Separate measured-answer dependencies from retained drawing validity

core/src/ui/active.rs

@@ -41,7 +41,7 @@ pub struct ActiveData {
     pub textures: Vec<TextureHandle>,
     pub primitives: Vec<RetainedPrimitive>,
     pub mask_region: Option<DrawRegion>,
-    pub(crate) extent_children: Vec<(WidgetId, ExtentPlacement)>,
+    pub inherited_children: Vec<WidgetId>,
     pub children: Vec<WidgetId>,
     /// The children whose size this widget read while drawing.
     pub size_deps: Vec<WidgetId>,
@@ -87,18 +87,3 @@ impl ActiveData {
         })
     }
 }
-
-#[derive(Clone, Copy, Debug)]
-pub(crate) enum ExtentPlacement {
-    Inherit,
-    Within(UiRegion),
-}
-
-impl ExtentPlacement {
-    pub fn resolve(self, extent: UiRegion) -> (UiRegion, [Option<crate::UiSpan>; 2]) {
-        match self {
-            Self::Inherit => (UiRegion::FULL, [Some(extent.x), Some(extent.y)]),
-            Self::Within(local) => (local.within(&extent), [None; 2]),
-        }
-    }
-}

core/src/ui/layout_holds.rs

@@ -15,6 +15,25 @@ impl LayoutHolds {
         placement: None,
     };
 
+    pub fn and(self, other: Self) -> Self {
+        debug_assert!(
+            self.placement.is_none()
+                || other.placement.is_none()
+                || self.placement == other.placement
+        );
+        Self {
+            frame: [
+                self.frame[0].and(other.frame[0]),
+                self.frame[1].and(other.frame[1]),
+            ],
+            extent: [
+                self.extent[0].and(other.extent[0]),
+                self.extent[1].and(other.extent[1]),
+            ],
+            placement: self.placement.or(other.placement),
+        }
+    }
+
     pub fn contains(self, px: PxVec2, placement: UiRegion) -> bool {
         self.placement.is_none_or(|old| old == placement)
             && [Axis::X, Axis::Y].into_iter().all(|axis| {

core/src/ui/painter.rs

@@ -1,9 +1,9 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
-    Axis, DrawRegion, ExtentPlacement, Holds, LayoutLen, Len, Px, PxVec2, RegionAlign,
+    Axis, DrawRegion, Holds, LayoutHolds, LayoutLen, Len, Px, PxVec2, RegionAlign, RenderedText,
-    RenderedText, RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData,
+    RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData, TextureHandle,
-    TextureHandle, UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
+    UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
     render::{
         GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveInst, PrimitiveKind,
         TexturePrimitive,
@@ -40,9 +40,10 @@ pub struct Painter<'a> {
     pub(super) textures: Vec<TextureHandle>,
     pub(super) primitives: Vec<RetainedPrimitive>,
     pub(super) mask_region: Option<DrawRegion>,
-    pub(super) extent_children: Vec<(WidgetId, ExtentPlacement)>,
+    pub(super) inherited_children: Vec<WidgetId>,
     pub(super) extent_own: [Holds; 2],
-    pub(super) extent_under: [Holds; 2],
+    /// Only children whose answers were read constrain this widget's answer.
+    pub(super) answer_under: LayoutHolds,
     pub(super) children: Vec<WidgetId>,
     /// The children asked about so far, so the first box each was asked in
     /// is the one recorded as its offer.
@@ -59,8 +60,8 @@ pub struct Painter<'a> {
     /// What this draw itself read of its box in pixels, per axis: every
     /// length until it reads one, then that one, unless it says otherwise.
     pub(super) own: [Holds; 2],
-    /// What the children it asked about and drew keep it to.
+    /// Dependencies of every child drawing, including unmeasured overlays.
-    pub(super) under: [Holds; 2],
+    pub(super) under: LayoutHolds,
     /// The movable region this widget's primitives are positioned through:
     /// its own when opted in, otherwise the nearest ancestor's.
     pub(super) move_idx: MoveIdx,
@@ -154,13 +155,7 @@ impl<'a> Painter<'a> {
     /// around one child wants, since its box is the child's.
     pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
         let own = self.placement;
-        self.widget_at_inner(
+        self.widget_at_inner(id, UiRegion::FULL, [Some(own.x), Some(own.y)], true, false)
-            id,
-            UiRegion::FULL,
-            [Some(own.x), Some(own.y)],
-            Some(ExtentPlacement::Inherit),
-            false,
-        )
     }
 
