Make Iris layout dependencies explicit

core/src/ui/active.rs

@@ -11,12 +11,16 @@ pub struct ActiveData {
     pub textures: Vec<TextureHandle>,
     pub primitives: Vec<PrimitiveHandle>,
     pub children: Vec<WidgetId>,
+    /// Direct children whose reported size this widget used during its
+    /// latest draw. Dirtiness propagates across these edges before layout
+    /// starts, so the resulting draw still travels only parent to child.
+    pub size_dependencies: Vec<WidgetId>,
     /// The inherited mask, not `own_mask`.
     pub mask: MaskIdx,
     /// The widget's retained mask slot, or `MaskIdx::NONE`.
     pub own_mask: MaskIdx,
     pub layer: LayerId,
-    /// The last `Widget::draw` result.
+    /// The size recorded by the last `Widget::draw` through its painter.
     pub size: Size,
     /// Retained so descendants' parent links stay valid across redraws.
     pub move_slot: MoveIdx,

core/src/ui/mod.rs

@@ -9,7 +9,7 @@ mod render_state;
 
 pub use access::*;
 pub use active::*;
-pub use painter::Painter;
+pub use painter::{DrawResult, Painter};
 pub use render_state::*;
 
 #[derive(Default)]

core/src/ui/painter.rs

@@ -25,12 +25,46 @@ pub struct Painter<'a> {
     /// Previous handles, consumed in draw order and freed if left over.
     pub(super) recycle: std::iter::Peekable<std::vec::IntoIter<PrimitiveHandle>>,
     pub(super) children: Vec<WidgetId>,
-    pub(super) reuse_child_sizes: bool,
+    pub(super) size_dependencies: Vec<WidgetId>,
+    pub(super) size: Option<Size>,
+    /// Whether a retained child's length on each axis is still valid. A
+    /// child's length may change when the parent's orthogonal extent changes
+    /// (most importantly, wrapped text gets taller when it gets narrower),
+    /// but not merely because a content-sized parent grew along that same
+    /// axis around one of its siblings.
+    pub(super) reuse_child_sizes: [bool; 2],
     pub layer: usize,
     pub(super) id: WidgetId,
 }
 
+/// A child draw whose size has not necessarily been observed by its parent.
+/// Holding this value keeps the painter borrowed, so `.size()` can only name
+/// the child from the immediately preceding draw.
+pub struct DrawResult<'p, 'a> {
+    painter: &'p mut Painter<'a>,
+    child: WidgetId,
+}
+
+impl DrawResult<'_, '_> {
+    /// Return the child's reported size and record the layout dependency.
+    pub fn size(self) -> Size {
+        if !self.painter.size_dependencies.contains(&self.child) {
+            self.painter.size_dependencies.push(self.child);
+        }
+        self.painter.state.active[&self.child].size
+    }
+}
+
 impl<'a> Painter<'a> {
+    /// Record the size this widget used. Every `Widget::draw` calls this
+    /// exactly once; parents observe it through [`DrawResult::size`].
+    pub fn set_size(&mut self, size: Size) {
+        assert!(
+            self.size.replace(size).is_none(),
+            "a widget set its size more than once during one draw"
+        );
+    }
+
     fn primitive_at<P: Primitive>(&mut self, primitive: P, region: UiRegion) {
         self.write_primitive(primitive, region, Drawn::Yes);
     }
@@ -207,15 +241,19 @@ impl<'a> Painter<'a> {
         self.mask = self.own_mask;
     }
 
-    /// Draws a widget within this widget's region, returning the size it
-    /// reported using.
-    pub fn widget<W: ?Sized>(&mut self, id: &StrongWidget<W>) -> Size {
+    /// Draw a widget within this widget's region. Reading the result's size
+    /// records that this widget's layout depends on the child.
+    pub fn widget<'p, W: ?Sized>(&'p mut self, id: &StrongWidget<W>) -> DrawResult<'p, 'a> {
         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>, region: UiRegion) -> Size {
+    pub fn widget_within<'p, W: ?Sized>(
+        &'p mut self,
+        id: &StrongWidget<W>,
+        region: UiRegion,
+    ) -> DrawResult<'p, 'a> {
         self.widget_at(id, region.within(&self.region))
     }
 
@@ -260,17 +298,29 @@ impl<'a> Painter<'a> {
         }
     }
 
-    pub fn known_len<W: ?Sized>(&self, id: &StrongWidget<W>, axis: Axis) -> Option<Len> {
-        if let Some(len) = self.rsc.widgets().get_dyn(id.id())?.size_hint(axis) {
-            return Some(len.fold_dp(self.density()));
+    pub fn known_len<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<Len> {
+        let len = if let Some(len) = self.rsc.widgets().get_dyn(id.id())?.size_hint(axis) {
+            Some(len.fold_dp(self.density()))
+        } else if !self.reuse_child_sizes[match axis {
+            Axis::X => 0,
+            Axis::Y => 1,
+        }] || self.rsc.widgets().needs_redraw.contains(&id.id())
+        {
+            None
+        } else {
+            self.state.active.get(&id.id()).map(|a| a.size.axis(axis))
+        };
+        if len.is_some() && !self.size_dependencies.contains(&id.id()) {
+            self.size_dependencies.push(id.id());
         }
-        if !self.reuse_child_sizes || self.rsc.widgets().needs_redraw.contains(&id.id()) {
-            return None;
-        }
-        self.state.active.get(&id.id()).map(|a| a.size.axis(axis))
+        len
     }
 
-    fn widget_at<W: ?Sized>(&mut self, id: &StrongWidget<W>, region: UiRegion) -> Size {
+    fn widget_at<'p, W: ?Sized>(
+        &'p mut self,
+        id: &StrongWidget<W>,
+        region: UiRegion,
+    ) -> DrawResult<'p, 'a> {
         self.children.push(id.id());
         // Passed directly rather than looked up from `self.active`: this
         // widget's own `ActiveData` (which would carry its `move_slot`) is
@@ -289,19 +339,26 @@ impl<'a> Painter<'a> {
             self.mask,
             Retained::default(),
             self.rsc,
-        )
+        );
+        DrawResult {
+            painter: self,
+            child: id.id(),
+        }
     }
 
