Stop composer layout recursion on spaces

docs/LAYOUT.md

@@ -289,7 +289,14 @@ 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.
+still take `draw_inner`'s retained fast path. Each downward visit is exactly
+one redraw, not another upward propagation: a wrapping child can have no fixed
+point when an ancestor shrink-wraps it (a trailing space alternated between one
+line in the offered width and two lines in its reported natural width). Feeding
+that answer back into the same branch recursively overflowed Android's native
+UI-thread stack before Rust could report a panic. A container that intends a
+wrapping child to occupy its width declares that constraint explicitly; the
+message composer does so on both sides of its vertical `ScrollArea`.
 
 ### 4. Wrapped text, and "needs child height before choosing width"