     /// What a widget's rules declare its lengths to be, which whoever draws
@@ -176,29 +171,19 @@ impl<'a> Painter<'a> {
     /// this frame; what it answered is still something this widget asked.
     pub fn undraw<W: ?Sized>(&mut self, id: &StrongWidget<W>) {
         self.children.retain(|child| *child != id.id());
-        self.extent_children.retain(|(child, _)| *child != id.id());
+        self.inherited_children.retain(|child| *child != id.id());
         self.state.undraw_rec(id.id(), self.rsc);
     }
 
-    /// Draws a child in a frame relative to this widget's frame or extent.
+    /// Draws a child in `region`, relative to this widget's frame. The child
-    /// A plain `UiRegion` is frame-relative. `DrawRegion::Extent` keeps the
+    /// resolves declared lengths and reports against that frame, then places
-    /// child's frame attached to the extent without reading `placement()`.
+    /// its drawing by its own alignment.
-    /// The child places its answer within that frame by its own alignment.
     pub fn widget_within<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
-        region: impl Into<DrawRegion>,
+        region: UiRegion,
     ) -> DrawResult<'s, 'a, W> {
-        match region.into() {
+        self.widget_at(id, region, [None; 2])
-            DrawRegion::Frame(region) => self.widget_at(id, region, [None; 2]),
-            DrawRegion::Extent(local) => self.widget_at_inner(
-                id,
-                local.within(&self.placement),
-                [None; 2],
-                Some(ExtentPlacement::Within(local)),
-                false,
-            ),
-        }
     }
 
     /// Draws a widget in `region`, saying where in it the drawing goes.
@@ -221,7 +206,7 @@ impl<'a> Painter<'a> {
         region: UiRegion,
         placement: [Option<UiSpan>; 2],
     ) -> DrawResult<'s, 'a, W> {
-        self.widget_at_inner(id, region, placement, None, false)
+        self.widget_at_inner(id, region, placement, false, false)
     }
 
     fn widget_at_inner<'s, W: ?Sized>(
@@ -229,12 +214,15 @@ impl<'a> Painter<'a> {
         id: &'s StrongWidget<W>,
         region: UiRegion,
         placement: [Option<UiSpan>; 2],
-        extent: Option<ExtentPlacement>,
+        inherited: bool,
         measuring: bool,
     ) -> DrawResult<'s, 'a, W> {
-        self.extent_children.retain(|(child, _)| *child != id.id());
+        if inherited {
-        if let Some(extent) = extent {
+            if !self.inherited_children.contains(&id.id()) {
-            self.extent_children.push((id.id(), extent));
+                self.inherited_children.push(id.id());
+            }
+        } else {
+            self.inherited_children.retain(|child| *child != id.id());
         }
         let region_node = self.rsc.widgets().is_region_node(id.id());
         let declared = self.declared_lens(id);
@@ -276,7 +264,7 @@ impl<'a> Painter<'a> {
         // The answer and what it holds for, both about the box asked in. The
         // child's record may say something else once its drawing has been
         // placed: a drawing made again in its placed box holds for that box.
-        let (size, holds) = self.state.draw_inner(
+        let (size, answer_holds, holds) = self.state.draw_inner(
             id.id(),
             within,
             DrawInfo {
@@ -297,53 +285,34 @@ impl<'a> Painter<'a> {
             measuring,
             self.rsc,
         );
+        let in_parent = |holds: LayoutHolds| {
+            let mut result = LayoutHolds::ANY;
             for axis in AXES {
                 let n = axis as usize;
-            match extent {
+                result.frame[n] = holds.frame[n].through(local.axis(axis).len());
-                Some(ExtentPlacement::Inherit) if declared[n].is_none() => {
+                if inherited && declared[n].is_none() {
-                    self.under[n] =
+                    result.extent[n] = holds.extent[n];
-                        self.under[n].and(holds.frame[n].through(local.axis(axis).len()));
+                    if holds.placement.is_some() {
-                    self.extent_under[n] = self.extent_under[n].and(holds.extent[n]);
+                        result.placement = Some(self.placement);
-                    self.reads_placement |= holds.placement.is_some();
                     }
-                Some(ExtentPlacement::Within(part))
+                } else {
-                    if declared[n].is_none()
-                        && part.axis(axis).start.rel == crate::Rel::ZERO
-                        && part.axis(axis).end.rel == crate::Rel::ONE =>
-                {
-                    let dependent = holds.frame[n]
-                        .and(holds.extent[n])
-                        .through(part.axis(axis).len());
-                    self.extent_under[n] = self.extent_under[n].and(dependent);
-                }
-                _ => {
                     let chosen = placement[n].unwrap_or(UiSpan::FULL).len();
-                    self.under[n] = self.under[n]
+                    result.frame[n] = result.frame[n].and(
-                        .and(holds.frame[n].through(local.axis(axis).len()))
-                        .and(
                         holds.extent[n]
                             .through(chosen)
                             .through(local.axis(axis).len()),
                     );
-                    // Fractional endpoints compose before pixel evaluation. Their
-                    // difference cannot be inverted through the extent length alone.
-                    if matches!(extent, Some(ExtentPlacement::Within(_))) && declared[n].is_none() {
-                        self.reads_placement = true;
                 }
             }
-            }
+            result
-        }
+        };
-        // A fractional report is composed into the parent frame, so its
+        self.under = self.under.and(in_parent(holds));
-        // value can change with the extent even when the drawing holds.
+        let answer_holds = in_parent(answer_holds);
-        let reads_placement = matches!(extent, Some(ExtentPlacement::Within(_)))
-            && AXES.into_iter().any(|axis| {
-                declared[axis as usize].is_none() && size.axis(axis).rel != crate::Rel::ZERO
-            });
         DrawResult {
             child: id,
             painter: self,
             size: in_parent_frame(size, local.size(), declared),
-            reads_placement,
+            answer_holds,
         }
     }
 
