Give a slot to the children a container places, and nothing else

A widget's region is now held in the coordinates of the slot it draws in
rather than the window's, and `Painter::place` is how a container asks for a
slot: it draws a child it decides the box of and may decide again. Everything
under that slot is a fraction of its box, so placing the child a second time
is one entry to write whether it moved or changed length. A child drawn any
other way has no slot and shares its nearest ancestor's.

That is what keeps the chain short. `chain_cost` measured depth as the cost
-- free to 8, +42.6% at 16 -- and a slot per widget put a transcript's glyphs
past that for nothing, since almost every slot was zero. `Span`, `Aligned`
and `Scroll` are the containers that re-place a child after drawing it, and
`tests/layout.rs` pins that four widgets between a span and a leaf leave the
leaf's chain one deep.

`UiRegion::stretch`, `UiRegion::stretchable` and `UiScalar::stretch` are
gone. Nothing is inverted any more: a box that changed length is written to
its slot, and the descendants recompose against it in the shader. That also
retires the case the guard existed for, where a fixed length has no fraction
to recover -- `tests/layout.rs` now stretches a 40-tall row on its other
axis, which `stretchable` refused outright.

What still walks the CPU is deciding who must draw again, which no chain can
answer: `mark_resized` descends from the widget whose box changed and marks
anything whose own box changed length and whose drawing reads it. A part of
a box with no relative extent on an axis is a fixed length, and composing
into it leaves none either, so the walk stops where a length did not change
-- an 80-wide child in a widened row is not redrawn though it says `Redraw`.

`Span`, `Pad`, `Stack`, `Offset`, `Aligned`, `SetSize` and `LayerOffset` say
`Scale`: each places in fractions and offsets of its own box and none reads
the box's pixel length. `Scroll` and `MaxSize` do read pixels and stay
`Redraw`.

45 tests pass, five of them new. Render verification comes after the CPU
side, per the owner.

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

core/src/orientation/pos.rs

@@ -202,17 +202,6 @@ impl UiScalar {
         }
     }
 
-    /// `within` undone against `from` and redone against `to`, for one axis.
-    /// `from`'s relative extent is the denominator, so it must not be zero.
-    fn stretch(&self, from: &UiSpan, to: &UiSpan) -> Self {
-        let frac = (self.rel - from.start.rel) / (from.end.rel - from.start.rel);
-        Self {
-            rel: frac.lerp(to.start.rel, to.end.rel),
-            abs: self.abs - frac.lerp(from.start.abs, from.end.abs)
-                + frac.lerp(to.start.abs, to.end.abs),
-        }
-    }
-
     pub fn within_len(&self, len: UiScalar) -> Self {
         self.within(&UiSpan {
             start: UiScalar::ZERO,
@@ -391,30 +380,6 @@ impl UiRegion {
             },
         }
     }
-
-    /// Whether a stretch out of this box can be expressed. Each part inside a
-    /// box is held as a fraction of it, and a fixed length has no fraction to
-    /// hold one by -- every part of it is just an offset from its start.
-    pub fn stretchable(&self) -> bool {
-        self.x.start.rel != self.x.end.rel && self.y.start.rel != self.y.end.rel
-    }
-
-    /// Re-expresses a region inside `from` as the same fractions of `to`.
-    /// `from` must be `stretchable`; a translation is `shift` instead, which
-    /// needs no fractions and works out of any box.
-    pub fn stretch(&self, from: &UiRegion, to: &UiRegion) -> UiRegion {
-        debug_assert!(from.stretchable(), "a fixed length has no fraction");
-        UiRegion {
-            x: UiSpan {
-                start: self.x.start.stretch(&from.x, &to.x),
-                end: self.x.end.stretch(&from.x, &to.x),
-            },
-            y: UiSpan {
-                start: self.y.start.stretch(&from.y, &to.y),
-                end: self.y.end.stretch(&from.y, &to.y),
-            },
-        }
-    }
 }
 
 impl Display for UiRegion {

core/src/render/data.rs

@@ -82,10 +82,7 @@ unsafe impl bytemuck::Pod for MoveOffset {}
 unsafe impl bytemuck::Zeroable for MoveOffset {}
 
 impl MoveOffset {
-    pub fn root(parent: MoveIdx) -> Self {
-        Self {
-            region: UiRegion::FULL,
-            parent,
-        }
+    pub fn new(parent: MoveIdx, region: UiRegion) -> Self {
+        Self { region, parent }
     }
 }

