as that content while the box it was
+/// offered is the viewport -- and with no padding to tell those two apart,
+/// the span inside it looked like it was still at its offer. So everything
+/// under the pad was asked again in the *placed* box, the offer resolving
+/// against the node's own entry, which holds that box: the texts kept the
+/// widths they had, the content stayed the length those widths make, and the
+/// old answer confirmed itself. What the branch adds is a tree that differs
+/// rather than a box that moved, since a probe measured at the wrong width
+/// takes the other side.
+fn plant_under_a_node(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget<Span>; 2]) {
+    let probe = rect(Color::RED).add(&mut h.rsc);
+    let wide = rect(Color::GREEN).add(&mut h.rsc);
+    let narrow = rect(Color::BLUE).add(&mut h.rsc);
+    let branch = Branch {
+        probe: probe.add_strong(&mut h.rsc),
+        wide: wide.add_strong(&mut h.rsc),
+        narrow: narrow.add_strong(&mut h.rsc),
+        threshold: 213.0,
+    }
+    .add(&mut h.rsc);
+    let wrapped = wtext(
+        "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.",
+    )
+    .size(16)
+    .wrap(true)
+    .add(&mut h.rsc);
+    let plain = wtext("one line, overflowing whatever it is given")
+        .size(16)
+        .wrap(false)
+        .add(&mut h.rsc);
+    let row = |h: &mut Harness, mut children: Vec<StrongWidget>| {
+        if swapped {
+            children.rotate_left(1);
+        }
+        Span {
+            children,
+            dir: Dir::RIGHT,
+            gap: Px::ZERO,
+        }
+        .add(&mut h.rsc)
+    };
+    let texts: Vec<StrongWidget> =
+        vec![wrapped.add_strong(&mut h.rsc), plain.add_strong(&mut h.rsc)];
+    let inner = row(h, texts);
+    let pair: Vec<StrongWidget> = vec![branch.add_strong(&mut h.rsc), inner.add_strong(&mut h.rsc)];
+    let outer = row(h, pair);
+    let pad = Pad {
+        padding: Padding::ZERO,
+        inner: outer.add_strong(&mut h.rsc),
+    }
+    .add(&mut h.rsc);
+    h.rsc.widgets_mut().set_region_node(pad.id(), true);
+    let root = Scroll::new(pad.add_strong(&mut h.rsc), Axis::X).add(&mut h.rsc);
+    h.set_root(root);
+    (
+        vec![
+            probe.id(),
+            wide.id(),
+            narrow.id(),
+            branch.id(),
+            wrapped.id(),
+            plain.id(),
+            inner.id(),
+            outer.id(),
+            pad.id(),
+            root.id(),
+        ],
+        [outer, inner],
+    )
+}
+
+#[test]
+fn a_widget_under_a_region_node_is_asked_in_the_box_that_node_was_offered() {
+    let mut warm = Harness::new((900, 1200));
+    let (ids, spans) = plant_under_a_node(&mut warm, false);
+    warm.frame();
+    for span in spans {
+        warm.rsc[span].children.rotate_left(1);
+    }
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let (cold_ids, _) = plant_under_a_node(&mut cold, true);
+    cold.frame();
+
+    let mut wrong = Vec::new();
+    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        if got != want {
+            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+}
+
+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
+/// the text below the span is then wrapped at that width -- so a width the
+/// shaper measured comes back to it as the box to break in.
+fn plant_a_measured_width(h: &mut Harness, swapped: bool) -> (WeakWidget<Span>, WidgetId) {
+    let first: StrongWidget = rect(Color::YELLOW).add_strong(&mut h.rsc);
+    let mut inner = Span::empty(Dir::UP);
+    inner.children = match swapped {
+        true => swapped_in(h),
+        false => vec![first],
+    };
+    let inner = inner.height(142).add(&mut h.rsc);
