Avoid speculative layout when retained answers suffice

core/src/ui/painter.rs

@@ -22,6 +22,8 @@ pub struct Painter<'a> {
     /// The children whose size this widget read while drawing.
     pub(super) size_deps: Vec<WidgetId>,
     pub(super) reads_output: bool,
+    /// Whether this widget is drawing in the same pixel-sized box as before.
+    pub(super) same_box: bool,
     /// The slot this widget's primitives are positioned through: its own if
     /// its parent placed it, otherwise the nearest ancestor that has one.
     pub(super) move_idx: MoveIdx,
@@ -144,6 +146,21 @@ impl<'a> Painter<'a> {
         Some(hint)
     }
 
+    /// A clean child's retained size, when this widget's own constraints are
+    /// unchanged. This is the answer from its last real draw, not a guess.
+    pub fn retained_size<W: ?Sized>(&mut self, id: &StrongWidget<W>) -> Option<Size> {
+        if !self.same_box || self.rsc.widgets().needs_redraw.contains(&id.id()) {
+            return None;
+        }
+        let active = self.state.active.get(&id.id())?;
+        if active.parent != Some(self.id) {
+            return None;
+        }
+        let size = active.size;
+        self.depend_on_size(id);
+        Some(size)
+    }
+
     fn depend_on_size<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
         if !self.size_deps.contains(&child.id()) {
             self.size_deps.push(child.id());