@@ -400,7 +369,7 @@ impl<'a> Painter<'a> {
                 .retained_size(child.id(), px, placement, self.move_idx, self.rsc.widgets());
         let Some((size, holds)) = retained else {
             return self
-                .widget_at_inner(child, region, offered, None, true)
+                .widget_at_inner(child, region, offered, false, true)
                 .len(axis);
         };
         #[cfg(feature = "layout-diagnostics")]
@@ -417,7 +386,7 @@ impl<'a> Painter<'a> {
             y: placement[1].unwrap_or(UiSpan::FULL),
         };
         let holds = holds.in_frame(placement);
-        for (axis, under) in AXES.into_iter().zip(self.under.iter_mut()) {
+        for (axis, under) in AXES.into_iter().zip(self.answer_under.frame.iter_mut()) {
             *under = under.and(holds[axis as usize].through(local.axis(axis).len()));
         }
         in_parent_frame(size, local.size(), declared).axis(axis)
@@ -641,7 +610,7 @@ pub struct DrawResult<'p, 'a, W: ?Sized> {
     painter: &'p mut Painter<'a>,
     child: &'p StrongWidget<W>,
     size: Size,
-    reads_placement: bool,
+    answer_holds: LayoutHolds,
 }
 
 impl<W: ?Sized> DrawResult<'_, '_, W> {
@@ -652,7 +621,7 @@ impl<W: ?Sized> DrawResult<'_, '_, W> {
             diag::size_read(self.child.id(), self.painter.id, self.size);
         }
         self.painter.depend_on(self.child);
-        self.painter.reads_placement |= self.reads_placement;
+        self.painter.answer_under = self.painter.answer_under.and(self.answer_holds);
         self.size
     }
 