     /// Place an already-drawn child's used area, redrawing only if its size changes.
-    pub fn place<W: ?Sized>(&mut self, id: &StrongWidget<W>, region: UiRegion) -> Size {
+    pub fn place<'p, W: ?Sized>(
+        &'p mut self,
+        id: &StrongWidget<W>,
+        region: UiRegion,
+    ) -> DrawResult<'p, 'a> {
         let region = region.within(&self.region);
         let retained = self
             .state
             .active
             .get(&id.id())
             .map(|active| (active.layer, active.mask));
-        if let Some(size) = self.state.place(id.id(), region, self.rsc) {
-            size
+        if self.state.place(id.id(), region, self.rsc).is_some() {
         } else if let Some((layer, mask)) = retained {
             self.children.push(id.id());
             self.rsc.widgets_mut().needs_redraw.insert(id.id());
@@ -315,9 +372,24 @@ impl<'a> Painter<'a> {
                 mask,
                 Retained::default(),
                 self.rsc,
-            )
+            );
         } else {
-            self.widget_at(id, region)
+            self.children.push(id.id());
+            let parent_move_slot = self.child_move_slot.unwrap_or(self.move_slot);
+            self.state.draw_inner(
+                self.layer,
+                id.id(),
+                region,
+                Some(self.id),
+                parent_move_slot.idx() as u32,
+                self.mask,
+                Retained::default(),
+                self.rsc,
+            );
+        }
+        DrawResult {
+            painter: self,
+            child: id.id(),
         }
     }
 
@@ -326,7 +398,7 @@ impl<'a> Painter<'a> {
         id: &StrongWidget<W>,
         used: Size,
         within: UiRegion,
-    ) -> Size {
+    ) -> DrawResult<'_, 'a> {
         let region = self.fit_region(used, within);
         self.place(id, region)
     }

core/src/ui/render_state.rs

@@ -535,8 +535,16 @@ impl UiRenderState {
         let move_slot = Self::move_slot_for(old_move_slot, parent_move_slot, rsc);
 
         let inherited_mask = mask;
-        let reuse_child_sizes =
-            old_region.is_some_and(|old| Self::same_size(old, region, self.output_size));
+        // `Painter::layer` is a cursor widgets advance while assigning
+        // layers to their children. Retain the layer this widget itself was
+        // entered on, not wherever that cursor finishes after `draw`.
+        let inherited_layer = layer;
+        let reuse_child_sizes = old_region.map_or([false; 2], |old| {
+            [
+                Self::same_axis_size(old, region, Axis::Y, self.output_size),
+                Self::same_axis_size(old, region, Axis::X, self.output_size),
+            ]
+        });
         let mut painter = Painter {
             state: self,
             region,
@@ -550,6 +558,8 @@ impl UiRenderState {
             primitives: Vec::new(),
             recycle: recycle.into_iter().peekable(),
             children: Vec::new(),
+            size_dependencies: Vec::new(),
+            size: None,
             reuse_child_sizes,
             rsc,
         };
@@ -561,7 +571,13 @@ impl UiRenderState {
             widget.size_hint(Axis::Y).map(|len| len.fold_dp(density)),
         ];
         painter.state.draw_count += 1;
-        let size = widget.draw(&mut painter);
+        widget.draw(&mut painter);
+        let size = painter.size.unwrap_or_else(|| {
+            panic!(
+                "widget '{}' ({id:?}) did not set its size during draw",
+                painter.rsc.widgets().label(id)
+            )
+        });
         debug_assert!(
             size.x.dp == 0.0 && size.y.dp == 0.0,
             "widget {id:?} reported an unresolved `dp` size ({size:?}); \
@@ -593,8 +609,10 @@ impl UiRenderState {
             primitives,
             recycle,
             children,
+            size_dependencies,
+            size: _,
             reuse_child_sizes: _,
-            layer,
+            layer: _,
             id,
         } = painter;
 
@@ -615,8 +633,9 @@ impl UiRenderState {
             textures,
             primitives,
             children,
+            size_dependencies,
             mask: inherited_mask,
-            layer,
+            layer: inherited_layer,
             size,
             move_slot,
             child_move_slot,
@@ -690,9 +709,13 @@ impl UiRenderState {
     }
 
     fn same_size(a: UiRegion, b: UiRegion, output: Vec2) -> bool {
-        let a = a.size().to_abs(output);
-        let b = b.size().to_abs(output);
-        (a.x - b.x).abs() < 0.01 && (a.y - b.y).abs() < 0.01
+        Self::same_axis_size(a, b, Axis::X, output) && Self::same_axis_size(a, b, Axis::Y, output)
+    }
+
+    fn same_axis_size(mut a: UiRegion, mut b: UiRegion, axis: Axis, output: Vec2) -> bool {
+        let a = a.axis(axis).len().to_abs(output.axis(axis));
+        let b = b.axis(axis).len().to_abs(output.axis(axis));
+        (a - b).abs() < 0.01
     }
 
     pub(super) fn place(
@@ -920,12 +943,76 @@ impl UiRenderState {
     }
 
     pub fn redraw_updates(&mut self, rsc: &mut dyn UiRsc) {
-        while let Some(&id) = rsc.widgets().needs_redraw.iter().next() {
-            self.redraw(id, rsc);
+        while rsc.widgets().has_updates() {
+            // Expand size dependencies before drawing anything. The parent
+            // links are the retained widget tree already used by hit testing
+            // and removal; only the direct-child dependency list is new.
+            let pending: Vec<_> = rsc.widgets().needs_redraw.iter().copied().collect();
+            for mut child in pending {
+                for _ in 0..PARENT_CHAIN_LIMIT {
+                    // An exact hint is the child's current answer without a
+                    // draw. If both axes still match the retained size, no
+                    // parent can observe a size change from this mutation.
+                    if self.size_matches_hints(child, rsc) {
+                        break;
+                    }
+                    let Some(parent) = self.active.get(&child).and_then(|active| active.parent)
+                    else {
+                        break;
+                    };
+                    let depends = self
+                        .active
+                        .get(&parent)
+                        .is_some_and(|active| active.size_dependencies.contains(&child));
+                    if !depends {
+                        break;
+                    }
+                    rsc.widgets_mut().needs_redraw.insert(parent);
+                    child = parent;
+                }
+            }
+
+            // A dirty ancestor draws its dirty descendants on the way down;
+            // starting those descendants separately would duplicate work.
+            let dirty: Vec<_> = rsc.widgets().needs_redraw.iter().copied().collect();
+            let mut roots = Vec::new();
+            for id in dirty {
+                let mut ancestor = self.active.get(&id).and_then(|active| active.parent);
+                let mut covered = false;
+                for _ in 0..PARENT_CHAIN_LIMIT {
+                    let Some(parent) = ancestor else { break };
+                    if rsc.widgets().needs_redraw.contains(&parent) {
+                        covered = true;
+                        break;
+                    }
+                    ancestor = self.active.get(&parent).and_then(|active| active.parent);
+                }
+                if !covered {
+                    roots.push(id);
+                }
+            }
+            for id in roots {
+                self.redraw(id, rsc);
+            }
         }
         rsc.free();
     }
 
+    fn size_matches_hints(&self, id: WidgetId, 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;
+        };
+        [Axis::X, Axis::Y].into_iter().all(|axis| {
+            widget
+                .size_hint(axis)
+                .map(|hint| hint.fold_dp(self.density))
+                == Some(active.size.axis(axis))
+        })
+    }
+
     pub fn root_changed<'a>(&self, root: impl Into<Option<&'a StrongWidget>>) -> bool {
         root.into().map(|r| r.id()) != self.old_root
     }
@@ -1207,61 +1294,18 @@ impl UiRenderState {
 