core/src/ui/active.rs

@@ -15,8 +15,11 @@ pub struct ActiveData {
     pub size_deps: Vec<WidgetId>,
     /// Whether it read the output's size, and so is wrong when that changes.
     pub reads_output: bool,
-    /// The move slot its primitives are positioned through.
+    /// The slot its primitives are positioned through: its own if its parent
+    /// placed it, otherwise the nearest ancestor that has one.
     pub move_idx: MoveIdx,
+    /// The slot `region` is given in, which is whatever its parent drew in.
+    pub parent_move: MoveIdx,
     pub mask: MaskIdx,
     pub layer: LayerId,
 }

core/src/ui/mod.rs

@@ -36,9 +36,19 @@ pub struct Moves {
 }
 
 impl Moves {
-    pub fn push(&mut self, parent: MoveIdx) -> MoveIdx {
+    pub fn push(&mut self, parent: MoveIdx, region: UiRegion) -> MoveIdx {
         self.changed = true;
-        MoveIdx::slot(self.arena.push(MoveOffset::root(parent)).idx())
+        MoveIdx::slot(self.arena.push(MoveOffset::new(parent, region)).idx())
+    }
+
+    /// Re-points a slot at a different parent, for a widget drawn somewhere
+    /// else in the tree than it was.
+    pub fn set_parent(&mut self, idx: MoveIdx, parent: MoveIdx) {
+        let entry = self.arena.get_mut(Id::preset(idx.idx() as u32));
+        if entry.parent != parent {
+            entry.parent = parent;
+            self.changed = true;
+        }
     }
 
     pub fn remove(&mut self, idx: MoveIdx) {
@@ -77,6 +87,18 @@ impl Moves {
         region
     }
 
+    /// How many slots a region in `idx` is composed through, which is what
+    /// the shader's walk costs per primitive.
+    pub fn depth(&self, idx: MoveIdx) -> usize {
+        let mut depth = 0;
+        let mut at = idx;
+        while at != MoveIdx::NONE && depth < CHAIN_LIMIT as usize {
+            at = self.arena[at.idx()].parent;
+            depth += 1;
+        }
+        depth
+    }
+
     pub fn entries(&self) -> &[MoveOffset] {
         &self.arena
     }

core/src/ui/painter.rs

@@ -13,6 +13,7 @@ pub struct Painter<'a> {
     pub(super) state: &'a mut UiRenderState,
     pub(super) rsc: &'a mut dyn UiRsc,
 
+    /// This widget's box, in the coordinates of `move_idx`.
     pub(super) region: UiRegion,
     pub(super) mask: MaskIdx,
     pub(super) textures: Vec<TextureHandle>,
@@ -21,7 +22,8 @@ pub struct Painter<'a> {
     /// The children whose size this widget read while drawing.
     pub(super) size_deps: Vec<WidgetId>,
     pub(super) reads_output: bool,
-    /// The move slot this widget's primitives are positioned through.
+    /// The slot this widget's primitives are positioned through: its own if
+    /// its parent placed it, otherwise the nearest ancestor that has one.
     pub(super) move_idx: MoveIdx,
     pub layer: usize,
     pub(super) id: WidgetId,
@@ -78,24 +80,39 @@ impl<'a> Painter<'a> {
 
     /// Draws a widget within this widget's region.
     pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
-        self.widget_at(id, self.region)
+        self.widget_at(id, self.region, false)
     }
 
-    /// Draws a widget somewhere within this one. Drawing one a second time
-    /// gives it a new box, keeping the drawing it already has where it can.
+    /// Draws a widget somewhere within this one.
     pub fn widget_within<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
         region: UiRegion,
     ) -> DrawResult<'s, 'a, W> {
         let region = region.within(&self.region);
-        self.widget_at(id, region)
+        self.widget_at(id, region, false)
+    }
+
+    /// Draws a child this widget decides the box of, and may decide again
+    /// once it knows what the child came to. The child gets a slot of its
+    /// own, so placing it a second time writes one entry however much it
+    /// drew -- moved or resized alike, since everything under the slot is
+    /// held as a fraction of its box. A child drawn any other way has no slot
+    /// and can only be given a different box by drawing again.
+    pub fn place<'s, W: ?Sized>(
+        &'s mut self,
+        id: &'s StrongWidget<W>,
+        region: UiRegion,
+    ) -> DrawResult<'s, 'a, W> {
+        let region = region.within(&self.region);
+        self.widget_at(id, region, true)
     }
 
