Settle growing layout branches in one frame

app-rust/tests/phone_screen.rs

@@ -258,17 +258,15 @@ fn a_newline_leaves_the_caret_inside_the_composers_padding() {
         screen.composer.field.edit(&mut h.rsc).insert("a\n");
         h.frame(PHONE_FRAME_MS);
     }
+    let message = h
+        .render
+        .debug(h.rsc.widgets(), "Message")
+        .find(|a| !a.primitives.is_empty())
+        .expect("the composer field is drawn");
     // The caret is the last primitive `TextEdit::draw` emits.
-    let caret = {
-        let slot = *h
-            .render
-            .debug(h.rsc.widgets(), "Message")
-            .flat_map(|a| a.primitives.iter().map(|p| p.slot))
-            .collect::<Vec<_>>()
-            .last()
-            .expect("the focused field draws a caret");
-        h.render.primitive_corners(slot, &h.rsc)
-    };
+    let caret = h
+        .render
+        .primitive_corners(message.primitives.last().unwrap().slot, &h.rsc);
     // The bar sits directly on the IME, so its inside edge is one
     // `FIELD_PAD_DP` above `height - ime`. Stated in pixels rather than
     // read back from the composer, which is the thing under test.
@@ -280,4 +278,22 @@ fn a_newline_leaves_the_caret_inside_the_composers_padding() {
          and its padding is {padding}px",
         caret.bot_right.y,
     );
+
+    // The old assertion only guarded the last line. A viewport one line
+    // shorter than the field still kept that caret above the bottom while
+    // moving the first line above the bar's mask, visibly slicing it off.
+    let mask = h.rsc.ui.masks[message.mask.idx()];
+    let bar = h.render.primitive_corners(mask.primitive, &h.rsc);
+    let visible_content_top = message
+        .primitives
+        .iter()
+        .map(|p| h.render.primitive_corners(p.slot, &h.rsc))
+        .filter(|r| r.bot_right.y > bar.top_left.y)
+        .map(|r| r.top_left.y)
+        .fold(f32::INFINITY, f32::min);
+    assert!(
+        visible_content_top > bar.top_left.y,
+        "the composer's first visible line is clipped above its bar: content starts at {visible_content_top}, bar starts at {}",
+        bar.top_left.y,
+    );
 }