+    let text = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let stack = Stack {
+        children: vec![inner.add_strong(&mut h.rsc), text.add_strong(&mut h.rsc)],
+        size: StackSize::Child(0),
+    }
+    .add(&mut h.rsc);
+    h.set_root((stack,).span(Dir::DOWN).width(195));
+    (inner, text.id())
+}
+
+/// What the span holds once its children have been swapped, which is what
+/// the warm tree is changed to and what the cold one is grown with.
+fn swapped_in(h: &mut Harness) -> Vec<StrongWidget> {
+    let paragraph = |h: &mut Harness| -> StrongWidget {
+        wtext(PARAGRAPH).size(16).wrap(true).add_strong(&mut h.rsc)
+    };
+    vec![
+        paragraph(h),
+        rect(Color::YELLOW).add_strong(&mut h.rsc),
+        paragraph(h),
+    ]
+}
+
+/// A text handed back the width it measured breaks there the way it broke
+/// when it measured it. The width the shaper answers is not on the grid, and
+/// a report rounded to the nearest step is under the longest line half the
+/// time: a warm tree then keeps a break made in a wider box while a cold one
+/// makes a narrower break in the same box, and the paragraph gains a line.
+#[test]
+fn a_text_is_given_back_a_box_the_line_it_measured_fits_in() {
+    let mut warm = Harness::new((900, 1200));
+    let (inner, text) = plant_a_measured_width(&mut warm, false);
+    warm.frame();
+    warm.rsc[inner].children = swapped_in(&mut warm);
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let (_, cold_text) = plant_a_measured_width(&mut cold, true);
+    cold.frame();
+
+    assert_eq!(warm.region(&text), cold.region(&cold_text));
+}
+
+#[test]
+fn adding_text_to_a_reverse_row_keeps_its_shared_height() {
+    fn build(
+        h: &mut Harness,
+        changed: bool,
+    ) -> (WeakWidget<Span>, WeakWidget<Text>, Vec<StrongWidget>) {
+        let wrap = wtext("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.").size(16).wrap(true).add_strong(&mut h.rsc);
+        let one = || {
+            wtext("one line, overflowing whatever it is given")
+                .size(16)
+                .wrap(false)
+        };
+        let plain = one().add_strong(&mut h.rsc);
+        let shared = one()
+            .width(LayoutLen::LEFTOVER)
+            .height(LayoutLen::LEFTOVER)
+            .add(&mut h.rsc);
+        let mut extra: Vec<StrongWidget> = vec![
+            rect(Color::RED).add_strong(&mut h.rsc),
+            one().add_strong(&mut h.rsc),
+            one().add_strong(&mut h.rsc),
+        ];
+        let children: Vec<StrongWidget> = if changed {
+            let mut children: Vec<StrongWidget> = vec![plain, shared.add_strong(&mut h.rsc)];
+            children.append(&mut extra);
+            children
+        } else {
+            vec![wrap, plain, shared.add_strong(&mut h.rsc)]
+        };
+        let row = Span {
+            children,
+            dir: Dir::LEFT,
+            gap: Px::ZERO,
+        }
+        .height(LayoutLen::rel(1.0))
+        .add(&mut h.rsc);
+        let fill: StrongWidget = rect(Color::BLUE).add_strong(&mut h.rsc);
+        let children: Vec<StrongWidget> = vec![fill, row.add_strong(&mut h.rsc)];
+        let root = Span {
+            children,
+            dir: Dir::RIGHT,
+            gap: Px::from_int(4),
+        }
+        .height(LayoutLen::rel(1.0))
+        .add(&mut h.rsc);
+        h.set_root(root);
+        (row, shared, extra)
+    }
+    let mut warm = Harness::new((900, 1200));
+    let (row, shared, extra) = build(&mut warm, false);
+    warm.rsc[row].children.remove(0);
+    warm.rsc[row].children.extend(extra);
+    warm.frame();
+    let mut cold = Harness::new((900, 1200));
+    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);
+}

