Take a strong widget to depend on, and use the question mark

`depend_on_size` took a bare `WidgetId`, which any id at all satisfies.
It takes a `&StrongWidget` now, so `DrawResult` carries the handle it
was drawn from rather than an id copied out of it and nothing can claim
a dependency on a widget it does not hold.

`UiSpan::outside` matched on a pair of `Option`s where `?` says it. It
was written that way while the method was still `const`, and it is not.

No call site changed. Checked: fmt, clippy and 35 tests; `tabs`, `view`,
`minimal` and `text` render unchanged, and the live sway resize round
trip still matches a cold start at each size.

core/src/orientation/pos.rs

@@ -56,13 +56,6 @@ impl UiVec2 {
         }
     }
 
-    pub const fn outside(&self, region: &UiRegion) -> UiVec2 {
-        UiVec2 {
-            x: self.x.outside(&region.x),
-            y: self.y.outside(&region.y),
-        }
-    }
-
     pub fn axis_mut(&mut self, axis: Axis) -> &mut UiScalar {
         match axis {
             Axis::X => &mut self.x,
@@ -209,10 +202,12 @@ impl UiScalar {
         }
     }
 
-    pub const fn outside(&self, span: &UiSpan) -> Self {
-        let rel = self.rel.lerp_inv(span.start.rel, span.end.rel);
+    /// Undoes `within`, and `None` where the span has a fixed length: every
+    /// fraction of it lands on the same `rel`, so none can be told apart.
+    pub fn outside(&self, span: &UiSpan) -> Option<Self> {
+        let rel = self.rel.lerp_inv(span.start.rel, span.end.rel)?;
         let abs = self.abs - rel.lerp(span.start.abs, span.end.abs);
-        Self { rel, abs }
+        Some(Self { rel, abs })
     }
 
     pub fn within_len(&self, len: UiScalar) -> Self {
@@ -283,11 +278,11 @@ impl UiSpan {
         }
     }
 
-    pub const fn outside(&self, parent: &Self) -> Self {
-        Self {
-            start: self.start.outside(parent),
-            end: self.end.outside(parent),
-        }
+    pub fn outside(&self, parent: &Self) -> Option<Self> {
+        Some(Self {
+            start: self.start.outside(parent)?,
+            end: self.end.outside(parent)?,
+        })
     }
 
     pub const fn len(&self) -> UiScalar {
@@ -324,14 +319,7 @@ impl UiRegion {
             y: self.y.within(&parent.y),
         }
     }
-    pub const fn outside(&self, parent: &Self) -> Self {
-        Self {
-            x: self.x.outside(&parent.x),
-            y: self.y.outside(&parent.y),
-        }
-    }
-
-    pub const fn axis(&mut self, axis: Axis) -> &UiSpan {
+    pub const fn axis(&self, axis: Axis) -> &UiSpan {
         match axis {
             Axis::X => &self.x,
             Axis::Y => &self.y,
@@ -409,6 +397,50 @@ impl UiRegion {
     }
 }
 
+/// Taking a drawing out of one box and putting it in another, checked once
+/// for a whole subtree so that applying it cannot fail.
+///
+/// A box of a fixed length holds each part as an offset from its start rather
+/// than as a fraction of it, so those parts can be carried to a box of the
+/// same length but never stretched to a different one.
+#[derive(Debug, Copy, Clone, PartialEq)]
+pub struct Remap {
+    from: UiRegion,
+    to: UiRegion,
+}
+
+impl Remap {
+    pub fn new(from: UiRegion, to: UiRegion) -> Option<Self> {
+        [Axis::X, Axis::Y]
+            .into_iter()
+            .all(|axis| {
+                let (from, to) = (from.axis(axis), to.axis(axis));
+                from.start.rel != from.end.rel || from.len() == to.len()
+            })
+            .then_some(Self { from, to })
+    }
+
+    pub fn apply(&self, region: UiRegion) -> UiRegion {
+        UiRegion {
+            x: Self::span(region.x, self.from.x, self.to.x),
+            y: Self::span(region.y, self.from.y, self.to.y),
+        }
+    }
+
+    fn span(span: UiSpan, from: UiSpan, to: UiSpan) -> UiSpan {
+        match span.outside(&from) {
+            Some(out) => out.within(&to),
+            // `new` admits this only where the two are the same length, so
+            // the difference between their starts is the whole move.
+            None => {
+                let mut span = span;
+                span.shift(to.start - from.start);
+                span
+            }
+        }
+    }
+}
+
 impl Display for UiRegion {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(

core/src/ui/active.rs

@@ -13,6 +13,8 @@ pub struct ActiveData {
     pub children: Vec<WidgetId>,
     /// The children whose size this widget read while drawing.
     pub size_deps: Vec<WidgetId>,
+    /// Whether it read the output's size, and so is wrong when that changes.
+    pub reads_output: bool,
     pub mask: MaskIdx,
     pub layer: LayerId,
 }

core/src/ui/painter.rs

@@ -20,7 +20,7 @@ pub struct Painter<'a> {
     pub(super) children: Vec<WidgetId>,
     /// The children whose size this widget read while drawing.
     pub(super) size_deps: Vec<WidgetId>,
-    pub(super) size: Option<Size>,
+    pub(super) reads_output: bool,
     pub layer: usize,
     pub(super) id: WidgetId,
 }
@@ -70,39 +70,31 @@ impl<'a> Painter<'a> {
         self.mask = self.rsc.ui_mut().masks.push(Mask { region });
     }
 