core/src/ui/render_state.rs

@@ -66,13 +66,6 @@ pub struct UiRenderState {
     /// A widget's move slot, which outlives any one `ActiveData`: a redraw
     /// replaces that while its children go on pointing at the slot.
     slots: HashMap<WidgetId, MoveIdx>,
-    /// Answers invalidated by a declared-length change below them. These are
-    /// replaced even when retained placement means the redraw is not at the
-    /// old offer.
-    answer_invalid: crate::util::HashSet<WidgetId>,
-    /// Whether this frame contains a declared-length change, so any dirty
-    /// dependent replaces its answer too.
-    replace_answers: bool,
     /// Widgets waiting for an ancestor to draw them, so the walk down the
     /// depths does not pick one up again at its own depth.
     deferred: crate::util::HashSet<WidgetId>,
@@ -87,8 +80,6 @@ impl UiRenderState {
             output_size: PxVec2::ZERO,
             old_root: None,
             slots: Default::default(),
-            answer_invalid: Default::default(),
-            replace_answers: false,
             deferred: Default::default(),
             moves: Default::default(),
             resized: false,
@@ -103,9 +94,8 @@ impl UiRenderState {
     /// retained entry at all.
     ///
     /// The root is the only widget a resize marks, and only where the new
-    /// output falls outside what its answer holds for: that range is the
+    /// output invalidates its answer or its drawing. The latter includes
-    /// intersection of everything under it, so admitting the new output says
+    /// children whose size it never read. Where either fails, the ordinary walk
-    /// the whole tree still stands. Where it does not, the ordinary walk
     /// draws the root, and each widget's own range decides how far down the
     /// new length reaches.
     pub fn resize(&mut self, size: impl Into<Vec2>, widgets: &mut Widgets) {
@@ -116,10 +106,10 @@ impl UiRenderState {
         self.output_size = size;
         self.resized = true;
         let Some(root) = self.old_root else { return };
-        let stands = self
+        let stands = self.active.get(&root).is_some_and(|active| {
-            .active
+            let px = active.given_region.size().to_px(size);
-            .get(&root)
+            active.answers_at(px) && active.holds.contains(px, active.placement)
-            .is_some_and(|active| active.answers_at(active.given_region.size().to_px(size)));
+        });
         if !stands {
             widgets.needs_redraw.insert(root);
         }
@@ -182,7 +172,6 @@ impl UiRenderState {
         if rsc.widgets().has_updates() {
             self.redraw_updates(rsc);
         }
-        self.replace_answers = false;
         self.free(rsc);
     }
 
@@ -215,7 +204,7 @@ impl UiRenderState {
         mut old: Option<ActiveData>,
         measuring: bool,
         rsc: &mut dyn UiRsc,
-    ) -> (Size, LayoutHolds) {
+    ) -> (Size, LayoutHolds, LayoutHolds) {
         #[cfg(feature = "layout-diagnostics")]
         {
             diag::bump(Counter::DrawRequests);
@@ -226,8 +215,7 @@ impl UiRenderState {
         // one bottom-up walk, so anything deeper has settled or deferred to
         // its own parent, and a deferred one leaves that parent marked.
         let stale = rsc.widgets().needs_redraw.contains(&id);
-        let replace_answer = self.answer_invalid.remove(&id) || (self.replace_answers && stale);
+        let retained = match stale {
-        let retained = match replace_answer || stale {
             true => None,
             false => self
                 .retained_answer(id, info)
@@ -284,7 +272,7 @@ impl UiRenderState {
         active.given_region = info.given_region;
         active.offer_len = info.offer_len;
         if info.placement == info.offer_placement && info.px == info.offered_px {
-            active.answer = Some(settled);
+            active.answer = Some(answer);
             active.offer_placement = info.offer_placement;
         }
         active.decided = info.decided();
@@ -300,9 +288,9 @@ impl UiRenderState {
             && let Some(old_parent) = self.active.get_mut(&old_parent)
         {
             old_parent.children.retain(|child| *child != id);
-            old_parent.extent_children.retain(|(child, _)| *child != id);
+            old_parent.inherited_children.retain(|child| *child != id);
         }
-        settled
+        (answer.0, answer.1, settled.1)
     }
 