     /// redraws a widget that's currently active (drawn)
     pub fn redraw(&mut self, id: WidgetId, rsc: &mut dyn UiRsc) {
-        self.redraw_and_settle(id, rsc);
-    }
-
-    /// Measure a changed branch toward the root, then revisit each widget
-    /// whose reported size changed on the way back down. The upward pass gives
-    /// every parent the new child size; the downward pass is what lets those
-    /// children draw inside the final boxes their parents chose. Without it a
-    /// newly grown subtree can retain the provisional (even inverted) region
-    /// it was measured in until an unrelated later update redraws it.
-    fn redraw_and_settle(&mut self, id: WidgetId, rsc: &mut dyn UiRsc) {
-        let Some((parent, changed)) = self.redraw_once(id, rsc) else {
-            return;
-        };
-        if changed {
-            if let Some(pid) = parent {
-                self.redraw_and_settle(pid, rsc);
-            }
-            // The parent pass above has now placed this widget in its final
-            // region. Draw it once more there; unchanged descendants still
-            // take draw_inner's retained fast path. This is deliberately one
-            // redraw rather than another settling pass: feeding its size
-            // back into the same upward walk can alternate between the
-            // provisional and final regions forever (a text edit first did
-            // that when an Android IME committed a space), overflowing the
-            // native thread's stack before Rust can report a panic.
-            let settled_size = self.active.get(&id).map(|active| active.size);
-            let _ = self.redraw_once(id, rsc);
-            debug_assert_eq!(
-                self.active.get(&id).map(|active| active.size),
-                settled_size,
-                "a widget changed size after its parent settled its final region"
-            );
-        }
-    }
-
-    /// Redraw `id` exactly once, returning its parent and whether the size it
-    /// reports changed. [`Self::redraw_and_settle`] owns any propagation; in
-    /// particular, its final downward redraw must not start another upward
-    /// pass through the same branch.
-    fn redraw_once(
-        &mut self,
-        id: WidgetId,
-        rsc: &mut dyn UiRsc,
-    ) -> Option<(Option<WidgetId>, bool)> {
         rsc.widgets_mut().needs_redraw.remove(&id);
         // An ancestor is drawing this widget right now, and that draw is
         // about to write fresh primitives for it. Drawing it a second time
         // here would leave one of the two copies on screen with nothing
         // owning it -- see `draw_started`'s own doc.
         if self.draw_started.contains(&id) {
-            return None;
+            return;
         }
 
-        let active = self.remove(id, false, true, rsc)?;
-        let old_size = active.size;
+        let Some(active) = self.remove(id, false, true, rsc) else {
+            return;
+        };
         let parent = active.parent;
         // `old_move_slot` being `Some` below means the slot is reused in
         // place rather than freshly parented, so this is only reached for
@@ -1285,13 +1329,6 @@ impl UiRenderState {
             },
             rsc,
         );
-        // If this widget's own reported size changed, its parent's layout
-        // (which placed it using the old size) is now stale and needs to
-        // relay out too. Checked after the real draw, not before it --
-        // there is no query left that answers "what size would this be"
-        // without actually drawing (LAYOUT.md section 5).
-        let changed = self.active.get(&id).map(|a| a.size) != Some(old_size);
-        Some((parent, changed))
     }
 }
 

core/src/widget/mod.rs

@@ -16,7 +16,7 @@ pub use view::*;
 pub use widgets::*;
 
 pub trait Widget: Any {
-    fn draw(&mut self, painter: &mut Painter) -> Size;
+    fn draw(&mut self, painter: &mut Painter);
 
     /// An exact, context-free length known without drawing or inspecting children.
     fn size_hint(&self, _axis: Axis) -> Option<Len> {
@@ -45,8 +45,8 @@ pub trait Widget: Any {
 }
 
 impl Widget for () {
-    fn draw(&mut self, _: &mut Painter) -> Size {
-        Size::ZERO
+    fn draw(&mut self, painter: &mut Painter) {
+        painter.set_size(Size::ZERO);
     }
 
     fn is_size_independent(&self) -> bool {

src/layout_tests.rs

@@ -5,6 +5,7 @@
 //! CPU-side layout/move machinery LAYOUT.md is about.
 
 use crate::prelude::*;
+use std::{cell::Cell, cell::RefCell, rc::Rc};
 
 /// The minimal `UiRsc` a test needs: just the shared `UiData`, none of the
 /// event/window/state plumbing `DefaultRsc` carries. `pub(crate)` so
@@ -26,14 +27,14 @@ impl UiRsc for TestRsc {
 struct FixedRect(f32);
 
 impl Widget for FixedRect {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         let size = Size::from_axis(Axis::Y, Len::abs(self.0), Len::REST);
         painter.primitive_within(
             RectPrimitive::color(UiColor::WHITE),
             size.to_uivec2(painter.density())
                 .align(RegionAlign::TOP_LEFT),
         );
-        size
+        painter.set_size(size);
     }
 }
 
@@ -43,12 +44,214 @@ struct ChildOffset {
 }
 
 impl Widget for ChildOffset {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         painter.set_child_offset(self.offset);
-        painter.widget(&self.child)
+        let size = painter.widget(&self.child).size();
+        painter.set_size(size);
     }
 }
 
