Give a slot to the children a container places, and nothing else

A widget's region is now held in the coordinates of the slot it draws in
rather than the window's, and `Painter::place` is how a container asks for a
slot: it draws a child it decides the box of and may decide again. Everything
under that slot is a fraction of its box, so placing the child a second time
is one entry to write whether it moved or changed length. A child drawn any
other way has no slot and shares its nearest ancestor's.

That is what keeps the chain short. `chain_cost` measured depth as the cost
-- free to 8, +42.6% at 16 -- and a slot per widget put a transcript's glyphs
past that for nothing, since almost every slot was zero. `Span`, `Aligned`
and `Scroll` are the containers that re-place a child after drawing it, and
`tests/layout.rs` pins that four widgets between a span and a leaf leave the
leaf's chain one deep.

`UiRegion::stretch`, `UiRegion::stretchable` and `UiScalar::stretch` are
gone. Nothing is inverted any more: a box that changed length is written to
its slot, and the descendants recompose against it in the shader. That also
retires the case the guard existed for, where a fixed length has no fraction
to recover -- `tests/layout.rs` now stretches a 40-tall row on its other
axis, which `stretchable` refused outright.

What still walks the CPU is deciding who must draw again, which no chain can
answer: `mark_resized` descends from the widget whose box changed and marks
anything whose own box changed length and whose drawing reads it. A part of
a box with no relative extent on an axis is a fixed length, and composing
into it leaves none either, so the walk stops where a length did not change
-- an 80-wide child in a widened row is not redrawn though it says `Redraw`.

`Span`, `Pad`, `Stack`, `Offset`, `Aligned`, `SetSize` and `LayerOffset` say
`Scale`: each places in fractions and offsets of its own box and none reads
the box's pixel length. `Scroll` and `MaxSize` do read pixels and stay
`Redraw`.

45 tests pass, five of them new. Render verification comes after the CPU
side, per the owner.

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

tests/retained.rs

@@ -251,9 +251,8 @@ fn a_change_two_levels_under_its_reader_still_reaches_it() {
     assert_corners!(h, below, (12, 232), (388, 388));
 }
 
-/// Claims its drawing may be stretched, and has a child so that the stretch
-/// has to reach one. No shipped container claims `Scale` -- `Rect`, `Image`
-/// and `()` are all childless -- so nothing else walks a subtree to remap it.
+/// Claims its drawing survives its box changing length, and has a child so
+/// that the walk looking for what does not has one to reach.
 struct Stretchy {
     inner: StrongWidget,
     draws: Rc<Cell<usize>>,
@@ -271,7 +270,7 @@ impl Widget for Stretchy {
 }
 
 #[test]
-fn stretching_a_subtree_remaps_the_children_in_it() {
+fn stretching_a_subtree_carries_the_children_in_it() {
     let mut h = Harness::new((400, 400));
     let first = rect(Color::RED).height(40).add(&mut h.rsc);
     let inner = rect(Color::BLUE).add(&mut h.rsc);
@@ -291,8 +290,50 @@ fn stretching_a_subtree_remaps_the_children_in_it() {
     assert_eq!(
         draws.get(),
         settled,
-        "its drawing is stretched, not redrawn"
+        "its drawing follows its box, rather than being made again"
     );
     assert_corners!(h, outer, (0, 80), (400, 400));
     assert_corners!(h, inner, (0, 80), (400, 400));
 }
+
+#[test]
+fn a_widened_row_redraws_what_reads_its_length_and_nothing_else() {
+    let mut h = Harness::new((400, 200));
+    // What a transcript row is: something whose shaping depends on the width
+    // it is given, beside something that only has to be the right shape.
+    let (wraps, wrap_draws) = counted(&mut h, Size::REST, OnResize::Redraw);
+    let (backing, back_draws) = counted(&mut h, Size::REST, OnResize::Scale);
+    let row = (backing, wraps).span(Dir::RIGHT).add(&mut h.rsc);
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, row).span(Dir::RIGHT));
+    let (settled_wrap, settled_back) = (wrap_draws.get(), back_draws.get());
+
+    h.rsc[bar].x = Some(Len::abs(200));
+    h.frame();
+
+    // The span reads every child's size, so redrawing one takes the span
+    // with it -- and the span then measures and places the redrawn child.
+    assert!(wrap_draws.get() > settled_wrap, "reads the width it got");
+    assert_eq!(back_draws.get(), settled_back, "only has to be the shape");
+    assert_corners!(h, backing, (200, 0), (300, 200));
+    assert_corners!(h, wraps, (300, 0), (400, 200));
+}
+
+#[test]
+fn a_fixed_length_child_is_not_redrawn_when_the_box_around_it_grows() {
+    let mut h = Harness::new((400, 200));
+    // It would be drawn again for a width it does not have: its own box is
+    // a fixed 80 wherever the row's edges end up.
+    let (fixed, draws) = counted(&mut h, Size::from((80, 200)), OnResize::Redraw);
+    let (rest, _) = counted(&mut h, Size::REST, OnResize::Scale);
+    let row = (fixed, rest).span(Dir::RIGHT).add(&mut h.rsc);
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, row).span(Dir::RIGHT));
+    let settled = draws.get();
+
+    h.rsc[bar].x = Some(Len::abs(200));
+    h.frame();
+
+    assert_eq!(draws.get(), settled, "its own length did not change");
+    assert_corners!(h, fixed, (200, 0), (280, 200));
+}