     /// Recompose retained geometry when the evaluation still holds at this extent.
@@ -373,16 +361,16 @@ impl UiRenderState {
             textures: Vec::new(),
             primitives: Vec::new(),
             mask_region: None,
-            extent_children: Vec::new(),
+            inherited_children: Vec::new(),
             children: Vec::new(),
             offered: Vec::new(),
             offered_px: info.offered_px,
             at_offer,
             size_deps: Vec::new(),
             own: [Holds::ANY; 2],
-            under: [Holds::ANY; 2],
+            under: LayoutHolds::ANY,
             extent_own: [Holds::ANY; 2],
-            extent_under: [Holds::ANY; 2],
+            answer_under: LayoutHolds::ANY,
             depth: info.depth,
             move_idx,
             rsc,
@@ -410,9 +398,9 @@ impl UiRenderState {
             textures,
             primitives,
             mask_region,
-            extent_children,
+            inherited_children,
             extent_own,
-            extent_under,
+            answer_under,
             children,
             offered: _,
             offered_px: _,
@@ -450,14 +438,13 @@ impl UiRenderState {
             "'{}' ({id:?}) clips to {px:?} and reports {size}",
             rsc.widgets().label(id),
         );
-        let holds = LayoutHolds {
+        let own_holds = LayoutHolds {
-            frame: [own[0].and(under[0]), own[1].and(under[1])],
+            frame: own,
-            extent: [
+            extent: extent_own,
-                extent_own[0].and(extent_under[0]),
-                extent_own[1].and(extent_under[1]),
-            ],
             placement: reads_placement.then_some(placement),
         };
+        let answer_holds = own_holds.and(answer_under);
+        let holds = answer_holds.and(under);
         debug_assert!(
             holds.contains(px, placement),
             "'{}' ({id:?}) drew in {px:?}, outside the ranges it reported: {holds:?}",
@@ -516,7 +503,7 @@ impl UiRenderState {
             textures,
             primitives,
             mask_region,
-            extent_children,
+            inherited_children,
             children,
             size_deps,
             declared: declared_lens(rsc.widgets(), id),
@@ -530,7 +517,7 @@ impl UiRenderState {
         };
         rsc.on_draw(&active);
         self.active.insert(id, active);
-        (size, holds)
+        (size, answer_holds)
     }
 
     /// Keeps a region node's entry across redraws because descendants retain
@@ -765,18 +752,19 @@ impl UiRenderState {
         }
         let parent_move = active.move_idx;
         let mask = active.mask;
-        let children = active.extent_children.len();
+        let children = active.inherited_children.len();
         for index in 0..children {
-            let (child, extent) = self.active[&id].extent_children[index];
+            let child = self.active[&id].inherited_children[index];
-            let (child_region, chosen) = extent.resolve(placement);
             let active = &self.active[&child];
-            let (child_local, chosen) =
+            let (child_local, chosen) = ask_box(
-                ask_box(child_region, active.declared, active.own_align, chosen);
+                UiRegion::FULL,
-            // Keep the slot chosen from measurement: the final draw can
+                active.declared,
-            // report a different size, for example after text reflows.
+                active.own_align,
+                [Some(placement.x), Some(placement.y)],
+            );
             let child_placement = UiRegion {
-                x: chosen[0].unwrap_or(active.placement.x),
+                x: chosen[0].unwrap_or(UiSpan::FULL),
-                y: chosen[1].unwrap_or(active.placement.y),
+                y: chosen[1].unwrap_or(UiSpan::FULL),
             };
             let child_info = DrawInfo {
                 layer: active.layer,
@@ -936,7 +924,7 @@ impl UiRenderState {
                 textures: Vec::new(),
                 primitives: Vec::new(),
                 mask_region: None,
-                extent_children: Vec::new(),
+                inherited_children: Vec::new(),
                 children: Vec::new(),
                 size_deps: Vec::new(),
                 move_idx: info.parent_move,
@@ -958,8 +946,6 @@ impl UiRenderState {
             }
         }
         self.slots.clear();
-        self.answer_invalid.clear();
-        self.replace_answers = false;
         self.moves.clear();
         self.layers.clear();
         rsc.widgets_mut().needs_redraw.clear();
@@ -973,7 +959,6 @@ impl UiRenderState {
             rsc.on_remove(id);
             self.remove(id, true, rsc);
             self.drop_slot(id);
-            self.answer_invalid.remove(&id);
         }
         rsc.ui_mut().textures.free();
     }
@@ -1110,15 +1095,6 @@ impl UiRenderState {
         if let Some(parent) = active.parent
             && (declared_changed || alignment_changed || !active.drawn || active.answer.is_none())
         {
-            if declared_changed {
-                self.replace_answers = true;
-                let mut at = Some(id);
-                while let Some(next) = at {
-                    self.answer_invalid.insert(next);
-                    rsc.widgets_mut().needs_redraw.insert(next);
-                    at = self.active[&next].parent;
-                }
-            }
             // Both stay marked: the parent because it has this to draw, and
             // this because the parent must draw it rather than keep what it
             // has. The mark comes off in `draw_at`, where the parent draws.
@@ -1172,7 +1148,7 @@ impl UiRenderState {
             placement: AXES
                 .map(|axis| active.decided[axis as usize].then(|| *active.placement.axis(axis))),
         };
-        let (given, was_answer) = (active.region, active.answer);
+        let (given, was_answer, was_holds) = (active.region, active.answer, active.holds);
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::LocalRedraws);
 