-    /// States what this widget uses of the box it was handed, once per draw.
-    /// This is the size a parent reads.
-    pub fn set_size(&mut self, size: impl Into<Size>) {
-        debug_assert!(
-            self.size.is_none(),
-            "{} set its size twice in one draw",
-            self.label(),
-        );
-        self.size = Some(size.into());
-    }
-
     /// Draws a widget within this widget's region.
-    pub fn widget<W: ?Sized>(&mut self, id: &StrongWidget<W>) -> DrawResult<'_, 'a> {
+    pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
         self.widget_at(id, self.region)
     }
 
-    /// Draws a widget somewhere within this one.
-    /// Useful for drawing child widgets in select areas.
-    pub fn widget_within<W: ?Sized>(
-        &mut self,
-        id: &StrongWidget<W>,
+    /// 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.
+    pub fn widget_within<'s, W: ?Sized>(
+        &'s mut self,
+        id: &'s StrongWidget<W>,
         region: UiRegion,
-    ) -> DrawResult<'_, 'a> {
+    ) -> DrawResult<'s, 'a, W> {
         let region = region.within(&self.region);
         self.widget_at(id, region)
     }
 
-    fn widget_at<W: ?Sized>(
-        &mut self,
-        id: &StrongWidget<W>,
+    fn widget_at<'s, W: ?Sized>(
+        &'s mut self,
+        id: &'s StrongWidget<W>,
         region: UiRegion,
-    ) -> DrawResult<'_, 'a> {
+    ) -> DrawResult<'s, 'a, W> {
+        // A child listed twice would be moved twice.
+        if !self.children.contains(&id.id()) {
             self.children.push(id.id());
+        }
         let size = self.state.draw_inner(
             self.layer,
             id.id(),
@@ -113,43 +105,24 @@ impl<'a> Painter<'a> {
             self.rsc,
         );
         DrawResult {
-            child: id.id(),
+            child: id,
             painter: self,
             size,
         }
     }
 
     /// What a child says its length is without being drawn, if it can say.
-    pub fn size_hint<W: ?Sized>(&self, id: &StrongWidget<W>, axis: Axis) -> Option<Len> {
-        self.rsc.widgets().get_dyn(id.id())?.size_hint(axis)
+    /// Asking counts as reading its size.
+    pub fn size_hint<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<Len> {
+        let hint = self.rsc.widgets().get_dyn(id.id())?.size_hint(axis)?;
+        self.depend_on_size(id);
+        Some(hint)
     }
 
-    /// Gives a child its final box once this widget knows what that is,
-    /// keeping the drawing it already has where it can.
-    pub fn place<W: ?Sized>(&mut self, id: &StrongWidget<W>, region: UiRegion) -> Size {
-        let region = region.within(&self.region);
-        let id = id.id();
-        // Choosing a child's box depends on its size, from a draw or a hint.
-        // Claiming it is separate: one left out of `children` is removed.
-        if !self.size_deps.contains(&id) {
-            self.size_deps.push(id);
+    fn depend_on_size<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
+        if !self.size_deps.contains(&child.id()) {
+            self.size_deps.push(child.id());
         }
-        if !self.children.contains(&id) {
-            self.children.push(id);
-        }
-        if self.state.active.contains_key(&id) {
-            return self.state.place(id, region, self.rsc);
-        }
-        // Not drawn yet: its length came from a hint, so this is its one draw.
-        self.state.draw_inner(
-            self.layer,
-            id,
-            region,
-            Some(self.id),
-            self.mask,
-            None,
-            self.rsc,
-        )
     }
 
     pub fn render_text(
@@ -190,11 +163,17 @@ impl<'a> Painter<'a> {
         self.region
     }
 
-    pub fn output_size(&self) -> Vec2 {
+    /// The output's size in pixels. A widget that reads it draws again when
+    /// the output changes, since nothing else can put that right.
+    pub fn output_size(&mut self) -> Vec2 {
+        self.reads_output = true;
         self.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.
     pub fn px_size(&mut self) -> Vec2 {
+        self.reads_output = true;
         self.region.size().to_abs(self.state.output_size)
     }
 
@@ -222,15 +201,15 @@ impl<'a> Painter<'a> {
 /// A child that has just been drawn. Reading its size records that this
 /// widget's own size depends on it; dropping it without reading draws the
 /// child and leaves the parent independent of what it came to.
-pub struct DrawResult<'p, 'a> {
+pub struct DrawResult<'p, 'a, W: ?Sized> {
     painter: &'p mut Painter<'a>,
-    child: WidgetId,
+    child: &'p StrongWidget<W>,
     size: Size,
 }
 
-impl DrawResult<'_, '_> {
+impl<W: ?Sized> DrawResult<'_, '_, W> {
     pub fn size(self) -> Size {
-        self.painter.size_deps.push(self.child);
+        self.painter.depend_on_size(self.child);
         self.size
     }
 

core/src/ui/render_state.rs

@@ -1,5 +1,5 @@
 use crate::{
-    ActiveData, Axis, DrawLayers, IdLike, MaskIdx, Painter, PixelRegion, Size, SizeDependence,
+    ActiveData, Axis, DrawLayers, IdLike, MaskIdx, OnResize, Painter, PixelRegion, Remap, Size,
     StrongWidget, UiRegion, UiRsc, WidgetId, Widgets,
     util::{HashMap, HashSet, Vec2, forget_ref},
 };
@@ -53,11 +53,21 @@ impl UiRenderState {
             );
         }
         let root = root.into();
-        if self.needs_full_redraw(root) {
+        if self.root_changed(root) {
             self.redraw_all(root, rsc);
             self.old_root = root.map(|r| r.id());
+        } else if self.resized {
+            // A region is a fraction of the output plus an offset, resolved
+            // against the window in the shader, so a resize moves the whole
+            // drawing on its own. Only a widget that read pixels can be wrong.
+            for (&id, active) in &self.active {
+                if active.reads_output {
+                    rsc.widgets_mut().needs_redraw.insert(id);
+                }
+            }
+        }
         self.resized = false;
-        } else if rsc.widgets().has_updates() {
+        if rsc.widgets().has_updates() {
             self.redraw_updates(rsc);
         }
     }
@@ -93,6 +103,7 @@ impl UiRenderState {
         }
 
