Let OnResize answer for the window too, and delete the second rule

A resize had its own mechanism: `reads_output` recorded that a widget had
looked at the output, `update` scanned every active widget for one whose
`output_px` had moved, marked it and its whole reader chain, and
`resize_marks` kept those marks from counting as content dirtiness --
while `on_resize` answered the same question for every other box. Two
answers to "does this drawing survive its box changing length", and the
one that applied to the window ignored what the widget had declared.

With the output held as the root of the chain there is one question. A
resize offers the root widget its box again, `try_reuse` answers per axis
from `on_resize`, and `redraws_under` prices the subtree. Gone with it:
`reads_output`, `resized`, `resize_marks`, the scan, the eager reader
marking, and the shallowest-first branch in `redraw_updates`, which only
existed because resize marking worked differently -- the settle loop now
has one order.

Two things this needed. An unslotted widget may be reused when only its
parent's box changed length: it has nothing of its own to write, and what
it drew is a fraction of that box, so the slot above it already carries
the change. And `root_readers` holds the widgets whose size came from the
output rather than their own box -- `MaxSize` -- since no box of theirs
need have changed; they are marked per axis, from a set kept as they draw
rather than by scanning.

`a_resize_does_not_redraw_what_the_shader_can_move` now says `Scale`,
which is what it was always describing, and `a_resize_redraws_what_does
_not_scale` is its other half. `ReadsWidth` declares `Scale` across the
axis it does not read, so per-axis precision comes from the widget rather
than from which output axis it happened to touch.

Resize phase, seed 1 depth 8: 6.45M instructions per frame to 5.84M.

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

tests/retained.rs

@@ -191,6 +191,8 @@ impl Widget for ReadsOutput {
     }
 }
 
+/// Reads the output across one axis only, and says so: its drawing follows
+/// a taller box on its own, so only a wider one is worth a draw.
 struct ReadsWidth {
     draws: Rc<Cell<usize>>,
 }
@@ -200,12 +202,19 @@ impl Widget for ReadsWidth {
         self.draws.set(self.draws.get() + 1);
         Size::px((painter.output_len(Axis::X) / 4.0, 20.0).into())
     }
+
+    fn on_resize(&self, axis: Axis) -> OnResize {
+        match axis {
+            Axis::X => OnResize::Redraw,
+            Axis::Y => OnResize::Scale,
+        }
+    }
 }
 
 #[test]
 fn a_resize_does_not_redraw_what_the_shader_can_move() {
     let mut h = Harness::new((400, 200));
-    let (leaf, draws) = counted(&mut h, Size::REST, OnResize::Redraw);
+    let (leaf, draws) = counted(&mut h, Size::REST, OnResize::Scale);
     h.set_root(leaf);
     let settled = draws.get();
 
@@ -216,11 +225,28 @@ fn a_resize_does_not_redraw_what_the_shader_can_move() {
     assert_eq!(
         draws.get(),
         settled,
-        "its box is the same fraction of a different output"
+        "a scaling drawing follows its box, and the output is one"
     );
     assert_corners!(h, leaf, (0, 0), (800, 100));
 }
 
+/// The output is the root of the box chain, so a resize is a box that changed
+/// length and `OnResize` answers for it -- there is not a second rule for the
+/// window. A drawing that does not scale is redrawn whichever box moved.
+#[test]
+fn a_resize_redraws_what_does_not_scale() {
+    let mut h = Harness::new((400, 200));
+    let (leaf, draws) = counted(&mut h, Size::REST, OnResize::Redraw);
+    h.set_root(leaf);
+    let settled = draws.get();
+
+    h.resize((800, 100));
+    h.frame();
+
+    assert_eq!(draws.get(), settled + 1, "its box is a different length");
+    assert_corners!(h, leaf, (0, 0), (800, 100));
+}
+
 #[test]
 fn a_resize_redraws_what_read_the_output() {
     let mut h = Harness::new((400, 200));