+struct TracedLeaf {
+    height: f32,
+    trace: Rc<RefCell<Vec<&'static str>>>,
+}
+
+impl Widget for TracedLeaf {
+    fn draw(&mut self, painter: &mut Painter) {
+        self.trace.borrow_mut().push("child");
+        painter.set_size(Size::from_axis(Axis::Y, Len::abs(self.height), Len::REST));
+    }
+}
+
+struct TracedParent {
+    child: StrongWidget,
+    reads_child_size: bool,
+    trace: Rc<RefCell<Vec<&'static str>>>,
+}
+
+struct CountedLeaf {
+    height: f32,
+    draws: Rc<Cell<u32>>,
+}
+
+impl Widget for CountedLeaf {
+    fn draw(&mut self, painter: &mut Painter) {
+        self.draws.set(self.draws.get() + 1);
+        painter.set_size(Size::from_axis(Axis::Y, Len::abs(self.height), Len::REST));
+    }
+}
+
+impl Widget for TracedParent {
+    fn draw(&mut self, painter: &mut Painter) {
+        self.trace.borrow_mut().push("parent");
+        let child = painter.widget(&self.child);
+        let size = if self.reads_child_size {
+            child.size()
+        } else {
+            Size::REST
+        };
+        painter.set_size(size);
+    }
+}
+
+/// Minimal reproduction for a container's child-layer cursor being retained
+/// as though it were the layer on which the container itself was entered.
+///
+/// `Stack` is drawn by `Sized` on layer 0. It advances its painter to layers
+/// 1 and 2 for its two children. The retained `ActiveData` must still say the
+/// stack itself is on layer 0; otherwise an ordinary redraw of `Sized` asks
+/// for the stack on 0 again and turns the invented 2 -> 0 change into a full
+/// redraw of the stack and both children.
+#[test]
+fn a_widget_retains_its_entry_layer_not_its_child_cursor() {
+    let mut rsc = TestRsc {
+        ui: UiData::default(),
+    };
+    let back = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
+    let front = rsc.ui.widgets.add_strong(Rect::new(UiColor::RED));
+    let stack = rsc.ui.widgets.add_strong(Stack {
+        children: vec![back.any(), front.any()],
+        size: StackSize::Default,
+    });
+    let stack_id = stack.id();
+    let outer = rsc.ui.widgets.add_strong(Sized {
+        inner: stack.any(),
+        x: None,
+        y: None,
+    });
+    let outer_weak = outer.weak();
+    let root = outer.any();
+    let mut render = UiRenderState::new();
+    render.resize((100.0, 100.0));
+    render.update(&root, &mut rsc);
+
+    assert_eq!(
+        render.active[&stack_id].layer, 0,
+        "Stack entered on layer 0; layers 1 and 2 belong only to its children"
+    );
+
+    render.take_counters();
+    rsc.ui.widgets.get_mut(&outer_weak).unwrap().x = None;
+    render.update(&root, &mut rsc);
+    assert_eq!(
+        render.take_counters().0,
+        1,
+        "redrawing the parent should retain the unchanged Stack subtree"
+    );
+}
+
+#[test]
+fn a_size_dependent_parent_is_invalidated_before_layout_runs_downward() {
+    let mut rsc = TestRsc {
+        ui: UiData::default(),
+    };
+    let trace = Rc::new(RefCell::new(Vec::new()));
+    let child = rsc.ui.widgets.add_strong(TracedLeaf {
+        height: 20.0,
+        trace: trace.clone(),
+    });
+    let child_weak = child.weak();
+    let parent = rsc.ui.widgets.add_strong(TracedParent {
+        child: child.any(),
+        reads_child_size: true,
+        trace: trace.clone(),
+    });
+    let parent_weak = parent.weak();
+    let root = parent.any();
+    let mut render = UiRenderState::new();
+    render.resize((100.0, 100.0));
+    render.update(&root, &mut rsc);
+    trace.borrow_mut().clear();
+
+    rsc.ui.widgets.get_mut(&child_weak).unwrap().height = 40.0;
+    render.update(&root, &mut rsc);
+
+    assert_eq!(&*trace.borrow(), &["parent", "child"]);
+    assert_eq!(render.active[&parent_weak.id()].size.y, Len::abs(40.0));
+}
+
+#[test]
+fn a_parent_that_ignores_child_size_is_not_invalidated_with_it() {
+    let mut rsc = TestRsc {
+        ui: UiData::default(),
+    };
+    let trace = Rc::new(RefCell::new(Vec::new()));
+    let child = rsc.ui.widgets.add_strong(TracedLeaf {
+        height: 20.0,
+        trace: trace.clone(),
+    });
+    let child_weak = child.weak();
+    let parent = rsc.ui.widgets.add_strong(TracedParent {
+        child: child.any(),
+        reads_child_size: false,
+        trace: trace.clone(),
+    });
+    let root = parent.any();
+    let mut render = UiRenderState::new();
+    render.resize((100.0, 100.0));
+    render.update(&root, &mut rsc);
+    trace.borrow_mut().clear();
+
+    rsc.ui.widgets.get_mut(&child_weak).unwrap().height = 40.0;
+    render.update(&root, &mut rsc);
+
+    assert_eq!(&*trace.borrow(), &["child"]);
+}
+
+/// A content-sized vertical container grows vertically when one child grows,
+/// but that does not invalidate another child's retained height. Its width is
+/// the context that could change that height (for example through wrapping),
+/// and that stayed fixed.
+#[test]
+fn a_span_reuses_unchanged_sibling_sizes_when_only_its_along_extent_changes() {
+    let mut rsc = TestRsc {
+        ui: UiData::default(),
+    };
+    let changed_draws = Rc::new(Cell::new(0));
+    let sibling_draws = Rc::new(Cell::new(0));
+    let changed = rsc.ui.widgets.add_strong(CountedLeaf {
+        height: 20.0,
+        draws: changed_draws.clone(),
+    });
+    let changed_weak = changed.weak();
+    let sibling = rsc.ui.widgets.add_strong(CountedLeaf {
+        height: 20.0,
+        draws: sibling_draws.clone(),
+    });
+    let span = rsc.ui.widgets.add_strong(Span {
+        children: vec![changed.any(), sibling.any()],
+        dir: Dir::DOWN,
+        gap: Len::ZERO,
+    });
+    // `Sized` settles its child from the full offered box into the content
+    // height, reproducing the retained-region change a nested content-sized
+    // row sees when one of its children grows.
+    let root = rsc
+        .ui
+        .widgets
+        .add_strong(Sized {
+            inner: span.any(),
+            x: None,
+            y: None,
+        })
+        .any();
+    let mut render = UiRenderState::new();
+    render.resize((100.0, 100.0));
+    render.update(&root, &mut rsc);
+    changed_draws.set(0);
+    sibling_draws.set(0);
+
+    rsc.ui.widgets.get_mut(&changed_weak).unwrap().height = 40.0;
+    render.update(&root, &mut rsc);
+
+    assert!(changed_draws.get() > 0, "the changed child was not redrawn");
+    assert_eq!(
+        sibling_draws.get(),
+        0,
+        "a vertical size change redrew a sibling whose width stayed fixed"
+    );
+}
+
 #[test]
 fn a_child_coordinate_offset_moves_only_the_child_subtree() {
     let mut rsc = TestRsc {
@@ -721,7 +924,7 @@ struct MoveThenPlace {
 }
 
 impl Widget for MoveThenPlace {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         let offer = UiRegion::new(
             UiSpan::FULL,
             UiSpan::new(
@@ -738,7 +941,7 @@ impl Widget for MoveThenPlace {
             ),
         );
         painter.place(&self.inner, place);
-        Size::default()
+        painter.set_size(Size::default());
     }
 }
 

src/sense.rs

@@ -21,7 +21,7 @@ pub enum CursorSense {
     HoverStart,
     Hovering,
     HoverEnd,
-    Scroll,
+    Scroll(Axis),
     /// Delivered exactly once, in place of `PressEnd`, to whichever widget
     /// currently holds pointer capture (`UiRenderState::capture_pointer`)
     /// when the button lifts -- see `iris::sense`'s pointer-capture doc
@@ -47,7 +47,10 @@ pub enum CursorSense {
 }
 