     fn widget_at<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
         region: UiRegion,
+        slotted: bool,
     ) -> DrawResult<'s, 'a, W> {
         // A child listed twice would be moved twice.
         if !self.children.contains(&id.id()) {
@@ -106,6 +123,8 @@ impl<'a> Painter<'a> {
             id.id(),
             region,
             Some(self.id),
+            self.move_idx,
+            slotted,
             self.mask,
             None,
             self.rsc,
@@ -165,6 +184,8 @@ impl<'a> Painter<'a> {
         }
     }
 
+    /// This widget's box, in the coordinates its own primitives are written
+    /// in -- so a region composed `within` it may be drawn directly.
     pub fn region(&self) -> UiRegion {
         self.region
     }
@@ -176,11 +197,13 @@ impl<'a> Painter<'a> {
         self.state.output_size
     }
 
-    /// This widget's box in pixels. Resolved against the output's size, so a
-    /// widget that reads it draws again when the output changes.
+    /// This widget's box in pixels. Resolved against the output's size and
+    /// the boxes it sits within, so a widget that reads it draws again when
+    /// the output changes.
     pub fn px_size(&mut self) -> Vec2 {
         self.reads_output = true;
-        self.region.size().to_abs(self.state.output_size)
+        let region = self.state.moves.resolve(self.move_idx, self.region);
+        region.size().to_abs(self.state.output_size)
     }
 
     pub fn text_data(&mut self) -> &mut TextData {

core/src/ui/render_state.rs

@@ -4,6 +4,8 @@ use crate::{
     util::{HashMap, HashSet, Vec2, forget_ref},
 };
 
+const AXES: [Axis; 2] = [Axis::X, Axis::Y];
+
 pub struct UiRenderState {
     pub active: HashMap<WidgetId, ActiveData>,
     pub layers: DrawLayers,
@@ -82,7 +84,17 @@ impl UiRenderState {
         self.clear(rsc);
         // free all resources & cache
         if let Some(id) = root {
-            self.draw_inner(0, id.id(), UiRegion::FULL, None, MaskIdx::NONE, None, rsc);
+            self.draw_inner(
+                0,
+                id.id(),
+                UiRegion::FULL,
+                None,
+                MoveIdx::NONE,
+                false,
+                MaskIdx::NONE,
+                None,
+                rsc,
+            );
         }
     }
 
@@ -94,13 +106,15 @@ impl UiRenderState {
         id: WidgetId,
         region: UiRegion,
         parent: Option<WidgetId>,
+        parent_move: MoveIdx,
+        slotted: bool,
         mask: MaskIdx,
         old_children: Option<Vec<WidgetId>>,
         rsc: &mut dyn UiRsc,
     ) -> Size {
         let mut old_children = old_children.unwrap_or_default();
         if self.active.contains_key(&id) {
-            if let Some(size) = self.try_reuse(id, region, rsc) {
+            if let Some(size) = self.try_reuse(id, region, parent_move, rsc) {
                 return size;
             }
             // if not, then maintain resize and track old children to remove unneeded
@@ -109,14 +123,21 @@ impl UiRenderState {
         }
 
         // draw widget
-        let move_idx = self.move_slot(id, parent);
-        self.moves.set(move_idx, UiRegion::FULL);
+        let (move_idx, local) = match slotted {
+            // Its box becomes its slot's, so it draws in the slot's own
+            // coordinates and the box it was given is one entry to rewrite.
+            true => (self.move_slot(id, parent_move, region), UiRegion::FULL),
+            false => {
+                self.drop_slot(id);
+                (parent_move, region)
+            }
+        };
         rsc.widgets_mut().needs_redraw.remove(&id);
         self.draw_started.insert(id);
 
         let mut painter = Painter {
             state: self,
-            region,
+            region: local,
             mask,
             layer,
             id,
@@ -136,7 +157,7 @@ impl UiRenderState {
         let Painter {
             state: _,
             rsc: _,
-            region,
+            region: _,
             mask,
             textures,
             primitives,
@@ -166,6 +187,7 @@ impl UiRenderState {
             size_deps,
             reads_output,
             move_idx,
+            parent_move,
             mask,
             layer,
         };
@@ -182,101 +204,133 @@ impl UiRenderState {
         size
     }
 
