Stop composer layout recursion on spaces

app-rust/src/ui/composer.rs

@@ -112,6 +112,13 @@ where
     // the transcript: measured at 58px of stray text for a 475px message
     // in a 417px box.
     let content = field
+        // A wrapping editor occupies the composer's width even when its
+        // current text is short. Without this inner constraint the vertical
+        // ScrollArea reports the text's narrow natural width to Pad; settling
+        // then offers that width back to the editor, where a trailing space
+        // wraps and doubles its height. The next parent pass restores the
+        // wide one-line answer, so the two layouts have no fixed point.
+        .width(rest(1))
         // `scrollable_to_end`: what is being typed is at the end, so a
         // message longer than the six lines shown holds that end.
         .scrollable(Axis::Y, Pin::End)

app-rust/tests/phone_screen.rs

@@ -225,6 +225,40 @@ fn the_composer_sits_above_a_simulated_ime_inset() {
     );
 }
 
+/// A trailing space is narrower than the composer's available width but can
+/// wrap when the field is measured again in its own reported width. The
+/// settling walk must not bounce forever between those one- and two-line
+/// answers. On Android that recursion exhausted the native UI thread's stack
+/// and ended in SIGSEGV, before Rust's panic hook could write anything.
+#[test]
+fn a_space_in_the_composer_finishes_layout() {
+    let (mut h, screen) = opened();
+    screen.composer.set_bottom_inset(&mut h.rsc, 1000.0);
+    h.frame(PHONE_FRAME_MS * 2);
+    h.state.set_focus(Some(screen.composer.field));
+    screen.composer.field.edit(&mut h.rsc).set_cursor_byte(0);
+
+    for text in ["h", "i", " "] {
+        screen.composer.field.edit(&mut h.rsc).insert(text);
+        h.frame(PHONE_FRAME_MS);
+    }
+
+    assert_eq!(screen.composer.field.edit(&mut h.rsc).text.text(), "hi ");
+    let region = h
+        .render
+        .window_region(&screen.composer.field, &h.rsc)
+        .expect("the composer field is drawn");
+    let size = region.bot_right - region.top_left;
+    assert!(
+        size.x > phone_size().x / 2.0,
+        "the field shrink-wrapped to the short message: {region:?}"
+    );
+    assert!(
+        size.y < 30.0 * PHONE_SCALE,
+        "the trailing space wrapped onto a second line: {region:?}"
+    );
+}
+
 /// A newline typed into the composer must leave the caret inside the
 /// bar's own padding, not flush against its bottom edge.
 ///

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"
 

iris/core/src/ui/render_state.rs

@@ -1217,18 +1217,50 @@ impl UiRenderState {
     /// newly grown subtree can retain the provisional (even inverted) region
     /// it was measured in until an unrelated later update redraws it.
     fn redraw_and_settle(&mut self, id: WidgetId, rsc: &mut dyn UiRsc) {
+        let Some((parent, changed)) = self.redraw_once(id, rsc) else {
+            return;
+        };
+        if changed {
+            if let Some(pid) = parent {
+                self.redraw_and_settle(pid, rsc);
+            }
+            // The parent pass above has now placed this widget in its final
+            // region. Draw it once more there; unchanged descendants still
+            // take draw_inner's retained fast path. This is deliberately one
+            // redraw rather than another settling pass: feeding its size
+            // back into the same upward walk can alternate between the
+            // provisional and final regions forever (a text edit first did
+            // that when an Android IME committed a space), overflowing the
+            // native thread's stack before Rust can report a panic.
+            let settled_size = self.active.get(&id).map(|active| active.size);
+            let _ = self.redraw_once(id, rsc);
+            debug_assert_eq!(
+                self.active.get(&id).map(|active| active.size),
+                settled_size,
+                "a widget changed size after its parent settled its final region"
+            );
+        }
+    }
+
+    /// Redraw `id` exactly once, returning its parent and whether the size it
+    /// reports changed. [`Self::redraw_and_settle`] owns any propagation; in
+    /// particular, its final downward redraw must not start another upward
+    /// pass through the same branch.
+    fn redraw_once(
+        &mut self,
+        id: WidgetId,
+        rsc: &mut dyn UiRsc,
+    ) -> Option<(Option<WidgetId>, bool)> {
         rsc.widgets_mut().needs_redraw.remove(&id);
         // An ancestor is drawing this widget right now, and that draw is
         // about to write fresh primitives for it. Drawing it a second time
         // here would leave one of the two copies on screen with nothing
         // owning it -- see `draw_started`'s own doc.
         if self.draw_started.contains(&id) {
-            return;
+            return None;
         }
 
-        let Some(active) = self.remove(id, false, true, rsc) else {
-            return;
-        };
+        let active = self.remove(id, false, true, rsc)?;
         let old_size = active.size;
         let parent = active.parent;
         // `old_move_slot` being `Some` below means the slot is reused in
@@ -1259,15 +1291,7 @@ impl UiRenderState {
         // there is no query left that answers "what size would this be"
         // without actually drawing (LAYOUT.md section 5).
         let changed = self.active.get(&id).map(|a| a.size) != Some(old_size);
-        if changed {
-            if let Some(pid) = parent {
-                self.redraw_and_settle(pid, rsc);
-            }
-            // The parent pass above has now placed this widget in its final
-            // region. Draw it once more there; unchanged descendants still
-            // take draw_inner's retained fast path.
-            self.redraw_and_settle(id, rsc);
-        }
+        Some((parent, changed))
     }
 }