         // draw widget
+        rsc.widgets_mut().needs_redraw.remove(&id);
         self.draw_started.insert(id);
 
         let mut painter = Painter {
@@ -105,12 +116,12 @@ impl UiRenderState {
             primitives: Vec::new(),
             children: Vec::new(),
             size_deps: Vec::new(),
-            size: None,
+            reads_output: false,
             rsc,
         };
 
         let mut widget = painter.rsc.widgets().get_dyn_dynamic(id);
-        widget.draw(&mut painter);
+        let size = widget.draw(&mut painter);
         drop(widget);
 
         let Painter {
@@ -122,17 +133,11 @@ impl UiRenderState {
             primitives,
             children,
             size_deps,
-            size,
+            reads_output,
             layer,
             id,
         } = painter;
 
-        let size = size.unwrap_or_else(|| {
-            panic!(
-                "'{}' ({id:?}) drew without a size; every widget calls Painter::set_size",
-                rsc.widgets().label(id)
-            )
-        });
         debug_assert!(
             Self::hints_agree(id, size, rsc),
             "'{}' ({id:?}) drew a size its size_hint disagrees with",
@@ -149,6 +154,7 @@ impl UiRenderState {
             primitives,
             children,
             size_deps,
+            reads_output,
             mask,
             layer,
         };
@@ -165,26 +171,6 @@ impl UiRenderState {
         size
     }
 
-    /// Gives an already drawn widget a new box, keeping its drawing if it can
-    /// and drawing it again if it cannot.
-    pub(super) fn place(&mut self, id: WidgetId, region: UiRegion, rsc: &mut dyn UiRsc) -> Size {
-        if let Some(size) = self.try_reuse(id, region, rsc) {
-            return size;
-        }
-        let Some(active) = self.remove(id, false, rsc) else {
-            return Size::ZERO;
-        };
-        self.draw_inner(
-            active.layer,
-            id,
-            region,
-            active.parent,
-            active.mask,
-            Some(active.children),
-            rsc,
-        )
-    }
-
     /// 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> {
@@ -192,43 +178,41 @@ impl UiRenderState {
             return None;
         }
         let active = self.active.get(&id)?;
-        let (size, was) = (active.size, active.region);
-        if was == region {
+        let (size, old) = (active.size, active.region);
+        if old == region {
             return Some(size);
         }
         // TODO: epsilon?
-        if was.size() == region.size() {
-            self.mov(id, was, region);
-            return Some(size);
+        if old.size() != region.size() && !self.reusable(id, region, rsc) {
+            return None;
         }
-        if self.reusable(id, region, rsc) {
-            // Its drawing stands; the box is written into the primitives.
-            self.mov(id, was, region);
-            return Some(size);
-        }
-        None
+        // Its drawing stands, if the new box can be reached from the old one.
+        self.mov(id, &Remap::new(old, region)?);
+        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, mut region: UiRegion, rsc: &dyn UiRsc) -> bool {
+    fn reusable(&self, id: WidgetId, region: UiRegion, rsc: &dyn UiRsc) -> bool {
         let Some(active) = self.active.get(&id) else {
             return false;
         };
         let Some(widget) = rsc.widgets().get_dyn(id) else {
             return false;
         };
-        let mut was = active.region;
         [Axis::X, Axis::Y].into_iter().all(|axis| {
-            let offered = region.axis_mut(axis).len();
-            let had = was.axis_mut(axis).len();
-            match widget.size_dependence(axis) {
-                SizeDependence::None => true,
-                // `Internal` could also keep its drawing when only the room
-                // around it changed, but that is a translation rather than a
-                // remap, so it waits for the move chain.
-                SizeDependence::Internal | SizeDependence::External => offered == had,
+            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,
             }
         })
     }
@@ -244,17 +228,17 @@ impl UiRenderState {
         })
     }
 
-    fn mov(&mut self, id: WidgetId, from: UiRegion, to: UiRegion) {
+    fn mov(&mut self, id: WidgetId, remap: &Remap) {
         let active = self.active.get_mut(&id).unwrap();
         for h in &active.primitives {
             let region = self.layers[h.layer].region_mut(h);
-            *region = region.outside(&from).within(&to);
+            *region = remap.apply(*region);
         }
-        active.region = active.region.outside(&from).within(&to);
+        active.region = remap.apply(active.region);
         // SAFETY: children cannot be recursive
         let children = unsafe { forget_ref(&active.children) };
         for child in children {
-            self.mov(*child, from, to);
+            self.mov(*child, remap);
         }
     }
 
@@ -307,17 +291,12 @@ impl UiRenderState {
         root.into().map(|r| r.id()) != self.old_root
     }
 
-    // Scheduling and drawing must use the same full-redraw predicate.
-    fn needs_full_redraw<'a>(&self, root: impl Into<Option<&'a StrongWidget>>) -> bool {
-        self.root_changed(root) || self.resized
-    }
-
     pub fn needs_redraw<'a>(
         &self,
         root: impl Into<Option<&'a StrongWidget>>,
         widgets: &Widgets,
     ) -> bool {
-        self.needs_full_redraw(root) || widgets.has_updates()
+        self.root_changed(root) || self.resized || widgets.has_updates()
     }
 
     pub fn active_widgets(&self) -> usize {
@@ -350,14 +329,19 @@ impl UiRenderState {
 