-    /// The slot a widget's drawing is positioned through, made on its first
-    /// draw and kept until it stops being drawn.
-    fn move_slot(&mut self, id: WidgetId, parent: Option<WidgetId>) -> MoveIdx {
+    /// The slot a widget's box is held in, made on its first placed draw and
+    /// kept until it stops being drawn -- a redraw replaces its `ActiveData`
+    /// while descendants go on naming the slot.
+    fn move_slot(&mut self, id: WidgetId, parent: MoveIdx, region: UiRegion) -> MoveIdx {
         if let Some(&idx) = self.slots.get(&id) {
+            self.moves.set_parent(idx, parent);
+            self.moves.set(idx, region);
             return idx;
         }
-        let parent = parent
-            .and_then(|p| self.slots.get(&p).copied())
-            .unwrap_or(MoveIdx::NONE);
-        let idx = self.moves.push(parent);
+        let idx = self.moves.push(parent, region);
         self.slots.insert(id, idx);
         idx
     }
 
+    /// Gives up a slot a widget no longer needs, because it is drawn somewhere
+    /// that does not place it. Its descendants name it, so this is only
+    /// reached where they are about to be drawn again.
+    fn drop_slot(&mut self, id: WidgetId) {
+        if let Some(idx) = self.slots.remove(&id) {
+            self.moves.remove(idx);
+        }
+    }
+
     /// The drawing a widget already has, kept for a new box if the box has not
     /// changed in a way it depends on.
-    fn try_reuse(&mut self, id: WidgetId, region: UiRegion, rsc: &dyn UiRsc) -> Option<Size> {
+    fn try_reuse(
+        &mut self,
+        id: WidgetId,
+        region: UiRegion,
+        parent_move: MoveIdx,
+        rsc: &mut dyn UiRsc,
+    ) -> Option<Size> {
         if rsc.widgets().needs_redraw.contains(&id) {
             return None;
         }
         let active = self.active.get(&id)?;
-        let (size, old, slot) = (active.size, active.region, active.move_idx);
-        // TODO: epsilon?
-        if old.size() == region.size() {
-            // The right shape and only somewhere else, which is one slot to
-            // write however much is under it. Both boxes are in the
-            // coordinates its parent drew, so the chain above applies alike.
-            let moved =
-                region.to_px(self.output_size).top_left - old.to_px(self.output_size).top_left;
-            self.moves.set(slot, UiRegion::FULL.offset(moved));
-            return Some(size);
-        }
-        if !self.reusable(id, region, rsc) || !old.stretchable() {
+        // Drawn somewhere else in the tree: its box is in coordinates it no
+        // longer sits in, and its slot names the wrong parent.
+        if active.parent_move != parent_move {
             return None;
         }
-        // Its drawing stands, re-expressed as the same fractions of the box.
-        self.stretch(id, old, region);
+        let (size, old, slot) = (active.size, active.region, active.move_idx);
+        if old == region {
+            return Some(size);
+        }
+        // Only a placed widget can be given a different box without drawing
+        // again: everything it drew is a fraction of its slot's box, so one
+        // entry says where all of it went.
+        if slot == parent_move {
+            return None;
+        }
+        let mut changed = [false; 2];
+        for (axis, c) in AXES.into_iter().zip(changed.iter_mut()) {
+            *c = region.axis(axis).len() != old.axis(axis).len();
+        }
+        if changed.iter().any(|&c| c) {
+            let widget = rsc.widgets().get_dyn(id)?;
+            let redraws = AXES
+                .into_iter()
+                .zip(changed)
+                .any(|(axis, c)| c && widget.on_resize(axis) != OnResize::Scale);
+            if redraws {
+                return None;
+            }
+        }
+        self.moves.set(slot, region);
+        self.active.get_mut(&id).unwrap().region = region;
+        if changed.iter().any(|&c| c) {
+            self.mark_resized(id, changed, rsc);
+        }
         Some(size)
     }
 