 #[derive(Clone)]
-pub struct CursorSenses(Vec<CursorSense>);
+pub struct CursorSenses {
+    senses: Vec<CursorSense>,
+    drag_axis: Option<Axis>,
+}
 
 impl Event for CursorSenses {
     type Data<'a> = CursorData<'a>;
@@ -74,7 +77,13 @@ impl Event for CursorSenses {
         if data.sense == CursorSense::Drop || data.sense == CursorSense::Cancel {
             return self.contains(&data.sense).then(|| data.clone());
         }
-        if let Some(sense) = should_run(self, &data.cursor, data.hover) {
+        if let Some(sense) = should_run(
+            self,
+            &data.cursor,
+            data.hover,
+            data.drag_axis,
+            data.captured,
+        ) {
             let mut data = data.clone();
             data.sense = sense;
             Some(data)
@@ -84,6 +93,30 @@ impl Event for CursorSenses {
     }
 }
 
+impl CursorSenses {
+    fn consumes(&self, data: &CursorData<'_>, momentary_active: bool) -> bool {
+        if !momentary_active {
+            return true;
+        }
+        let Some(sense) = should_run(
+            self,
+            &data.cursor,
+            data.hover,
+            data.drag_axis,
+            data.captured,
+        ) else {
+            return false;
+        };
+        match (self.drag_axis, sense) {
+            // Directional drags are candidates until movement chooses an
+            // axis. Only the matching one consumes its visual layer.
+            (Some(axis), CursorSense::Pressing(_)) => data.captured || data.drag_axis == Some(axis),
+            (Some(_), CursorSense::PressStart(_) | CursorSense::PressEnd(_)) => false,
+            _ => sense.is_momentary(),
+        }
+    }
+}
+
 impl CursorSense {
     pub fn click() -> Self {
         Self::PressStart(CursorButton::Left)
@@ -109,6 +142,16 @@ impl CursorSense {
     pub fn drag_senses() -> CursorSenses {
         Self::click_or_drag() | Self::unclick() | Self::Drop | Self::Cancel
     }
+
+    /// The frames of a drag along one axis. Before the gesture crosses
+    /// [`DRAG_SLOP`], directional listeners observe without consuming a
+    /// visual layer; once its direction is known, only listeners for that
+    /// axis receive and consume it.
+    pub fn drag(axis: Axis) -> CursorSenses {
+        let mut senses = Self::drag_senses();
+        senses.drag_axis = Some(axis);
+        senses
+    }
     pub fn is_dragging(&self) -> bool {
         matches!(self, CursorSense::Pressing(CursorButton::Left))
     }
@@ -261,6 +304,11 @@ pub struct CursorData<'a> {
     pub scroll_delta: Vec2,
     pub hover: ActivationState,
     pub cursor: CursorState,
+    /// The direction selected after this press crossed [`DRAG_SLOP`].
+    /// `None` while the gesture is still only a press.
+    pub drag_axis: Option<Axis>,
+    /// Whether this sample bypassed hit testing for the pointer holder.
+    pub captured: bool,
     /// the first sense that triggered this
     pub sense: CursorSense,
     pub render: &'a UiRenderState,
@@ -294,6 +342,8 @@ pub struct CursorData<'a> {
 pub struct PointerInput {
     captured: Option<WidgetId>,
     pressed: Vec<WidgetId>,
+    press_origin: Option<Vec2>,
+    drag_axis: Option<Axis>,
 }
 
 impl PointerInput {
@@ -393,6 +443,22 @@ impl SensorUi for UiRenderState {
             holder: std::cell::Cell::new(pointer.captured),
         };
         let button_down = cursor.buttons.select(&CursorButton::Left).is_on();
+        if cursor.buttons.left.is_start() {
+            pointer.press_origin = Some(cursor.pos);
+            pointer.drag_axis = None;
+        } else if button_down
+            && pointer.drag_axis.is_none()
+            && let Some(origin) = pointer.press_origin
+        {
+            let moved = cursor.pos - origin;
+            if moved.x.abs().max(moved.y.abs()) > DRAG_SLOP {
+                pointer.drag_axis = Some(if moved.x.abs() > moved.y.abs() {
+                    Axis::X
+                } else {
+                    Axis::Y
+                });
+            }
+        }
 
         // The platform took the gesture away (`CursorState::cancelled`).
         // Everybody still tracking this press hears about it -- the
@@ -410,6 +476,8 @@ impl SensorUi for UiRenderState {
                 }
             }
             requests.release();
+            pointer.press_origin = None;
+            pointer.drag_axis = None;
             for id in told {
                 deliver_cancel(self, rsc, state, id, &cursor, window_size, &requests);
             }
@@ -458,6 +526,8 @@ impl SensorUi for UiRenderState {
                 scroll_delta: cursor.scroll_delta,
                 hover: ActivationState::On,
                 cursor: cursor.clone(),
+                drag_axis: pointer.drag_axis,
+                captured: true,
                 sense,
                 render: self,
                 pointer: &requests,
@@ -466,6 +536,8 @@ impl SensorUi for UiRenderState {
             if !button_down {
                 requests.release();
                 pointer.pressed.clear();
+                pointer.press_origin = None;
+                pointer.drag_axis = None;
             }
             pointer.captured = requests.holder();
             rsc.events_mut().get_type::<CursorSense>().global = pointer;
@@ -524,20 +596,6 @@ impl SensorUi for UiRenderState {
                 // widget that registered a matching non-hover sense
                 // consumes it -- a button that only registered `click()`
                 // must not block a scroll meant for the list behind it.
-                let consumed = if momentary_active {
-                    rsc.events_mut()
-                        .get_type::<CursorSense>()
-                        .registered(*id)
-                        .any(|senses| {
-                            matches!(should_run(senses, &cursor, sensor.hover), Some(s) if s.is_momentary())
-                        })
-                } else {
-                    true
-                };
-                if consumed {
-                    sensed = true;
-                }
-
                 let cursor = cursor.clone();
 