     /// redraws a widget that's currently active (drawn)
     pub fn redraw(&mut self, id: WidgetId, rsc: &mut dyn UiRsc) {
-        rsc.widgets_mut().needs_redraw.remove(&id);
         self.draw_started.remove(&id);
         // Whoever read this widget's size may be a different size now, so the
-        // highest reader is what draws; it reaches this one on the way down.
-        if let Some(top) = self.highest_reader(id) {
+        // highest reader is what draws. Everything between the two is marked
+        // as well: their own boxes have not changed, so the mark is the only
+        // thing stopping the draw reusing its way past this widget.
+        if let Some(top) = self.mark_readers(id, rsc) {
             self.redraw(top, rsc);
+            // Cleared by that draw if it reached here; if it did not, this is
+            // no longer drawn and asking again would not end.
+            rsc.widgets_mut().needs_redraw.remove(&id);
             return;
         }
+        rsc.widgets_mut().needs_redraw.remove(&id);
 
         if self.draw_started.contains(&id) {
             return;
@@ -379,8 +363,8 @@ impl UiRenderState {
     }
 
     /// The furthest ancestor that read this widget's size, directly or through
-    /// widgets that did the same.
-    fn highest_reader(&self, id: WidgetId) -> Option<WidgetId> {
+    /// widgets that did the same, marking everything below it on the way.
+    fn mark_readers(&self, id: WidgetId, rsc: &mut dyn UiRsc) -> Option<WidgetId> {
         let mut top = None;
         let mut at = id;
         while let Some(active) = self.active.get(&at)
@@ -390,6 +374,7 @@ impl UiRenderState {
                 .get(&parent)
                 .is_some_and(|p| p.size_deps.contains(&at))
         {
+            rsc.widgets_mut().needs_redraw.insert(at);
             top = Some(parent);
             at = parent;
         }

core/src/util/math.rs

@@ -1,33 +1,21 @@
-use std::ops::*;
-
-pub const trait LerpUtil {
+pub const trait LerpUtil: Sized {
     fn lerp(self, from: Self, to: Self) -> Self;
-    fn lerp_inv(self, from: Self, to: Self) -> Self;
+    fn lerp_inv(self, from: Self, to: Self) -> Option<Self>;
 }
 
-pub const trait DivOr {
-    fn div_or(self, rhs: Self, other: Self) -> Self;
-}
-
-const impl DivOr for f32 {
-    fn div_or(self, rhs: Self, other: Self) -> Self {
-        let res = self / rhs;
-        if res.is_nan() { other } else { res }
-    }
-}
-
-const impl<
-    T: const Add<Output = T> + const Sub<Output = T> + const Mul<Output = T> + const DivOr + Copy,
-> LerpUtil for T
-{
+const impl LerpUtil for f32 {
     /// linear interpolation
     /// from * (1.0 - self) + to * self
     fn lerp(self, from: Self, to: Self) -> Self {
         from + (to - from) * self
     }
-    /// inverse of lerp
-    fn lerp_inv(self, from: Self, to: Self) -> Self {
-        (self - from).div_or(to - from, from)
+    /// inverse of lerp, and `None` where `from` and `to` are the same point:
+    /// every input lerps to it, so there is no one answer to come back to.
+    fn lerp_inv(self, from: Self, to: Self) -> Option<Self> {
+        match to == from {
+            true => None,
+            false => Some((self - from) / (to - from)),
+        }
     }
 }
 

core/src/util/vec2.rs

@@ -1,4 +1,4 @@
-use crate::util::{DivOr, impl_op};
+use crate::util::impl_op;
 use std::{hash::Hash, ops::*};
 
 #[repr(C)]
@@ -67,15 +67,6 @@ impl_op!(Vec2 Sub sub; x y);
 impl_op!(Vec2 Mul mul; x y);
 impl_op!(Vec2 Div div; x y);
 
-const impl DivOr for Vec2 {
-    fn div_or(self, rhs: Self, other: Self) -> Self {
-        Self {
-            x: self.x.div_or(rhs.x, other.x),
-            y: self.y.div_or(rhs.y, other.y),
-        }
-    }
-}
-
 impl Neg for Vec2 {
     type Output = Self;
 

core/src/widget/mod.rs

@@ -15,25 +15,20 @@ pub use tag::*;
 pub use view::*;
 pub use widgets::*;
 
-/// How much of the box a widget was handed its drawing depends on, and so
-/// what has to change before it must be drawn again. Asked per axis, because
-/// wrapped text depends on the width it is offered and not on the height.
+/// What may be done to a widget's drawing when the box it was given changes
+/// on this axis, instead of drawing it again. Asked per axis, because wrapped
+/// text reads the width it is offered and not the height.
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
-pub enum SizeDependence {
-    /// None of it: the box only says where the primitives go, so a new one is
-    /// written into them instead of drawn.
-    None,
-    /// Its own extent, whatever box that sits in. Reusable in any box that
-    /// leaves that extent unchanged, including a larger one it does not fill.
+pub enum OnResize {
+    Scale,
+    Translate,
     #[default]
-    Internal,
-    /// The extent of the box itself, used or not.
-    External,
+    Redraw,
 }
 
 pub trait Widget: Any {
-    /// Draws the widget, and states what it used with `Painter::set_size`.
-    fn draw(&mut self, painter: &mut Painter);
+    /// Draws the widget, and returns what it used of the box it was given.
+    fn draw(&mut self, painter: &mut Painter) -> Size;
 
     /// An exact length the widget can give without a painter or its children.
     /// Optional, and saves a draw rather than changing one: a hint that
@@ -42,22 +37,23 @@ pub trait Widget: Any {
         None
     }
 