-    /// Whether the widget can keep the drawing it has and be given `region`
-    /// instead, asked one axis at a time: a change on an axis it does not
-    /// depend on costs nothing, whatever it depends on elsewhere.
-    fn reusable(&self, id: WidgetId, region: UiRegion, rsc: &dyn UiRsc) -> bool {
+    /// Marks every descendant whose drawing cannot survive the box it is a
+    /// fraction of changing length, `changed` saying which axes of that box
+    /// did.
+    ///
+    /// A part of a box with no relative extent on an axis is a fixed length,
+    /// held as offsets from that box's start, and composing anything into it
+    /// leaves no relative extent either. So a widget whose own box did not
+    /// change length has no descendant whose box did, and the walk stops
+    /// there.
+    fn mark_resized(&mut self, id: WidgetId, changed: [bool; 2], rsc: &mut dyn UiRsc) {
         let Some(active) = self.active.get(&id) else {
-            return false;
+            return;
         };
-        let Some(widget) = rsc.widgets().get_dyn(id) else {
-            return false;
-        };
-        [Axis::X, Axis::Y].into_iter().all(|axis| {
-            let offered = region.axis(axis).len();
-            let had = active.region.axis(axis).len();
-            match widget.on_resize(axis) {
-                OnResize::Scale => true,
-                // `Translate` is not acted on yet, and cannot be until a
-                // drawing can sit somewhere other than its box. `region` is
-                // both the box a widget was given and the box its primitives
-                // are in, and `mov` remaps from it -- so carrying a drawing at
-                // its old size while the box grows makes the next move stretch
-                // it. The offset chain is what separates the two.
-                OnResize::Translate | OnResize::Redraw => offered == had,
+        // SAFETY: children cannot be recursive
+        let children = unsafe { forget_ref(&active.children) };
+        for &child in children {
+            let Some(data) = self.active.get(&child) else {
+                continue;
+            };
+            let region = data.region;
+            let mut own = changed;
+            for (axis, c) in AXES.into_iter().zip(own.iter_mut()) {
+                *c &= region.axis(axis).len().rel != 0.0;
             }
-        })
+            if !own.iter().any(|&c| c) {
+                continue;
+            }
+            let redraws = match rsc.widgets().get_dyn(child) {
+                Some(widget) => AXES
+                    .into_iter()
+                    .zip(own)
+                    .any(|(axis, c)| c && widget.on_resize(axis) != OnResize::Scale),
+                None => true,
+            };
+            match redraws {
+                true => {
+                    rsc.widgets_mut().needs_redraw.insert(child);
+                }
+                false => self.mark_resized(child, own, rsc),
+            }
+        }
     }
 
     fn hints_agree(id: WidgetId, size: Size, rsc: &dyn UiRsc) -> bool {
         let Some(widget) = rsc.widgets().get_dyn(id) else {
             return true;
         };
-        [Axis::X, Axis::Y].into_iter().all(|axis| {
+        AXES.into_iter().all(|axis| {
             widget
                 .size_hint(axis)
                 .is_none_or(|hint| hint == size.axis(axis))
         })
     }
 
-    /// Rewrites a subtree's regions as the same fractions of a new box, for a
-    /// change of length that a slot cannot express. Every region it rewrites
-    /// is a region some slot was a delta from, so those go back to zero.
-    fn stretch(&mut self, id: WidgetId, from: UiRegion, to: UiRegion) {
-        let active = self.active.get_mut(&id).unwrap();
-        for h in &active.primitives {
-            let region = self.layers[h.layer].region_mut(h);
-            *region = region.stretch(&from, &to);
-        }
-        active.region = active.region.stretch(&from, &to);
-        self.moves.set(active.move_idx, UiRegion::FULL);
-        // SAFETY: children cannot be recursive
-        let children = unsafe { forget_ref(&active.children) };
-        for child in children {
-            self.stretch(*child, from, to);
-        }
-    }
-
     /// NOTE: instance textures are cleared and self.textures freed
     fn remove(&mut self, id: WidgetId, undraw: bool, rsc: &mut dyn UiRsc) -> Option<ActiveData> {
         let mut active = self.active.remove(&id);
@@ -363,11 +417,11 @@ impl UiRenderState {
         }
     }
 