@@ -1187,9 +1163,9 @@ impl UiRenderState {
         if info.placement != offered.placement {
             self.draw_inner(id, given, info, None, false, rsc);
         }
-        if Some(answer) != was_answer {
+        if Some((answer.0, answer.1)) != was_answer || self.active[&id].holds != was_holds {
-            // Its parent chose its box knowing the old answer, so it lays out
+            // The parent retains both the answer and the drawing's validity;
-            // again and chooses the box the new one asks for.
+            // even an unchanged size can narrow the range safe for a resize.
             #[cfg(feature = "layout-diagnostics")]
             {
                 diag::bump(Counter::SizeChanges);

src/widget/position/pad.rs

@@ -13,7 +13,7 @@ impl Widget for Pad {
         // it; where the box is bigger -- a share of a row, a rule over this
         // widget -- the slack is the inner's to sit in, and forcing the near
         // edge pinned it to a corner it had not asked for.
-        let inside = DrawRegion::Extent(self.padding.region());
+        let inside = self.padding.region_of(painter.placement());
         let inner = painter.widget_within(&self.inner, inside).size();
         Size {
             x: LayoutLen {

src/widget/position/stack.rs

@@ -13,6 +13,7 @@ impl Widget for Stack {
             StackSize::Default => None,
             StackSize::Child(i) => Some(i),
         };
+        let placement = painter.placement();
         // Whichever child sizes the stack keeps the stack's whole region as
         // its own -- the stack is the length that child asked for, so taking
         // the fraction of the stack's box again would take it twice -- and is
@@ -23,7 +24,13 @@ impl Widget for Stack {
             // drawing belongs to the layer it was made on.
             Some((i, child)) => {
                 painter.child_layer_at(i);
-                painter.widget(child).size()
+                painter
+                    .widget_at(
+                        child,
+                        UiRegion::FULL,
+                        [Some(placement.x), Some(placement.y)],
+                    )
+                    .size()
             }
             None => Size::LEFTOVER,
         };
@@ -35,7 +42,7 @@ impl Widget for Stack {
             // Every other child has the stack's own box for its region, since
             // the stack is what contains it, and where it sits in one bigger
             // than itself is its own business.
-            painter.widget_within(child, DrawRegion::Extent(UiRegion::FULL));
+            painter.widget_within(child, placement);
         }
         size
     }