-    fn size_dependence(&self, _axis: Axis) -> SizeDependence {
-        SizeDependence::Internal
+    fn on_resize(&self, _axis: Axis) -> OnResize {
+        OnResize::default()
     }
 }
 
 impl Widget for () {
-    fn draw(&mut self, painter: &mut Painter) {
-        painter.set_size(Size::ZERO);
+    /// A gap: nothing drawn, at the default length, so a span gives it a share.
+    fn draw(&mut self, _: &mut Painter) -> Size {
+        Size::default()
     }
 
     fn size_hint(&self, _axis: Axis) -> Option<Len> {
-        Some(Len::ZERO)
+        Some(Len::default())
     }
 
-    fn size_dependence(&self, _axis: Axis) -> SizeDependence {
-        SizeDependence::None
+    fn on_resize(&self, _axis: Axis) -> OnResize {
+        OnResize::Scale
     }
 }
 

src/widget/image.rs

@@ -6,17 +6,17 @@ pub struct Image {
 }
 
 impl Widget for Image {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         painter.primitive(&self.handle);
-        painter.set_size(Size::abs(self.handle.size()));
+        Size::abs(self.handle.size())
     }
 
     fn size_hint(&self, axis: Axis) -> Option<Len> {
         Some(Len::abs(self.handle.size().axis(axis)))
     }
 
-    fn size_dependence(&self, _: Axis) -> SizeDependence {
-        SizeDependence::None
+    fn on_resize(&self, _: Axis) -> OnResize {
+        OnResize::Scale
     }
 }
 

src/widget/mask.rs

@@ -5,14 +5,13 @@ pub struct Masked {
 }
 
 impl Widget for Masked {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         painter.set_mask(painter.region());
-        let size = painter.widget(&self.inner).size();
-        painter.set_size(size);
+        painter.widget(&self.inner).size()
     }
 
     /// It clips to the box it was given, not to the part its child used.
-    fn size_dependence(&self, _: Axis) -> SizeDependence {
-        SizeDependence::External
+    fn on_resize(&self, _: Axis) -> OnResize {
+        OnResize::Redraw
     }
 }

src/widget/position/align.rs

@@ -6,7 +6,7 @@ pub struct Aligned {
 }
 
 impl Widget for Aligned {
-    fn draw(&mut self, painter: &mut Painter) {
+    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();
@@ -16,7 +16,7 @@ impl Widget for Aligned {
             (None, Some(y)) => UiRegion::new(UiSpan::FULL, size.y.apply_rest().align(y)),
             (None, None) => UiRegion::FULL,
         };
-        painter.place(&self.inner, region);
-        painter.set_size(size);
+        painter.widget_within(&self.inner, region);
+        size
     }
 }

src/widget/position/layer.rs

@@ -6,11 +6,10 @@ pub struct LayerOffset {
 }
 
 impl Widget for LayerOffset {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         for _ in 0..self.offset {
             painter.next_layer();
         }
-        let size = painter.widget(&self.inner).size();
-        painter.set_size(size);
+        painter.widget(&self.inner).size()
     }
 }

src/widget/position/max_size.rs

@@ -6,16 +6,14 @@ pub struct MaxSize {
     pub y: Option<Len>,
 }
 
-impl MaxSize {}
-
 impl Widget for MaxSize {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         let child = painter.widget(&self.inner).size();
         let output = painter.output_size();
-        painter.set_size(Size {
+        Size {
             x: capped(child.x, self.x, output.x),
             y: capped(child.y, self.y, output.y),
-        });
+        }
     }
 }
 

src/widget/position/offset.rs

@@ -6,9 +6,8 @@ pub struct Offset {
 }
 
 impl Widget for Offset {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         let region = UiRegion::FULL.offset(self.amt);
-        let size = painter.widget_within(&self.inner, region).size();
-        painter.set_size(size);
+        painter.widget_within(&self.inner, region).size()
     }
 }

src/widget/position/pad.rs

@@ -6,11 +6,11 @@ pub struct Pad {
 }
 
 impl Widget for Pad {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         let inner = painter
             .widget_within(&self.inner, self.padding.region())
             .size();
-        painter.set_size(Size {
+        Size {
             x: Len {
                 abs: inner.x.abs + self.padding.left + self.padding.right,
                 ..inner.x
@@ -19,7 +19,7 @@ impl Widget for Pad {
                 abs: inner.y.abs + self.padding.top + self.padding.bottom,
                 ..inner.y
             },
-        });
+        }
     }
 }
 

src/widget/position/scroll.rs

@@ -10,7 +10,7 @@ pub struct Scroll {
 }
 
 impl Widget for Scroll {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         let output_len = painter.output_size().axis(self.axis);
         let container_len = painter.region().axis(self.axis).len();
         // Drawn in the whole container to learn its length, then placed at
@@ -31,8 +31,8 @@ impl Widget for Scroll {
 
         let mut region = UiRegion::FULL.offset(Vec2::from_axis(self.axis, -self.amt, 0.0));
         region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
-        painter.place(&self.inner, region);
-        painter.set_size(child);
+        painter.widget_within(&self.inner, region);
+        child
     }
 }
 

src/widget/position/set_size.rs

@@ -6,15 +6,13 @@ pub struct SetSize {
     pub y: Option<Len>,
 }
 
-impl SetSize {}
-
 impl Widget for SetSize {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         let child = painter.widget(&self.inner).size();
-        painter.set_size(Size {
+        Size {
             x: self.x.unwrap_or(child.x),
             y: self.y.unwrap_or(child.y),
-        });
+        }
     }
 