-    /// Where a widget is on screen: the box it drew against plus whatever the
-    /// chain above has moved since, which is the walk the vertex shader does.
+    /// Where a widget is on screen: its box composed through the boxes it
+    /// sits within, which is the walk the vertex shader does.
     pub fn window_region(&self, id: &impl IdLike) -> Option<PixelRegion> {
         let active = self.active.get(&id.id())?;
-        let region = self.moves.resolve(active.move_idx, active.region);
+        let region = self.moves.resolve(active.parent_move, active.region);
         Some(region.to_px(self.output_size))
     }
 
@@ -400,6 +454,8 @@ impl UiRenderState {
             id,
             active.region,
             active.parent,
+            active.parent_move,
+            active.move_idx != active.parent_move,
             active.mask,
             Some(active.children),
             rsc,

src/widget/position/align.rs

@@ -9,14 +9,20 @@ impl Widget for Aligned {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         // Drawn where it may be too big, then given its aligned box once its
         // size is known.
-        let size = painter.widget(&self.inner).size();
+        let size = painter.place(&self.inner, UiRegion::FULL).size();
         let region = match self.align.tuple() {
             (Some(x), Some(y)) => size.to_uivec2().align(RegionAlign { x, y }),
             (Some(x), None) => UiRegion::new(size.x.apply_rest().align(x), UiSpan::FULL),
             (None, Some(y)) => UiRegion::new(UiSpan::FULL, size.y.apply_rest().align(y)),
             (None, None) => UiRegion::FULL,
         };
-        painter.widget_within(&self.inner, region);
+        painter.place(&self.inner, region);
         size
     }
+
+    /// The aligned box is a fraction of its own, so the child keeps its
+    /// length and stays against the edge it was aligned to.
+    fn on_resize(&self, _: Axis) -> OnResize {
+        OnResize::Scale
+    }
 }

src/widget/position/layer.rs

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

src/widget/position/offset.rs

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

src/widget/position/pad.rs

@@ -21,6 +21,12 @@ impl Widget for Pad {
             },
         }
     }
+
+    /// The padding is an offset from each edge, so a longer box pads the same
+    /// amount and the child takes the rest.
+    fn on_resize(&self, _: Axis) -> OnResize {
+        OnResize::Scale
+    }
 }
 
 pub struct Padding {

src/widget/position/scroll.rs

@@ -12,10 +12,10 @@ pub struct Scroll {
 impl Widget for Scroll {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let output_len = painter.output_size().axis(self.axis);
-        let container_len = painter.region().axis(self.axis).len();
+        let container_len = UiScalar::abs(painter.px_size().axis(self.axis));
         // Drawn in the whole container to learn its length, then placed at
         // the scrolled offset.
-        let child = painter.widget(&self.inner).size();
+        let child = painter.place(&self.inner, UiRegion::FULL).size();
         let content_len = child
             .axis(self.axis)
             .apply_rest()
@@ -31,7 +31,7 @@ impl Widget for Scroll {
 
         let mut region = UiRegion::FULL.offset(Vec2::from_axis(self.axis, -self.amt, 0.0));
         region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
-        painter.widget_within(&self.inner, region);
+        painter.place(&self.inner, region);
         child
     }
 }

src/widget/position/set_size.rs

@@ -23,4 +23,8 @@ impl Widget for SetSize {
             Axis::Y => self.y,
         }
     }
+
+    fn on_resize(&self, _: Axis) -> OnResize {
+        OnResize::Scale
+    }
 }

src/widget/position/span.rs

@@ -17,7 +17,7 @@ impl Widget for Span {
             .iter()
             .map(|child| match painter.size_hint(child, axis) {
                 Some(len) => len,
-                None => painter.widget(child).len(axis),
+                None => painter.place(child, UiRegion::FULL).len(axis),
             })
             .collect();
 
@@ -42,7 +42,7 @@ impl Widget for Span {
             if self.dir.sign == Sign::Neg {
                 region.flip(axis);
             }
-            let used = painter.widget_within(child, region).size().axis(!axis);
+            let used = painter.place(child, region).size().axis(!axis);
             // TODO: rel shouldn't do this, but no easy way before actually calculating pixels
             if used.rel > 0.0 || used.rest > 0.0 {
                 ortho = Len::REST;
@@ -58,6 +58,12 @@ impl Widget for Span {
         };
         Size::from_axis(axis, along, ortho)
     }
+
+    /// Every child is placed in fractions and offsets of the span's own box,
+    /// so a longer box holds the same layout and the children follow it.
+    fn on_resize(&self, _: Axis) -> OnResize {
+        OnResize::Scale
+    }
 }
 