tests/layout_diagnostics.rs

@@ -5,11 +5,11 @@
 //!     cargo test --release --features layout-diagnostics \
 //!       --test layout_diagnostics -- --ignored --nocapture
 //!
-//! Uninstrumented hardware totals for one phase:
+//! Build the uninstrumented test with `cargo test --release --test
+//! layout_diagnostics --no-run`, then run the emitted executable directly:
 //!
-//!     IRIS_PHASE=resize IRIS_FRAMES=1000 perf stat \
-//!       -e cycles:u,instructions:u cargo test --release \
-//!       --test layout_diagnostics -- --ignored --nocapture
+//!     IRIS_PHASE=resize IRIS_FRAMES=10000 perf stat -r 7 \
+//!       -e cycles:u,instructions:u /path/to/layout_diagnostics --ignored --nocapture
 //!
 //! `IRIS_PHASE` is `cold`, `repaint`, `many`, `size`, `scroll`, `resize`, or
 //! `all`. `IRIS_SEED`, `IRIS_DEPTH`, and `IRIS_FRAMES` select the load, and
@@ -134,6 +134,9 @@ fn report(label: &str, mut elapsed: Vec<f64>, _harness: &Harness) {
     {
         let diagnostics = iris::core::layout_diagnostics::take();
         print!("{}", diagnostics.per_frame(frames));
+        for event in diagnostics.traces() {
+            println!("    {event:?}");
+        }
         for callsite in diagnostics.hot_text().iter().take(3) {
             let mut ancestry = Vec::new();
             let mut id = Some(callsite.id);

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

@@ -42,13 +42,6 @@ const OUTER: (f32, f32) = (1920.0, 1200.0);
 const INNER: (f32, f32) = (640.0, 900.0);
 const STILL: (f32, f32) = (900.0, 1200.0);
 
-/// The same box, to a step of the grid per level of nesting between the two
-/// ways of reaching it. A move, a repaint and a row of shares land on the
-/// same number; what is left is a box centred in a fraction of its parent
-/// against the same box centred in its own pixels. A step is a thousandth of
-/// a pixel, where this was a twentieth of one before any of it was on a grid.
-const AGREE_STEPS: i32 = 2;
-
 /// A way of changing what a span holds. Each is a shape worth its own case:
 /// taking a child out of the middle is not the same as emptying a span, and
 /// adding one is not the same as adding three.
@@ -110,6 +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
@@ -126,12 +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,
@@ -153,6 +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",
@@ -177,6 +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) {
@@ -291,7 +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()
         },
@@ -391,18 +400,15 @@ fn describe_widget(id: WidgetId, h: &Harness) -> String {
     label
 }
 
-fn same_region(got: Option<PixelRegion>, want: Option<PixelRegion>) -> bool {
-    match (got, want) {
-        (Some(got), Some(want)) => {
-            let same = |a: Px, b: Px| (a - b).abs() <= Px::STEP.mul_int(AGREE_STEPS);
-            same(got.top_left.x, want.top_left.x)
-                && same(got.top_left.y, want.top_left.y)
-                && same(got.bot_right.x, want.bot_right.x)
-                && same(got.bot_right.y, want.bot_right.y)
-        }
-        (None, None) => true,
-        _ => false,
-    }
+/// 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!(
+        "frame {} ask {} box {} size {}",
+        active.frame, active.part, active.extent, active.size,
+    )
 }
 
 /// Runs `case` on the tree `plan` describes, warm and cold, and says where
@@ -421,22 +427,40 @@ 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);
     cold.state.root = Some(root);
     cold.frame();
 
+    let places: HashMap<WidgetId, usize> = tree
+        .ids
+        .iter()
+        .enumerate()
+        .map(|(i, &id)| (id, i))
+        .collect();
     let mut drawn = 0;
     for (i, (&w, &c)) in tree.ids.iter().zip(&cold_tree.ids).enumerate() {
         let (got, want) = (warm.region(&w), cold.region(&c));
         drawn += got.is_some() as usize;
-        if same_region(got, want) {
+        if got == want {
             continue;
         }
         // 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];
@@ -445,11 +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  {}",
-            chain.join(" < ")
+            "widget {i}\n  warm {got:?}\n  cold {want:?}\n  {}\n{}",
+            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(),