     /// A declared axis is known without looking at the child, which is what

src/widget/position/span.rs

@@ -8,7 +8,7 @@ pub struct Span {
 }
 
 impl Widget for Span {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         let axis = self.dir.axis;
         // A length for every child before any is placed: from its own hint
         // where it has one, and from drawing it where it does not.
@@ -42,7 +42,7 @@ impl Widget for Span {
             if self.dir.sign == Sign::Neg {
                 region.flip(axis);
             }
-            let used = painter.place(child, region).axis(!axis);
+            let used = painter.widget_within(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;
@@ -56,7 +56,7 @@ impl Widget for Span {
             true => total,
             false => Len::default(),
         };
-        painter.set_size(Size::from_axis(axis, along, ortho));
+        Size::from_axis(axis, along, ortho)
     }
 }
 

src/widget/position/stack.rs

@@ -8,7 +8,7 @@ pub struct Stack {
 }
 
 impl Widget for Stack {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         let sizing = match self.size {
             StackSize::Default => None,
             StackSize::Child(i) => Some(i),
@@ -26,7 +26,7 @@ impl Widget for Stack {
                 size = drawn.size();
             }
         }
-        painter.set_size(size);
+        size
     }
 }
 

src/widget/ptr.rs

@@ -6,12 +6,11 @@ pub struct WidgetPtr {
 }
 
 impl Widget for WidgetPtr {
-    fn draw(&mut self, painter: &mut Painter) {
-        let size = match &self.inner {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        match &self.inner {
             Some(id) => painter.widget(id).size(),
-            None => Size::ZERO,
-        };
-        painter.set_size(size);
+            None => Size::default(),
+        }
     }
 }
 

src/widget/rect.rs

@@ -28,14 +28,14 @@ impl Rect {
 }
 
 impl Widget for Rect {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         painter.primitive(RectPrimitive {
             color: self.color,
             radius: self.radius,
             thickness: self.thickness,
             inner_radius: self.inner_radius,
         });
-        painter.set_size(Size::REST);
+        Size::REST
     }
 
     fn size_hint(&self, _: Axis) -> Option<Len> {
@@ -43,8 +43,8 @@ impl Widget for Rect {
     }
 
     /// Its box is its primitive's own region, so a new one is written there.
-    fn size_dependence(&self, _: Axis) -> SizeDependence {
-        SizeDependence::None
+    fn on_resize(&self, _: Axis) -> OnResize {
+        OnResize::Scale
     }
 }
 

src/widget/text/edit.rs

@@ -55,41 +55,47 @@ impl TextEdit {
 }
 
 impl Widget for TextEdit {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         let base = painter.layer;
         painter.child_layer();
         let (_, size) = self.view.draw(painter);
-        painter.set_size(size);
         painter.layer = base;
         let region = self.region();
 
         let Some(selection) = self.selection else {
-            return;
+            return size;
         };
         let layout = self.view.buf.layout();
 
         // parley reports selection as boxes in layout space, so bidi and
         // wrapped lines come out right without this code knowing about either.
         for (rect, _) in selection.geometry(layout) {
-            let size = vec2(rect.width() as f32, rect.height() as f32);
+            let rect_size = vec2(rect.width() as f32, rect.height() as f32);
             let top_left = vec2(rect.x0 as f32, rect.y0 as f32);
             painter.primitive_within(
                 RectPrimitive::color(Color::SKY),
-                size.align(Align::TOP_LEFT).offset(top_left).within(&region),
+                rect_size
+                    .align(Align::TOP_LEFT)
+                    .offset(top_left)
+                    .within(&region),
             );
         }
 
         let caret = selection.focus().geometry(layout, CARET_WIDTH);
-        let size = vec2(caret.width() as f32, caret.height() as f32);
+        let caret_size = vec2(caret.width() as f32, caret.height() as f32);
         let top_left = vec2(caret.x0 as f32, caret.y0 as f32);
         painter.primitive_within(
             RectPrimitive::color(Color::WHITE),
-            size.align(Align::TOP_LEFT).offset(top_left).within(&region),
+            caret_size
+                .align(Align::TOP_LEFT)
+                .offset(top_left)
+                .within(&region),
         );
+        size
     }
 
-    fn size_dependence(&self, axis: Axis) -> SizeDependence {
-        self.view.size_dependence(axis)
+    fn on_resize(&self, axis: Axis) -> OnResize {
+        self.view.on_resize(axis)
     }
 }
 

src/widget/text/mod.rs

@@ -89,12 +89,19 @@ impl TextView {
         (region, size)
     }
 
-    /// Wrapping reads the width it is offered, so a wider box reshapes it; a
-    /// taller one never does.
-    pub fn size_dependence(&self, axis: Axis) -> SizeDependence {
-        match axis == Axis::X && self.attrs.wrap {
-            true => SizeDependence::External,
-            false => SizeDependence::Internal,
+    /// Wrapping reads the width it is offered, so a wider box reshapes it and
+    /// a taller one does not. Alignment matters too, and separately: glyphs
+    /// anchored to the start of an axis stay put when that extent changes,
+    /// but centred or end-aligned ones move even though the shaping stands.
+    pub fn on_resize(&self, axis: Axis) -> OnResize {
+        let reshapes = axis == Axis::X && self.attrs.wrap;
+        let anchored = match axis {
+            Axis::X => self.align.x,
+            Axis::Y => self.align.y,
+        } == AxisAlign::Neg;
+        match reshapes || !anchored {
+            true => OnResize::Redraw,
+            false => OnResize::Translate,
         }
     }
 
@@ -120,14 +127,13 @@ impl Text {
 }
 
 impl Widget for Text {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         self.update_buf();
-        let (_, size) = self.view.draw(painter);
-        painter.set_size(size);
+        self.view.draw(painter).1
     }
 
