Hold the output as the box every chain bottoms out in

A position was composed up the slot chain to a normalized region and then
multiplied by the output's size, so the window was the one box in the
system that was not a box. Seeding the chain with a root slot holding it
in pixels makes composing through it leave everything below in pixels,
which is what the multiplication was doing.

`within` already does the arithmetic: a child at `rel` 1 inside a span of
`px` 0 to `px` 1920 composes to `px` 1920 and `rel` 0, so the trailing
`to_px` becomes the identity rather than a step. The shader walks the
same chain and needs no change for the same reason.

This is the shape the resize machinery wants before it can be deleted: a
resize becomes one slot written, which `try_reuse` and `redraws_under`
already carry. Nothing is removed yet.

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

core/src/ui/render_state.rs

@@ -2,7 +2,7 @@
 use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
 use crate::{
     ActiveData, Axis, DrawLayers, IdLike, MaskIdx, MoveIdx, Moves, OnResize, Painter, PixelRegion,
-    Size, StrongWidget, UiRegion, UiRsc, WidgetId, Widgets,
+    Size, StrongWidget, UiRegion, UiRsc, UiScalar, UiSpan, WidgetId, Widgets,
     util::{HashMap, HashSet, Vec2},
 };
 
@@ -19,6 +19,8 @@ pub struct UiRenderState {
     pub(super) output_size: Vec2,
 
     old_root: Option<WidgetId>,
+    /// The slot every chain bottoms out in, holding the output as a box.
+    root_move: MoveIdx,
     resized: [bool; 2],
     /// Content/state dirtiness whose retained size cannot answer a layout
     /// question until that widget has drawn again.
@@ -50,6 +52,22 @@ impl UiRenderState {
             draw_started: Default::default(),
             slots: Default::default(),
             moves: Default::default(),
+            root_move: MoveIdx::NONE,
+        }
+    }
+
+    /// The window as a box, so a chain bottoms out in one rather than in a
+    /// multiplication applied after it. Composing through a box held in
+    /// pixels leaves everything below it in pixels, which is why nothing
+    /// downstream has to know the output's size to resolve a position.
+    fn write_root(&mut self) {
+        let region = UiRegion::new(
+            UiSpan::new(UiScalar::ZERO, UiScalar::px(self.output_size.x)),
+            UiSpan::new(UiScalar::ZERO, UiScalar::px(self.output_size.y)),
+        );
+        match self.root_move == MoveIdx::NONE {
+            true => self.root_move = self.moves.push(MoveIdx::NONE, region),
+            false => self.moves.set(self.root_move, region),
         }
     }
 
@@ -59,6 +77,7 @@ impl UiRenderState {
             *resized |= size.axis(axis) != self.output_size.axis(axis);
         }
         self.output_size = size;
+        self.write_root();
     }
 
     pub fn output_size(&self) -> Vec2 {
@@ -149,6 +168,7 @@ impl UiRenderState {
         let _layout = diag::timer(TimerKind::FullLayout);
         self.clear(rsc);
         // free all resources & cache
+        self.write_root();
         if let Some(id) = root {
             self.draw_inner(
                 0,
@@ -156,7 +176,7 @@ impl UiRenderState {
                 UiRegion::FULL,
                 None,
                 1,
-                MoveIdx::NONE,
+                self.root_move,
                 false,
                 MaskIdx::NONE,
                 None,
@@ -589,6 +609,7 @@ impl UiRenderState {
         }
         self.slots.clear();
         self.moves.clear();
+        self.root_move = MoveIdx::NONE;
         self.layers.clear();
         self.invalid_sizes.clear();
         self.resize_marks.clear();

tests/layout.rs

@@ -180,5 +180,9 @@ fn only_a_container_that_places_its_children_lengthens_the_chain() {
     h.set_root((bar, buried).span(Dir::RIGHT));
 
     let slot = h.render.active[&leaf.id()].parent_move;
-    assert_eq!(h.render.moves.depth(slot), 1, "one span above the leaf");
+    assert_eq!(
+        h.render.moves.depth(slot),
+        2,
+        "the span above the leaf, and the root the window is held in"
+    );
 }