Hold a clipping widget to its box, and check that it is

`Scroll` reports `LEFTOVER` on both axes because it clips its content to its
box: it can neither take less of one nor honestly ask for more. `Masked` is
the other widget that clips and was passing its inner's size up, so a mask
over something taller than its box asked to be placed at the length it had
just cut off. It reports its box now, for the same reason.

The `debug_assert` the handoff has been asking for is the one that would have
caught both, narrowed to what is actually true: a widget that set a mask this
draw has to report inside the box it drew in. Reported as "does not exceed
the box" it fires on ordinary overflow instead -- measured, a hundred fuzzer
trees produce thousands of them, every one a text too tall for the box it was
offered, which is what a text is meant to say.

`tests/cases/scroll.rs` has a clipping widget that reports its content, to
show the assertion catches it.

Checked: fmt, clippy, 104 tests, all five shrinker cases at 300 seeds, 100
generated seeds, five examples byte-identical at 1920x1200.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/cases/scroll.rs

@@ -59,3 +59,26 @@ fn a_wheel_scrolls_the_content_and_stops_at_its_end() {
     h.frame();
     assert_corners!(h, top, (0, 0), (400, 200));
 }
+
+/// A widget that clips to its box may not report more than the box: its
+/// parent would place the part it cut off, and the framework would put a
+/// drawing longer than its box somewhere. `Masked` is the second of these
+/// after `Scroll`, and the assertion in `draw_at` is what says so.
+#[test]
+#[should_panic = "clips to"]
+fn a_clipping_widget_reporting_more_than_its_box_is_caught() {
+    struct Clipper(StrongWidget);
+
+    impl Widget for Clipper {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter.set_mask(painter.region());
+            painter.widget(&self.0).size()
+        }
+    }
+
+    let mut h = Harness::new((100, 100));
+    let tall = rect(Color::RED).height(400).add_strong(&mut h.rsc);
+    let clipper = Clipper(tall).add(&mut h.rsc);
+    h.set_root(clipper);
+    h.frame();
+}