 impl Span {

src/widget/position/stack.rs

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

tests/chain_cost.rs

@@ -77,7 +77,7 @@ fn fill(ui: &mut UiData, render: &mut UiRenderState, depth: usize) {
 
     let mut slot = MoveIdx::NONE;
     for _ in 0..depth {
-        slot = render.moves.push(slot);
+        slot = render.moves.push(slot, UiRegion::FULL);
     }
 
     let px = |v: f32| UiScalar { rel: 0.0, abs: v };

tests/layout.rs

@@ -126,3 +126,59 @@ fn a_moved_subtree_takes_its_children_with_it() {
     // `inner`'s own region was never rewritten.
     assert_corners!(h, inner, (10, 90), (390, 110));
 }
+
+#[test]
+fn a_fixed_length_child_keeps_it_when_the_box_around_it_grows() {
+    let mut h = Harness::new((400, 200));
+    let fixed = rect(Color::BLUE).width(50).add(&mut h.rsc);
+    let rest = rect(Color::GREEN).add(&mut h.rsc);
+    let panel = (fixed, rest).span(Dir::RIGHT).add(&mut h.rsc);
+    // Changing the bar's width is the only thing that changes the box the
+    // panel and everything under it was drawn for.
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, panel).span(Dir::RIGHT));
+    assert_corners!(h, fixed, (100, 0), (150, 200));
+    assert_corners!(h, rest, (150, 0), (400, 200));
+
+    h.rsc[bar].x = Some(Len::abs(200));
+    h.frame();
+
+    // The panel's box is 100 shorter, so the fixed child is the same 50 wide
+    // against its new start and the one taking the rest absorbs the change.
+    assert_corners!(h, fixed, (200, 0), (250, 200));
+    assert_corners!(h, rest, (250, 0), (400, 200));
+}
+
+#[test]
+fn a_box_with_a_fixed_length_can_be_stretched_on_its_other_axis() {
+    let mut h = Harness::new((400, 200));
+    // The row is 40 tall whatever happens, which used to make its drawing
+    // impossible to take out of: recovering a fraction of a box needs a
+    // relative extent, and it has none on that axis.
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let row = inner.pad(10).height(40).add(&mut h.rsc);
+    let filler = rect(Color::GREEN).add(&mut h.rsc);
+    let column = (row, filler).span(Dir::DOWN).add(&mut h.rsc);
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, column).span(Dir::RIGHT));
+    assert_corners!(h, inner, (110, 10), (390, 30));
+
+    h.rsc[bar].x = Some(Len::abs(200));
+    h.frame();
+
+    assert_corners!(h, inner, (210, 10), (390, 30));
+}
+
+#[test]
+fn only_a_container_that_places_its_children_lengthens_the_chain() {
+    let mut h = Harness::new((400, 200));
+    let leaf = rect(Color::BLUE).add(&mut h.rsc);
+    // Four widgets between the span and the leaf, none of which places what
+    // it draws, so all of them share the span's slot.
+    let buried = leaf.pad(4).pad(4).pad(4).pad(4).add(&mut h.rsc);
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, buried).span(Dir::RIGHT));
+
+    let slot = h.render.active[&leaf.id()].parent_move;
+    assert_eq!(h.render.moves.depth(slot), 1, "one span above the leaf");
+}

tests/retained.rs

@@ -251,9 +251,8 @@ fn a_change_two_levels_under_its_reader_still_reaches_it() {
     assert_corners!(h, below, (12, 232), (388, 388));
 }
 
-/// Claims its drawing may be stretched, and has a child so that the stretch
-/// has to reach one. No shipped container claims `Scale` -- `Rect`, `Image`
-/// and `()` are all childless -- so nothing else walks a subtree to remap it.
+/// Claims its drawing survives its box changing length, and has a child so
+/// that the walk looking for what does not has one to reach.
 struct Stretchy {
     inner: StrongWidget,
     draws: Rc<Cell<usize>>,
@@ -271,7 +270,7 @@ impl Widget for Stretchy {
 }
 