                 let data = CursorData {
@@ -546,12 +604,22 @@ impl SensorUi for UiRenderState {
                     scroll_delta: cursor.scroll_delta,
                     hover: sensor.hover,
                     cursor,
+                    drag_axis: pointer.drag_axis,
+                    captured: false,
                     // this does not have any meaning;
                     // might wanna set up Event to have a prepare stage
                     sense: CursorSense::Hovering,
                     render: self,
                     pointer: &requests,
                 };
+                let consumes = rsc
+                    .events_mut()
+                    .get_type::<CursorSense>()
+                    .registered(*id)
+                    .any(|senses| senses.consumes(&data, momentary_active));
+                if consumes {
+                    sensed = true;
+                }
                 rsc.run_event::<CursorSense>(*id, data, state);
                 // Anything handed a frame while the button is down may
                 // have opened a gesture on it, and is owed a `Cancel` if
@@ -563,7 +631,7 @@ impl SensorUi for UiRenderState {
                     pointer.pressed.push(*id);
                 }
             }
-            if sensed {
+            if sensed || requests.holder().is_some() {
                 break;
             }
         }
@@ -597,6 +665,10 @@ impl SensorUi for UiRenderState {
         // A cancel handler may itself have captured (a widget deciding
         // the gesture is now its own); `requests` is still the truth.
         pointer.captured = requests.holder();
+        if !button_down {
+            pointer.press_origin = None;
+            pointer.drag_axis = None;
+        }
         rsc.events_mut().get_type::<CursorSense>().global = pointer;
     }
 }
@@ -625,6 +697,8 @@ fn deliver_cancel<Rsc: HasEvents>(
         scroll_delta: cursor.scroll_delta,
         hover: ActivationState::On,
         cursor: cursor.clone(),
+        drag_axis: None,
+        captured: false,
         sense: CursorSense::Cancel,
         render,
         pointer,
@@ -636,6 +710,8 @@ pub fn should_run(
     senses: &CursorSenses,
     cursor: &CursorState,
     hover: ActivationState,
+    drag_axis: Option<Axis>,
+    captured: bool,
 ) -> Option<CursorSense> {
     // Every sense below that is about the *pointer* rather than about
     // hovering needs the pointer to actually be on this widget, and
@@ -670,7 +746,12 @@ pub fn should_run(
             CursorSense::HoverStart => hover.is_start(),
             CursorSense::Hovering => hover.is_on(),
             CursorSense::HoverEnd => hover.is_end(),
-            CursorSense::Scroll => on_this && cursor.scroll_delta != Vec2::ZERO,
+            CursorSense::Scroll(axis) => {
+                on_this
+                    && cursor.scroll_delta.axis(*axis) != 0.0
+                    && cursor.scroll_delta.axis(*axis).abs()
+                        >= cursor.scroll_delta.axis(!*axis).abs()
+            }
             // Never derived here -- `Drop` only ever fires through
             // `CursorSenses::should_run`'s own special case, ahead of this
             // loop, for the one widget `run_sensors`' capture branch is
@@ -683,7 +764,11 @@ pub fn should_run(
             // note above; both are set by `run_sensors` alone, for the one
             // widget it is delivering to this frame.
             CursorSense::Drop | CursorSense::Cancel => false,
-        } {
+        } && (captured
+            || !matches!(sense, CursorSense::Pressing(_))
+            || senses.drag_axis.is_none()
+            || senses.drag_axis == drag_axis)
+        {
             return Some(*sense);
         }
     }
@@ -744,19 +829,22 @@ impl Deref for CursorSenses {
     type Target = Vec<CursorSense>;
 
     fn deref(&self) -> &Self::Target {
-        &self.0
+        &self.senses
     }
 }
 
 impl DerefMut for CursorSenses {
     fn deref_mut(&mut self) -> &mut Self::Target {
-        &mut self.0
+        &mut self.senses
     }
 }
 
 impl From<CursorSense> for CursorSenses {
     fn from(val: CursorSense) -> Self {
-        CursorSenses(vec![val])
+        CursorSenses {
+            senses: vec![val],
+            drag_axis: None,
+        }
     }
 }
 
@@ -764,7 +852,10 @@ impl BitOr for CursorSense {
     type Output = CursorSenses;
 
     fn bitor(self, rhs: Self) -> Self::Output {
-        CursorSenses(vec![self, rhs])
+        CursorSenses {
+            senses: vec![self, rhs],
+            drag_axis: None,
+        }
     }
 }
 
@@ -772,7 +863,7 @@ impl BitOr<CursorSense> for CursorSenses {
     type Output = Self;
 
     fn bitor(mut self, rhs: CursorSense) -> Self::Output {
-        self.0.push(rhs);
+        self.senses.push(rhs);
         self
     }
 }

src/sense_tests.rs

@@ -73,9 +73,13 @@ fn a_button_over_a_list_scrolls_the_list_and_still_clicks() {
     let clicked = Rc::new(Cell::new(false));
     {
         let scrolled = scrolled.clone();
-        rsc.register_event(list_weak, CursorSense::Scroll, move |_ctx, _rsc| {
-            scrolled.set(true);
-        });
+        rsc.register_event(
+            list_weak,
+            CursorSense::Scroll(Axis::Y),
+            move |_ctx, _rsc| {
+                scrolled.set(true);
+            },
+        );
     }
     {
         let clicked = clicked.clone();
@@ -461,10 +465,10 @@ fn taking_the_pointer_cancels_everyone_else_tracking_the_press() {
         events: EventManager::default(),
     };
 
-    // The bystander *contains* the capturer, which is the real shape: a
-    // transcript's `LazySpan` and one row's own text both track the same
-    // press, and a `Stack`'s siblings would be on separate layers where
-    // only the topmost is dispatched to at all.
+    // The bystander contains the capturer on a lower visual layer: the
+    // shape of a vertical transcript scroller with a higher horizontal
+    // scroller inside one row. Both observe the undecided press, then only
+    // the recognizer matching its direction may consume it.
     let capturer = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
     let capturer_weak = capturer.weak();
     let bystander = rsc.ui.widgets.add_strong(Stack {
@@ -478,7 +482,7 @@ fn taking_the_pointer_cancels_everyone_else_tracking_the_press() {
         let capturer_saw = capturer_saw.clone();
         rsc.register_event(
             capturer_weak,
-            CursorSense::drag_senses(),
+            CursorSense::drag(Axis::X),
             move |ctx, _rsc| {
                 capturer_saw.set(capturer_saw.get() + 1);
                 if matches!(ctx.data.sense, CursorSense::Pressing(_)) {
@@ -493,7 +497,7 @@ fn taking_the_pointer_cancels_everyone_else_tracking_the_press() {
         let (cancelled, ended) = (cancelled.clone(), ended.clone());
         rsc.register_event(
             bystander_weak,
-            CursorSense::drag_senses(),
+            CursorSense::drag(Axis::Y),
             move |ctx, _rsc| match ctx.data.sense {
                 CursorSense::Cancel => cancelled.set(cancelled.get() + 1),
                 CursorSense::PressEnd(_) | CursorSense::Drop => ended.set(ended.get() + 1),
@@ -515,7 +519,7 @@ fn taking_the_pointer_cancels_everyone_else_tracking_the_press() {
     render.update(&root, &mut rsc);
     assert_eq!(cancelled.get(), 0, "nothing has captured yet");
 
