Retain frame and extent dependencies independently

Keep the original measurement placement separate from the assigned slot.
Validate frame and extent lengths before reusing an answer or drawing, and
represent hint-only records as having no measured answer.

Retain primitive and mask coordinates with their frame/extent reference.
Forwarded children follow a reused wrapper's placement without rerunning
valid draw bodies. Keep the single Widget::draw API.

Restore the eight failing suite cases from the region/placement prototype,
with regressions for mixed coordinate references, a changed inherited
extent, the sizing-stack fraction, and an undrawn share becoming visible.

This remains experimental: nested container updates do substantially more
work than e44dea3 despite restoring the leaf and wrapper reuse guarantees.
Do not merge it as a performance improvement.

core/src/ui/active.rs

@@ -1,6 +1,6 @@
 use crate::{
-    Holds, LayerId, LayoutLen, MaskIdx, MoveIdx, PrimitiveHandle, RegionAlign, Size, TextureHandle,
-    UiRegion, UiVec2, WidgetId,
+    DrawRegion, LayerId, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, RegionAlign, RetainedPrimitive,
+    Size, TextureHandle, UiRegion, UiVec2, WidgetId,
 };
 
 /// What is kept of a widget its parent has asked about. `drawn` says whether
@@ -15,10 +15,6 @@ pub struct ActiveData {
     pub region: UiRegion,
     /// Where its drawing sits inside that box, in the box's own coordinates.
     pub placement: UiRegion,
-    /// Whether its drawing read that placement, which is what says whether
-    /// moving it within the region is a redraw or only a different claim on
-    /// the same drawing.
-    pub reads_placement: bool,
     /// The same box as lengths of its parent's box, which is the one route
     /// to a box in pixels: a draw threads these down a level at a time, and
     /// [`crate::UiRenderState::redraw`] takes the same steps back up.
@@ -29,13 +25,14 @@ pub struct ActiveData {
     /// asked again -- and a chain of fractions has no frame in it, which is
     /// why a region node between two widgets cannot break it.
     pub offer_len: UiVec2,
-    /// What it answered there: the size and what that held for.
-    pub answer: (Size, [Holds; 2]),
+    pub offer_placement: [Option<crate::UiSpan>; 2],
+    /// The measured answer and its dependencies. A hint-only dependency or
+    /// a widget first encountered during placement has no measurement yet.
+    pub answer: Option<(Size, LayoutHolds)>,
     /// What the widget said it used of its box, the last time it drew.
     pub size: Size,
-    /// The pixel lengths of `region`, per axis, that its drawing and `size`
-    /// hold for.
-    pub holds: [Holds; 2],
+    /// The frame, extent and explicit placement reads that this drawing holds for.
+    pub holds: LayoutHolds,
     pub drawn: bool,
     pub parent: Option<WidgetId>,
     /// How far down the tree it was drawn, the root being 1. Carried down a
@@ -43,7 +40,9 @@ pub struct ActiveData {
     /// widget a frame visits and cannot drift while one is being drawn.
     pub depth: usize,
     pub textures: Vec<TextureHandle>,
-    pub primitives: Vec<PrimitiveHandle>,
+    pub primitives: Vec<RetainedPrimitive>,
+    pub mask_region: Option<DrawRegion>,
+    pub inherited_children: Vec<WidgetId>,
     pub children: Vec<WidgetId>,
     /// The children whose size this widget read while drawing.
     pub size_deps: Vec<WidgetId>,
@@ -74,16 +73,18 @@ pub struct ActiveData {
 }
 
 impl ActiveData {
-    /// Whether its drawing and size hold for a box of these pixel lengths.
-    pub fn holds_at(&self, px: crate::PxVec2) -> bool {
-        self.holds[0].contains(px.x) && self.holds[1].contains(px.y)
-    }
-
     /// Whether what it answered still stands for a box of these pixel
     /// lengths -- the box it was asked in, where `holds` is about the box its
     /// answer then chose.
     pub fn answers_at(&self, px: crate::PxVec2) -> bool {
-        let (_, holds) = self.answer;
-        holds[0].contains(px.x) && holds[1].contains(px.y)
+        self.answer.is_some_and(|(_, holds)| {
+            holds.contains(
+                px,
+                UiRegion {
+                    x: self.offer_placement[0].unwrap_or(crate::UiSpan::FULL),
+                    y: self.offer_placement[1].unwrap_or(crate::UiSpan::FULL),
+                },
+            )
+        })
     }
 }