 #[test]
-fn stretching_a_subtree_remaps_the_children_in_it() {
+fn stretching_a_subtree_carries_the_children_in_it() {
     let mut h = Harness::new((400, 400));
     let first = rect(Color::RED).height(40).add(&mut h.rsc);
     let inner = rect(Color::BLUE).add(&mut h.rsc);
@@ -291,8 +290,50 @@ fn stretching_a_subtree_remaps_the_children_in_it() {
     assert_eq!(
         draws.get(),
         settled,
-        "its drawing is stretched, not redrawn"
+        "its drawing follows its box, rather than being made again"
     );
     assert_corners!(h, outer, (0, 80), (400, 400));
     assert_corners!(h, inner, (0, 80), (400, 400));
 }
+
+#[test]
+fn a_widened_row_redraws_what_reads_its_length_and_nothing_else() {
+    let mut h = Harness::new((400, 200));
+    // What a transcript row is: something whose shaping depends on the width
+    // it is given, beside something that only has to be the right shape.
+    let (wraps, wrap_draws) = counted(&mut h, Size::REST, OnResize::Redraw);
+    let (backing, back_draws) = counted(&mut h, Size::REST, OnResize::Scale);
+    let row = (backing, wraps).span(Dir::RIGHT).add(&mut h.rsc);
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, row).span(Dir::RIGHT));
+    let (settled_wrap, settled_back) = (wrap_draws.get(), back_draws.get());
+
+    h.rsc[bar].x = Some(Len::abs(200));
+    h.frame();
+
+    // The span reads every child's size, so redrawing one takes the span
+    // with it -- and the span then measures and places the redrawn child.
+    assert!(wrap_draws.get() > settled_wrap, "reads the width it got");
+    assert_eq!(back_draws.get(), settled_back, "only has to be the shape");
+    assert_corners!(h, backing, (200, 0), (300, 200));
+    assert_corners!(h, wraps, (300, 0), (400, 200));
+}
+
+#[test]
+fn a_fixed_length_child_is_not_redrawn_when_the_box_around_it_grows() {
+    let mut h = Harness::new((400, 200));
+    // It would be drawn again for a width it does not have: its own box is
+    // a fixed 80 wherever the row's edges end up.
+    let (fixed, draws) = counted(&mut h, Size::from((80, 200)), OnResize::Redraw);
+    let (rest, _) = counted(&mut h, Size::REST, OnResize::Scale);
+    let row = (fixed, rest).span(Dir::RIGHT).add(&mut h.rsc);
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, row).span(Dir::RIGHT));
+    let settled = draws.get();
+
+    h.rsc[bar].x = Some(Len::abs(200));
+    h.frame();
+
+    assert_eq!(draws.get(), settled, "its own length did not change");
+    assert_corners!(h, fixed, (200, 0), (280, 200));
+}

tests/stretch.rs

@@ -1,46 +0,0 @@
-//! What a drawing can be taken out of, and what it cannot.
-
-use iris::core::{UiRegion, UiScalar, UiSpan};
-
-/// A box `size` tall whose top is `rel` of the way down the window.
-fn fixed(rel: f32, size: f32) -> UiRegion {
-    UiRegion::new(
-        UiSpan::FULL,
-        UiSpan::new(UiScalar { rel, abs: 0.0 }, UiScalar { rel, abs: size }),
-    )
-}
-
-#[test]
-fn a_fixed_length_cannot_be_stretched_out_of() {
-    assert!(!fixed(0.0, 164.0).stretchable());
-    assert!(!fixed(0.5, 164.0).stretchable());
-    assert!(UiRegion::FULL.stretchable());
-}
-
-#[test]
-fn a_stretch_keeps_each_part_at_its_fraction() {
-    let to = fixed(0.0, 98.0);
-    // A part filling the window fills what replaced it.
-    assert_eq!(UiRegion::FULL.stretch(&UiRegion::FULL, &to), to);
-    // And the middle half of it stays the middle half.
-    let half = UiRegion::new(
-        UiSpan::FULL,
-        UiSpan::new(UiScalar::rel(0.25), UiScalar::rel(0.75)),
-    );
-    assert_eq!(
-        half.stretch(&UiRegion::FULL, &to),
-        UiRegion::new(
-            UiSpan::FULL,
-            UiSpan::new(
-                UiScalar {
-                    rel: 0.0,
-                    abs: 24.5
-                },
-                UiScalar {
-                    rel: 0.0,
-                    abs: 73.5
-                }
-            )
-        )
-    );
-}