-    let mut moved = cursor_at((50.0, 20.0).into());
+    let mut moved = cursor_at((80.0, 50.0).into());
     moved.buttons.left = ActivationState::On;
     render.run_sensors(&mut rsc, &mut state, moved, win);
     render.update(&root, &mut rsc);
@@ -590,6 +594,9 @@ fn a_drag_pans_whichever_nested_scroll_area_owns_its_axis() {
                 record.set(Some(id));
                 id
             })
+            // The horizontal area is visually above the vertical one, as
+            // it is when a raised transcript row contains sideways content.
+            .layer_offset(1)
             .scrollable(Axis::Y, Pin::Start)
             .add_strong(&mut rsc);
         let inner = seen.get().unwrap();

src/widget/image.rs

@@ -6,10 +6,10 @@ pub struct Image {
 }
 
 impl Widget for Image {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         let size = self.handle.size();
         painter.texture_within(&self.handle, size.align(Align::TOP_LEFT));
-        Size::abs(size)
+        painter.set_size(Size::abs(size));
     }
 
     fn size_hint(&self, axis: Axis) -> Option<Len> {

src/widget/mask.rs

@@ -6,7 +6,7 @@ pub struct Masked {
 }
 
 impl Widget for Masked {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         match &self.shape {
             Some(shape) => {
                 painter.child_layer();
@@ -16,9 +16,9 @@ impl Widget for Masked {
             }
             None => painter.set_mask(painter.region()),
         }
-        let used = painter.widget(&self.inner);
+        let used = painter.widget(&self.inner).size();
         painter.place_used(&self.inner, used, UiRegion::FULL);
-        used
+        painter.set_size(used);
     }
 
     fn requires_exact_region(&self) -> bool {

src/widget/position/align.rs

@@ -6,8 +6,8 @@ pub struct Aligned {
 }
 
 impl Widget for Aligned {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
-        let used = painter.widget(&self.inner);
+    fn draw(&mut self, painter: &mut Painter) {
+        let used = painter.widget(&self.inner).size();
         let density = painter.density();
         let (x, y) = self.align.tuple();
         let region = UiRegion::new(
@@ -19,6 +19,6 @@ impl Widget for Aligned {
                 .align(y.unwrap_or(AxisAlign::Neg)),
         );
         painter.place(&self.inner, region);
-        used
+        painter.set_size(used);
     }
 }

src/widget/position/layer.rs

@@ -6,12 +6,12 @@ pub struct LayerOffset {
 }
 
 impl Widget for LayerOffset {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         for _ in 0..self.offset {
             painter.next_layer();
         }
-        let used = painter.widget(&self.inner);
+        let used = painter.widget(&self.inner).size();
         painter.place_used(&self.inner, used, UiRegion::FULL);
-        used
+        painter.set_size(used);
     }
 }

src/widget/position/lazy_span.rs

@@ -939,7 +939,7 @@ impl LazySpan {
             (Some(h), None) => {
                 let (lead, trail) = placement.edges(h);
                 let region = self.row_region(lead, trail);
-                let used = painter.widget_within(self.slot_widget(slot), region);
+                let used = painter.widget_within(self.slot_widget(slot), region).size();
                 let height = resolve(used);
                 if height != h {
                     let (new_lead, new_trail) = placement.edges(height);
@@ -957,7 +957,7 @@ impl LazySpan {
                     Placement::Trailing(_) => 0.0,
                 };
                 let first = self.row_region(measure_from, measure_from + GENEROUS_PADDING);
-                let height = resolve(painter.widget_within(self.slot_widget(slot), first));
+                let height = resolve(painter.widget_within(self.slot_widget(slot), first).size());
                 let (lead, trail) = placement.edges(height);
                 if is_anchor {
                     self.stabilize_lead(painter, measure_from, lead);
@@ -1072,7 +1072,7 @@ impl Widget for LazySpan {
         self.tick_fling(now)
     }
 
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         let axis = self.dir.axis;
         let output_len = painter.output_size().axis(axis);
         self.viewport_len = painter.region().axis(axis).len().to_abs(output_len);
@@ -1081,7 +1081,8 @@ impl Widget for LazySpan {
         self.repair_anchor();
         if self.anchor.is_none() {
             self.extents.clear();
-            return Size::REST;
+            painter.set_size(Size::REST);
+            return;
         }
 
         // What a wheel, a drag or a fling asked for since the last frame,
@@ -1154,7 +1155,7 @@ impl Widget for LazySpan {
         self.rehome_anchor();
         self.update_snap_end();
         self.ctl.set_travel(self.travel());
-        Size::REST
+        painter.set_size(Size::REST);
     }
 
     fn size_hint(&self, _axis: Axis) -> Option<Len> {

src/widget/position/max_size.rs

@@ -35,7 +35,7 @@ impl MaxSize {
 }
 
 impl Widget for MaxSize {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         let output = painter.output_size();
         let density = painter.density();
         let offered = painter.px_size();
@@ -43,12 +43,12 @@ impl Widget for MaxSize {
             x: Self::clamp_region(offered.x, self.x, output.x, density),
             y: Self::clamp_region(offered.y, self.y, output.y, density),
         };
-        let used = painter.widget_within(&self.inner, region);
+        let used = painter.widget_within(&self.inner, region).size();
         let size = Size {
             x: Self::clamp(used.x, self.x, output.x, density),
             y: Self::clamp(used.y, self.y, output.y, density),
         };
         painter.place_used(&self.inner, size, UiRegion::FULL);
-        size
+        painter.set_size(size);
     }
 }

src/widget/position/offset.rs

@@ -6,10 +6,10 @@ pub struct Offset {
 }
 
 impl Widget for Offset {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         let region = UiRegion::FULL.offset(self.amt);
-        let used = painter.widget_within(&self.inner, region);
+        let used = painter.widget_within(&self.inner, region).size();
         painter.place_used(&self.inner, used, region);
-        used
+        painter.set_size(used);
     }
 }

src/widget/position/pad.rs

@@ -8,11 +8,11 @@ pub struct Pad {
 }
 
 impl Widget for Pad {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         let density = painter.density();
         let offered = painter.px_size();
         let region = self.padding.region(density);
-        let used = painter.widget_within(&self.inner, region);
+        let used = painter.widget_within(&self.inner, region).size();
         painter.place_used(&self.inner, used, region);
         let width =
             self.padding.left.apply_rest(density).abs + self.padding.right.apply_rest(density).abs;
@@ -26,7 +26,7 @@ impl Widget for Pad {
         if needed.x <= offered.x + 0.01 && needed.y <= offered.y + 0.01 {
             self.exact_region = false;
         }
-        size
+        painter.set_size(size);
     }
 
     fn requires_exact_region(&self) -> bool {

src/widget/position/scroll_area.rs

@@ -30,7 +30,7 @@ impl Widget for ScrollArea {
         self.tick_fling(now)
     }
 
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         let axis = self.ctl.axis();
         let container_len = painter.px_size().axis(axis);
         self.container_len = container_len;
@@ -41,7 +41,9 @@ impl Widget for ScrollArea {
         self.ctl.set_amt(travelled);
 
         let hint = self.content_len.unwrap_or(container_len);
-        let used = painter.widget_within(&self.inner, self.child_region(hint));
+        let used = painter
+            .widget_within(&self.inner, self.child_region(hint))
+            .size();
 
         let measured = used
             .axis(axis)
@@ -62,7 +64,10 @@ impl Widget for ScrollArea {
             fwd: range - amt,
         });
 
