Settle growing layout branches in one frame

docs/LAYOUT.md

@@ -281,12 +281,15 @@ always a leaf: `Rect`, `Image`, a fixed glyph). A widget that returns
 `false` (the default) is redrawn in full whenever `available` changes,
 which is correct always, just not free.
 
-**Ancestor propagation** (a resized child changing its own reported size,
-requiring its parent to re-lay-out) is unchanged in spirit from today's
-`redraw` (`render_state.rs:270-305`), which already walks up exactly the
-ancestors whose cached size differs from the new one and stops as soon as
-a size is unchanged (`:274-286`). That loop moves from consulting
-`Cache.size` to consulting `ActiveData.size` (§5) but keeps its shape.
+**Size propagation goes both ways in the same frame.** A resized child first
+walks upward through exactly the ancestors whose cached size changes. That
+measurement pass gives each parent the new size but necessarily drew the
+branch in its old boxes. As the recursion returns, `redraw_and_settle` revisits
+those changed widgets from the outside in, after their parents have assigned
+the final boxes. Otherwise a newly appended child can retain the provisional
+(even inverted) region it was measured in until another update happens. The
+downward work is confined to the branch that changed; unchanged descendants
+still take `draw_inner`'s retained fast path.
 
 ### 4. Wrapped text, and "needs child height before choosing width"
 
@@ -667,9 +670,11 @@ A move alone cannot fix a changed size; `Painter::place` redraws in that
 case.
 
 The cost is bounded and worth stating, because it is what makes the rule
-safe to apply everywhere: the second draw happens only on the frame a
+safe to apply everywhere: the settling draw happens only on the frame a
 widget's own size actually changes, which is a frame that was already
-redrawing it. A widget whose reported size is a function of the box it
-was *offered* would disagree every frame and redraw every frame — which
-is why `LazySpan` requires content-sized rows, and has since long before
-this.
+redrawing it. `Sized` also requires its final region before retaining its
+children: its own reported size may be known exactly while a descendant was
+drawn in the provisional box, so moving only the wrapper is insufficient. A
+widget whose reported size is a function of the box it was *offered* would
+disagree every frame and redraw every frame — which is why `LazySpan` requires
+content-sized rows, and has since long before this.