tests/cases/retained.rs

@@ -1022,210 +1022,86 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
 }
 
 #[test]
-fn padding_and_stack_frames_follow_the_extent_without_drawing_again() {
+fn resizing_does_not_remeasure_a_fixed_stack_for_its_unmeasured_overlay() {
-    struct Observed<W> {
+    let mut h = Harness::new((400, 200));
-        widget: W,
+    let (sizing, _) = counted(&mut h, Size::from((100, 80)), false);
+    let (overlay, draws) = counted(&mut h, Size::LEFTOVER, true);
+    h.set_root((sizing, overlay).stack().size(StackSize::Child(0)));
+    let settled = draws.get();
+
+    h.resize((800, 300));
+    h.frame();
+
+    assert_eq!(draws.get(), settled);
+    assert_corners!(h, overlay, (350, 110), (450, 190));
+}
+
+struct Unmeasured {
+    child: StrongWidget,
     draws: Rc<Cell<usize>>,
 }
-    impl<W: Widget> Widget for Observed<W> {
+
+impl Widget for Unmeasured {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
-            self.widget.draw(painter)
+        painter.widget(&self.child);
-        }
-    }
-    struct Frame {
-        child: StrongWidget,
-        extent: UiRegion,
-    }
-    impl Widget for Frame {
-        fn draw(&mut self, painter: &mut Painter) -> Size {
-            painter.widget_at(
-                &self.child,
-                UiRegion::FULL,
-                [Some(self.extent.x), Some(self.extent.y)],
-            );
         Size::LEFTOVER
     }
 }
-    for node in [false, true] {
-        let plant = |h: &mut Harness, extent| {
-            let draws = Rc::new(Cell::new(0));
-            let leaf = rect(Color::BLUE).masked().add(&mut h.rsc);
-            h.rsc.widgets_mut().set_region_node(leaf, node);
-            let fixed = rect(Color::RED).width(31).height(19).add(&mut h.rsc);
-            let stack = Observed {
-                widget: Stack {
-                    children: vec![leaf.add_strong(&mut h.rsc), fixed.add_strong(&mut h.rsc)],
-                    size: StackSize::Default,
-                },
-                draws: draws.clone(),
-            }
-            .add_strong(&mut h.rsc);
-            let pad = Observed {
-                widget: Pad {
-                    inner: stack,
-                    padding: Padding::uniform(7).with_left(13),
-                },
-                draws: draws.clone(),
-            }
-            .add_strong(&mut h.rsc);
-            let root = Frame { child: pad, extent }.add(&mut h.rsc);
-            h.set_root(root);
-            (root, leaf, fixed, draws)
-        };
-        let mut warm = Harness::new((403, 211));
-        let (root, leaf, fixed, draws) = plant(&mut warm, UiRegion::FULL);
-        for (start, end) in [(0.13, 0.83), (-0.17, 1.23), (0.31, 0.67)] {
-            let extent = UiRegion::new(
-                UiSpan::new(Len::rel(start) + Len::px(3.125), Len::rel(end)),
-                UiSpan::new(Len::px(11.25), Len::rel(end)),
-            );
-            let before = draws.get();
-            warm.rsc[root].extent = extent;
-            warm.frame();
-            assert_eq!(draws.get(), before);
-            let mut cold = Harness::new((403, 211));
-            let (_, other, other_fixed, _) = plant(&mut cold, extent);
-            for (a, b) in [(leaf.id(), other.id()), (fixed.id(), other_fixed.id())] {
-                assert_eq!(warm.region(&a), cold.region(&b));
-                assert_eq!(primitive_bounds(&warm, a), primitive_bounds(&cold, b));
-            }
-            let mask = |h: &Harness, id: WidgetId| {
-                let active = &h.render.active[&id];
-                let mask = &h.rsc.ui().masks[active.mask.idx()];
-                h.render
-                    .moves
-                    .resolve(mask.move_idx, mask.region)
-                    .to_px(h.render.output_size())
-            };
-            assert_eq!(mask(&warm, leaf.id()), mask(&cold, other.id()));
-        }
-    }
-}
 
 #[test]
-fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
+fn a_declared_size_change_stops_at_an_independent_parent() {
-    struct Measured;
-    impl Widget for Measured {
-        fn draw(&mut self, painter: &mut Painter) -> Size {
-            let width = painter.px_len(Axis::X);
-            painter.primitive(RectPrimitive::color(Color::BLUE));
-            Size::from((80, if width > Px::from_int(100) { 40 } else { 60 }))
-        }
-    }
-    struct Frame {
-        child: StrongWidget,
-        start: f32,
-    }
-    impl Widget for Frame {
-        fn draw(&mut self, painter: &mut Painter) -> Size {
-            painter.widget_at(
-                &self.child,
-                UiRegion::FULL,
-                [
-                    Some(UiSpan::new(
-                        Len::px(self.start),
-                        Len::px(self.start + 200.0),
-                    )),
-                    Some(UiSpan::FULL),
-                ],
-            );
-            Size::LEFTOVER
-        }
-    }
     let mut h = Harness::new((400, 200));
-    let leaf = Measured.add(&mut h.rsc);
+    let leaf = rect(Color::RED).width(100).add(&mut h.rsc);
-    let stack = (leaf,).stack().add_strong(&mut h.rsc);
+    let parent = Unmeasured {
-    let root = Frame {
+        child: leaf.add_strong(&mut h.rsc),
-        child: stack,
+        draws: Rc::new(Cell::new(0)),
-        start: 0.0,
     }
-    .add(&mut h.rsc);
+    .add_strong(&mut h.rsc);
-    h.set_root(root);
+    let draws = Rc::new(Cell::new(0));
-    assert_corners!(h, leaf, (60, 80), (140, 120));
+    h.set_root(Unmeasured {
-    h.rsc[root].start = 30.0;
+        child: parent,
+        draws: draws.clone(),
+    });
+    let settled = draws.get();
+
+    h.set_len(leaf, Axis::X, 150);
     h.frame();
-    assert_corners!(h, leaf, (90, 80), (170, 120));
+
-    assert_eq!(
+    assert_corners!(h, leaf, (125, 0), (275, 200));
-        primitive_bounds(&h, leaf.id()),
+    assert_eq!(draws.get(), settled);
-        vec![h.region(&leaf).unwrap()]
-    );
 }
 