-        painter.place(&self.inner, self.child_region(measured))
+        let size = painter
+            .place(&self.inner, self.child_region(measured))
+            .size();
+        painter.set_size(size);
     }
 }
 

src/widget/position/scrollable.rs

@@ -502,11 +502,11 @@ where
     W: Widget + Scrollable,
     WL: WidgetLike<Rsc, Tag, Widget = W>,
 {
-    w.on(CursorSense::Scroll, move |ctx, rsc| {
+    w.on(CursorSense::Scroll(axis), move |ctx, rsc| {
         let delta = ctx.data.scroll_delta.axis(axis) * 50.0;
         ctx.widget(rsc).scroll(delta);
     })
-    .on(CursorSense::drag_senses(), |ctx, rsc: &mut Rsc| {
+    .on(CursorSense::drag(axis), |ctx, rsc: &mut Rsc| {
         let id = ctx.widget.id();
         let (sense, pos) = (ctx.data.sense, ctx.data.cursor.pos);
         let flung = ctx

src/widget/position/sized.rs

@@ -7,7 +7,7 @@ pub struct Sized {
 }
 
 impl Widget for Sized {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         let density = painter.density();
         let mut region = UiRegion::FULL;
         if let Some(x) = self.x {
@@ -16,13 +16,13 @@ impl Widget for Sized {
         if let Some(y) = self.y {
             region.y = y.apply_rest(density).align(AxisAlign::Neg);
         }
-        let used = painter.widget_within(&self.inner, region);
+        let used = painter.widget_within(&self.inner, region).size();
         let size = Size {
             x: self.x.map(|x| x.fold_dp(density)).unwrap_or(used.x),
             y: self.y.map(|y| y.fold_dp(density)).unwrap_or(used.y),
         };
         painter.place_used(&self.inner, size, UiRegion::FULL);
-        size
+        painter.set_size(size);
     }
 
     fn size_hint(&self, axis: Axis) -> Option<Len> {

src/widget/position/span.rs

@@ -8,7 +8,7 @@ pub struct Span {
 }
 
 impl Widget for Span {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         let axis = self.dir.axis;
         let gap = self.gap.apply_rest(painter.density()).abs;
 
@@ -29,7 +29,7 @@ impl Widget for Span {
                         slot.flip();
                     }
                     let region = UiRegion::from_axis(axis, slot, UiSpan::FULL);
-                    let len = painter.widget_within(child, region).axis(axis);
+                    let len = painter.widget_within(child, region).size().axis(axis);
                     lens[i] = Some(len);
                     drawn[i] = true;
                     len
@@ -65,9 +65,9 @@ impl Widget for Span {
                 child_region.flip(axis);
             }
             let used = if drawn[i] {
-                painter.place(child, child_region)
+                painter.place(child, child_region).size()
             } else {
-                painter.widget_within(child, child_region)
+                painter.widget_within(child, child_region).size()
             };
             placed.push(child_region);
             start.abs += gap;
@@ -97,7 +97,7 @@ impl Widget for Span {
             Len::default()
         };
 
-        Size::from_axis(axis, along, ortho_len)
+        painter.set_size(Size::from_axis(axis, along, ortho_len));
     }
 }
 

src/widget/position/stack.rs

@@ -8,7 +8,7 @@ pub struct Stack {
 }
 
 impl Widget for Stack {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         let density = painter.density();
         let known = match self.size {
             StackSize::Default => Some(Size::REST),
@@ -25,15 +25,15 @@ impl Widget for Stack {
         if let Some(child) = iter.next() {
             painter.child_layer();
             used.push(match region {
-                Some(region) => painter.widget_within(child, region),
-                None => painter.widget(child),
+                Some(region) => painter.widget_within(child, region).size(),
+                None => painter.widget(child).size(),
             });
         }
         for child in iter {
             painter.next_layer();
             used.push(match region {
-                Some(region) => painter.widget_within(child, region),
-                None => painter.widget(child),
+                Some(region) => painter.widget_within(child, region).size(),
+                None => painter.widget(child).size(),
             });
         }
         let size = match self.size {
@@ -50,7 +50,7 @@ impl Widget for Stack {
                 painter.place(child, child_region);
             }
         }
-        size
+        painter.set_size(size);
     }
 
     fn size_hint(&self, _axis: Axis) -> Option<Len> {

src/widget/ptr.rs

@@ -6,14 +6,15 @@ pub struct WidgetPtr {
 }
 
 impl Widget for WidgetPtr {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
-        if let Some(id) = &self.inner {
-            let used = painter.widget(id);
+    fn draw(&mut self, painter: &mut Painter) {
+        let size = if let Some(id) = &self.inner {
+            let used = painter.widget(id).size();
             painter.place_used(id, used, UiRegion::FULL);
             used
         } else {
             Size::ZERO
-        }
+        };
+        painter.set_size(size);
     }
 
     fn is_size_independent(&self) -> bool {

src/widget/rect.rs

@@ -28,14 +28,14 @@ impl Rect {
 }
 
 impl Widget for Rect {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         painter.primitive(RectPrimitive {
             color: self.color,
             radius: self.radius.fold_dp(painter.density()).abs,
             thickness: self.thickness,
             inner_radius: self.inner_radius,
         });
-        Size::REST
+        painter.set_size(Size::REST);
     }
 
     fn size_hint(&self, _axis: Axis) -> Option<Len> {

src/widget/text/edit.rs

@@ -92,7 +92,7 @@ impl TextEdit {
 }
 
 impl Widget for TextEdit {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         let base = painter.layer;
         painter.child_layer();
         let used = self.view.draw(painter);
@@ -100,7 +100,8 @@ impl Widget for TextEdit {
         let region = self.region();
 
         let Some(selection) = self.selection else {
-            return used;
+            painter.set_size(used);
+            return;
         };
         let layout = self.view.buf.layout();
 
@@ -122,7 +123,7 @@ impl Widget for TextEdit {
             RectPrimitive::color(Color::WHITE),
             size.align(Align::TOP_LEFT).offset(top_left).within(&region),
         );
-        used
+        painter.set_size(used);
     }
 
     fn requires_exact_region(&self) -> bool {

src/widget/text/mod.rs

@@ -111,7 +111,7 @@ impl TextView {
         if self.is_blank()
             && let Some(hint) = &self.hint
         {
-            return painter.widget(hint);
+            return painter.widget(hint).size();
         }
         let region = tex.size.align(self.align);
         let within = region.within(&painter.region());
@@ -141,9 +141,10 @@ impl Text {
 }
 
 impl Widget for Text {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         self.update_buf();
-        self.view.draw(painter)
+        let size = self.view.draw(painter);
+        painter.set_size(size);
     }
 
     fn requires_exact_region(&self) -> bool {