-    fn size_dependence(&self, axis: Axis) -> SizeDependence {
-        self.view.size_dependence(axis)
+    fn on_resize(&self, axis: Axis) -> OnResize {
+        self.view.on_resize(axis)
     }
 }
 

tests/layout.rs

@@ -32,3 +32,80 @@ fn resizing_relays_out_against_the_new_output() {
     assert_corners!(h, left, (0, 0), (100, 100));
     assert_corners!(h, right, (100, 0), (800, 100));
 }
+
+#[test]
+fn an_empty_widget_takes_a_share_of_a_span() {
+    let mut h = Harness::new((400, 200));
+    let gap = ().add(&mut h.rsc);
+    let right = rect(Color::BLUE).width(100).add(&mut h.rsc);
+    h.set_root((gap, right).span(Dir::RIGHT));
+
+    assert_corners!(h, gap, (0, 0), (300, 200));
+    assert_corners!(h, right, (300, 0), (400, 200));
+}
+
+#[test]
+fn a_child_drawn_twice_moves_once() {
+    let mut h = Harness::new((400, 200));
+    // `Aligned` draws its child twice; listing it twice would move it twice.
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let centered = inner.center().width(200).add(&mut h.rsc);
+    let left = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((left, centered).span(Dir::RIGHT));
+    assert_corners!(h, inner, (100, 0), (300, 200));
+
+    h.rsc[left].x = Some(Len::abs(150));
+    h.frame();
+
+    assert_corners!(h, inner, (150, 0), (350, 200));
+}
+
+#[test]
+fn a_resize_lands_where_a_cold_start_would() {
+    let build = |h: &mut Harness| {
+        let para = wtext(
+            "Wrapping shapes one source into as many lines as its container leaves room \
+             for, so the height of a paragraph is an answer rather than a setting.",
+        )
+        .size(20)
+        .wrap(true)
+        .pad(16)
+        .add(&mut h.rsc);
+        let below = rect(Color::RED).add(&mut h.rsc);
+        let root = (para, below).span(Dir::DOWN).pad(12);
+        h.set_root(root);
+        (para, below)
+    };
+
+    let mut cold = Harness::new((900, 1200));
+    let (cold_para, cold_below) = build(&mut cold);
+
+    let mut resized = Harness::new((1920, 1200));
+    let (para, below) = build(&mut resized);
+    resized.resize((900, 1200));
+    resized.frame();
+
+    assert_eq!(resized.region(&para), cold.region(&cold_para), "paragraph");
+    assert_eq!(resized.region(&below), cold.region(&cold_below), "below");
+}
+
+#[test]
+fn a_fixed_box_is_drawn_again_rather_than_stretched() {
+    let mut h = Harness::new((400, 400));
+    // The panel fills a stack sized by its sibling, so it is drawn in the
+    // whole box and then placed in the shorter one. Reusing it in that fixed
+    // box afterwards would leave it whatever height it happened to have.
+    let panel = rect(Color::BLUE).add(&mut h.rsc);
+    let leaf = rect(Color::RED).height(100).add(&mut h.rsc);
+    let stack = (panel, leaf)
+        .stack()
+        .size(StackSize::Child(1))
+        .add(&mut h.rsc);
+    h.set_root(stack.align(Align::TOP));
+    assert_corners!(h, panel, (0, 0), (400, 100));
+
+    h.rsc[leaf].y = Some(Len::abs(250));
+    h.frame();
+
+    assert_corners!(h, panel, (0, 0), (400, 250));
+}

tests/remap.rs

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

tests/retained.rs

@@ -10,16 +10,16 @@ use iris::prelude::*;
 struct Counted {
     draws: Rc<Cell<usize>>,
     size: Size,
-    dependence: SizeDependence,
+    dependence: OnResize,
 }
 
 impl Widget for Counted {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, _: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
-        painter.set_size(self.size);
+        self.size
     }
 
-    fn size_dependence(&self, _: Axis) -> SizeDependence {
+    fn on_resize(&self, _: Axis) -> OnResize {
         self.dependence
     }
 }
@@ -32,11 +32,7 @@ impl Counts {
     }
 }
 
-fn counted(
-    h: &mut Harness,
-    size: Size,
-    dependence: SizeDependence,
-) -> (WeakWidget<Counted>, Counts) {
+fn counted(h: &mut Harness, size: Size, dependence: OnResize) -> (WeakWidget<Counted>, Counts) {
     let draws = Rc::new(Cell::new(0));
     let id = Counted {
         draws: draws.clone(),
@@ -49,8 +45,8 @@ fn counted(
 
 /// A fixed-width leaf beside one that takes the rest, so changing the first
 /// hands the second a different box without the output changing.
-fn pair(h: &mut Harness, rest: SizeDependence) -> (WeakWidget<Counted>, Counts, WidgetId) {
-    let (first, _) = counted(h, Size::from((100, 200)), SizeDependence::Internal);
+fn pair(h: &mut Harness, rest: OnResize) -> (WeakWidget<Counted>, Counts, WidgetId) {
+    let (first, _) = counted(h, Size::from((100, 200)), OnResize::Translate);
     let (second, draws) = counted(h, Size::REST, rest);
     h.set_root((first, second).span(Dir::RIGHT));
     (first, draws, second.id())
@@ -59,7 +55,7 @@ fn pair(h: &mut Harness, rest: SizeDependence) -> (WeakWidget<Counted>, Counts,
 #[test]
 fn a_leaf_that_ignores_its_box_is_not_drawn_again_when_the_box_changes() {
     let mut h = Harness::new((400, 200));
-    let (first, draws, second) = pair(&mut h, SizeDependence::None);
+    let (first, draws, second) = pair(&mut h, OnResize::Scale);
     let settled = draws.get();
     assert_corners!(h, second, (100, 0), (400, 200));
 