 #[test]
-fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
+fn an_unmeasured_child_still_invalidates_its_parents_drawing_on_resize() {
-    struct Container {
+    let mut h = Harness::new((400, 200));
-        child: StrongWidget,
+    let draws = Rc::new(Cell::new(0));
-        region: UiRegion,
+    let leaf = ReadsWidth {
-    }
+        draws: draws.clone(),
-    impl Widget for Container {
-        fn draw(&mut self, painter: &mut Painter) -> Size {
-            painter
-                .widget_within(&self.child, DrawRegion::Extent(self.region))
-                .size()
-        }
-    }
-    struct Frame {
-        child: StrongWidget,
-        extent: UiRegion,
-        answer: Rc<Cell<Size>>,
-    }
-    impl Widget for Frame {
-        fn draw(&mut self, painter: &mut Painter) -> Size {
-            self.answer.set(
-                painter
-                    .widget_at(
-                        &self.child,
-                        UiRegion::FULL,
-                        [Some(self.extent.x), Some(self.extent.y)],
-                    )
-                    .size(),
-            );
-            Size::LEFTOVER
-        }
-    }
-    for fractional in [false, true] {
-        for region in [
-            UiRegion::FULL,
-            UiRegion::new(UiSpan::new(Len::rel(0.13), Len::rel(0.79)), UiSpan::FULL),
-        ] {
-            let plant = |h: &mut Harness, extent| {
-                let size = if fractional {
-                    Size {
-                        x: rel(0.5),
-                        y: LayoutLen::px(27),
-                    }
-                } else {
-                    Size::from((80, 27))
-                };
-                let (leaf, _) = counted(h, size, !fractional);
-                let child = Container {
-                    child: leaf.add_strong(&mut h.rsc),
-                    region,
-                }
-                .add_strong(&mut h.rsc);
-                let answer = Rc::new(Cell::new(Size::ZERO));
-                let root = Frame {
-                    child,
-                    extent,
-                    answer: answer.clone(),
     }
     .add(&mut h.rsc);
-                h.set_root(root);
+    h.set_root((leaf,).stack());
-                (root, leaf, answer)
+    let settled = draws.get();
-            };
+
-            let mut warm = Harness::new((403, 211));
+    h.resize((800, 200));
-            let (root, leaf, answer) = plant(&mut warm, UiRegion::FULL);
+    h.frame();
-            for width in [191.125, 297.25, 83.75] {
+
-                let extent =
+    assert!(draws.get() > settled);
-                    UiRegion::new(UiSpan::new(Len::px(13.125), Len::px(width)), UiSpan::FULL);
+    assert_corners!(h, leaf, (300, 90), (500, 110));
-                warm.rsc[root].extent = extent;
-                warm.frame();
-                let mut cold = Harness::new((403, 211));
-                let (_, other, other_answer) = plant(&mut cold, extent);
-                assert_eq!(answer.get(), other_answer.get());
-                assert_eq!(warm.region(&leaf), cold.region(&other));
-            }
-        }
 }
+
+#[test]
+fn changed_drawing_dependencies_reach_ancestors_without_a_size_change() {
+    let mut h = Harness::new((400, 200));
+    let (leaf, draws) = counted(&mut h, Size::LEFTOVER, false);
+    h.set_root(((leaf,).stack(),).stack());
+
+    h.rsc[leaf].reads_box = true;
+    h.frame();
+    let settled = draws.get();
+    h.resize((800, 200));
+    h.frame();
+
+    assert_eq!(draws.get(), settled + 1);
+    assert_corners!(h, leaf, (0, 0), (800, 200));
 }