Say what the fuzzer's branching widget branched on

Branch reads a measurement in pixels and draws a different subtree either
side of a threshold, and it left that read as a pin on the window, so
every one of them redrew on every resize: at depth 8 that was seed 1's
resize going from 40 widget draws to 131 and seed 13's from nothing to
828. It now states the range it actually branched on, the way Span states
the one that decides whether its shares have room. A fixture that redraws
everything on a resize cannot tell a change that reuses well from one
that does not.

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

src/random.rs

@@ -122,11 +122,21 @@ impl Widget for Branch {
         let measured = painter
             .widget_at(&self.probe, [None; 2], [Place::Within(Part::All), top])
             .len(Axis::X);
-        let px = painter.to_px(measured.apply_leftover(), Axis::X);
+        let len = measured.apply_leftover();
+        let px = painter.to_px(len, Axis::X);
+        // The range it actually branched on, said the way a container says
+        // one: pinning the window instead would redraw this widget on every
+        // resize, which is a fixture that never exercises reuse.
+        let threshold = Px::from_f32(self.threshold);
+        let holds = match px > threshold {
+            true => Holds::from(threshold + Px::STEP..=Px::MAX),
+            false => Holds::from(Px::MIN..=threshold),
+        };
+        painter.window_holds(Axis::X, holds.through(len));
 
         let below = Place::Within(Part::From(UiSpan::new(cut, painter.extent_len(Axis::Y))));
         let place = [Place::Within(Part::All), below];
-        match px > Px::from_f32(self.threshold) {
+        match px > threshold {
             true => painter.widget_at(&self.wide, [None; 2], place),
             false => painter.widget_at(&self.narrow, [None; 2], place),
         };