@@ -77,7 +73,7 @@ fn a_leaf_that_ignores_its_box_is_not_drawn_again_when_the_box_changes() {
 #[test]
 fn a_leaf_that_depends_on_its_box_is_drawn_again_when_the_box_changes() {
     let mut h = Harness::new((400, 200));
-    let (first, draws, second) = pair(&mut h, SizeDependence::External);
+    let (first, draws, second) = pair(&mut h, OnResize::Redraw);
     let settled = draws.get();
 
     h.rsc[first].size = Size::from((150, 200));
@@ -93,8 +89,8 @@ fn a_leaf_that_depends_on_its_box_is_drawn_again_when_the_box_changes() {
 #[test]
 fn a_span_child_that_declares_its_length_is_drawn_once() {
     let mut h = Harness::new((400, 200));
-    let (told, told_draws) = counted(&mut h, Size::from((100, 200)), SizeDependence::Internal);
-    let (asked, asked_draws) = counted(&mut h, Size::from((100, 200)), SizeDependence::Internal);
+    let (told, told_draws) = counted(&mut h, Size::from((100, 200)), OnResize::Translate);
+    let (asked, asked_draws) = counted(&mut h, Size::from((100, 200)), OnResize::Translate);
     // The span takes one child's length from its hint and has to draw the
     // other to find out, so only the second is drawn before it is placed.
     let hinted = told.width(100).add(&mut h.rsc);
@@ -111,7 +107,7 @@ fn a_span_child_that_declares_its_length_is_drawn_once() {
 #[test]
 fn a_span_relays_out_when_a_child_it_measured_changes() {
     let mut h = Harness::new((400, 200));
-    let (first, _, second) = pair(&mut h, SizeDependence::Internal);
+    let (first, _, second) = pair(&mut h, OnResize::Translate);
 
     h.rsc[first].size = Size::from((250, 200));
     h.frame();
@@ -135,3 +131,122 @@ fn a_placed_child_survives_the_next_frame() {
     assert_corners!(h, top, (0, 0), (400, 80));
     assert_corners!(h, bottom, (0, 80), (400, 200));
 }
+
+/// Lays its child out from the hint alone, never reading what it drew.
+struct FromHint {
+    inner: StrongWidget,
+}
+
+impl Widget for FromHint {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        let len = painter.size_hint(&self.inner, Axis::Y).unwrap();
+        let mut region = UiRegion::FULL;
+        region.y.end = region.y.start.offset(len.abs);
+        painter.widget_within(&self.inner, region);
+        Size::REST
+    }
+}
+
+#[test]
+fn a_parent_that_only_read_a_hint_relays_out_when_the_hint_changes() {
+    let mut h = Harness::new((400, 200));
+    let inner = rect(Color::RED).height(80).add(&mut h.rsc);
+    let parent = FromHint {
+        inner: inner.add_strong(&mut h.rsc),
+    }
+    .add(&mut h.rsc);
+    h.set_root(parent);
+    assert_corners!(h, inner, (0, 0), (400, 80));
+
+    h.rsc[inner].y = Some(Len::abs(120));
+    h.frame();
+
+    assert_corners!(h, inner, (0, 0), (400, 120));
+}
+
+/// Reads the output's size, which nothing but its own draw can put right.
+struct ReadsOutput {
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for ReadsOutput {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        Size::abs(painter.output_size() / 4.0)
+    }
+}
+
+#[test]
+fn a_resize_does_not_redraw_what_the_shader_can_move() {
+    let mut h = Harness::new((400, 200));
+    let (leaf, draws) = counted(&mut h, Size::REST, OnResize::Redraw);
+    h.set_root(leaf);
+    let settled = draws.get();
+
+    h.resize((800, 100));
+    assert!(h.needs_redraw());
+    h.frame();
+
+    assert_eq!(
+        draws.get(),
+        settled,
+        "its box is the same fraction of a different output"
+    );
+    assert_corners!(h, leaf, (0, 0), (800, 100));
+}
+
+#[test]
+fn a_resize_redraws_what_read_the_output() {
+    let mut h = Harness::new((400, 200));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = ReadsOutput {
+        draws: draws.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root(leaf);
+    let settled = draws.get();
+
+    h.resize((800, 100));
+    h.frame();
+
+    assert_eq!(draws.get(), settled + 1);
+}
+
+#[test]
+fn narrowing_the_output_reflows_text_and_relays_out_around_it() {
+    let mut h = Harness::new((600, 400));
+    let para = wtext(
+        "Wrapping shapes one source into as many lines as its container leaves \
+        room for, so the height of a paragraph is an answer rather than a setting.",
+    )
+    .size(20)
+    .wrap(true)
+    .add(&mut h.rsc);
+    let below = rect(Color::RED).add(&mut h.rsc);
+    h.set_root((para, below).span(Dir::DOWN));
+    let top = h.region(&below).expect("drew nothing").top_left.y;
+
+    h.resize((300, 400));
+    h.frame();
+
+    let lower = h.region(&below).expect("drew nothing").top_left.y;
+    assert!(lower > top, "same words, half the width: {top} -> {lower}");
+}
+
+#[test]
+fn a_change_two_levels_under_its_reader_still_reaches_it() {
+    let mut h = Harness::new((400, 400));
+    // Every wrapper up to the outer pad read the size below it, so the outer
+    // pad is what draws again -- and the span it hands the box to is the same
+    // size as before, which is what lets a draw reuse its way past the leaf.
+    let (leaf, _) = counted(&mut h, Size::abs((100, 100).into()), OnResize::Redraw);
+    let padded = leaf.pad(10).add(&mut h.rsc);
+    let below = rect(Color::RED).add(&mut h.rsc);
+    h.set_root((padded, below).span(Dir::DOWN).pad(12));
+    assert_corners!(h, below, (12, 132), (388, 388));
+
+    h.rsc[leaf].size = Size::abs((100, 200).into());
+    h.frame();
+
+    assert_corners!(h, below, (12, 232), (388, 388));
+}