Print the box a widget drew in, not the rel base labelled as it

`DrawInfo::px` was the child's rel base in pixels, resolved at all four
construction sites on every draw and read only by three diagnostics --
each of which called it the box: `diag::draw_request`'s `pixel_size`,
printed by `trace_unsettled` as "draw in"; and two `debug_assert`
messages saying "clips to" and "drew in". A rel base and a box differ
wherever a parent hands down part of its own, which is every child of a
span, so all three said something that was not true.

The field is gone and each site reads `region.to_px(window)`, which is
the box it claimed to be printing and costs nothing outside a failing
assert. The trace field is `region_px`. `Placing::window` existed only
to resolve that value and follows it out.

The 23-line counter block inside `try_reuse`'s "outside its range"
branch is `diag::outside`, beside the other diagnostics, so the decision
reads as its six checks.

fmt, workspace clippy under `-D warnings` with and without
`layout-diagnostics`, and the workspace tests under both are clean. The
cold dump over 400 depth-5 trees is byte-identical: 34,492 boxes. The
trace now prints "draw in 189.00x176.00" beside a region 189 by 176.

core/src/layout_diagnostics.rs

@@ -15,7 +15,7 @@
 //! reuse, size, placement, and text events for one suspicious widget. The
 //! selection is a set and survives [`take`] until cleared.
 
-use crate::{Axis, LayoutLen, PxVec2, Size, UiRegion, WidgetId};
+use crate::{Axis, LayoutHolds, LayoutLen, PxVec2, Size, UiRegion, UiVec2, WidgetId};
 use std::{
     cell::RefCell,
     collections::{HashMap, HashSet},
@@ -264,7 +264,7 @@ pub enum TraceEvent {
         id: WidgetId,
         parent: Option<WidgetId>,
         region: UiRegion,
-        pixel_size: PxVec2,
+        region_px: PxVec2,
         region_node: bool,
     },
     Reuse {
@@ -360,7 +360,7 @@ pub(crate) fn draw_request(
     id: WidgetId,
     parent: Option<WidgetId>,
     region: UiRegion,
-    pixel_size: PxVec2,
+    region_px: PxVec2,
     region_node: bool,
 ) {
     trace(
@@ -369,7 +369,7 @@ pub(crate) fn draw_request(
             id,
             parent,
             region,
-            pixel_size,
+            region_px,
             region_node,
         },
     );
@@ -379,6 +379,39 @@ pub(crate) fn reuse(id: WidgetId, outcome: ReuseOutcome) {
     trace(id, TraceEvent::Reuse { id, outcome });
 }
 
+/// A drawing that cannot be reused because the box on offer is outside what
+/// it holds for, and which of the three contracts said so. They overlap: a
+/// drawing can be outside two of them at once, and counting each is what
+/// says where a rel base redrawing more than it should is coming from.
+pub(crate) fn outside(
+    id: WidgetId,
+    holds: LayoutHolds,
+    region: UiRegion,
+    rel_base: UiVec2,
+    window: PxVec2,
+) {
+    for axis in Axis::BOTH {
+        let holds = holds[axis];
+        let len = region[axis].len();
+        let window = window[axis];
+        if holds.region_len.is_some_and(|pinned| pinned != len) {
+            bump(Counter::OutsidePinnedLen);
+        }
+        if !holds.window.contains(window)
+            || holds
+                .rel_base
+                .is_some_and(|pinned| pinned != rel_base[axis])
+        {
+            bump(Counter::OutsideRelBase);
+        }
+        if !holds.region.contains(len.to_px(window)) {
+            bump(Counter::OutsideRegion);
+        }
+    }
+    bump(Counter::ReuseOutside);
+    reuse(id, ReuseOutcome::Outside);
+}
+
 pub(crate) fn size_reported(id: WidgetId, size: Size) {
     trace(id, TraceEvent::SizeReported { id, size });
 }

core/src/ui/painter.rs

@@ -196,7 +196,6 @@ impl<'a> Painter<'a> {
         if !re_asked {
             self.children.push(id.id());
         }
-        let px = rel_base.to_px(self.window);
         let drawn = self.state.draw_inner(
             id.id(),
             DrawInfo {
@@ -211,7 +210,6 @@ impl<'a> Painter<'a> {
                 placed: place,
                 asked: place,
                 re_asked,
-                px,
             },
             None,
             self.rsc,
@@ -280,7 +278,6 @@ impl<'a> Painter<'a> {
             id: self.id,
             region: self.region,
             rel_base: self.rel_base,
-            window: self.window,
             depth: self.depth,
             move_idx: self.move_idx,
             mask: self.mask,

core/src/ui/render_state.rs

@@ -30,8 +30,6 @@ pub(super) struct DrawInfo {
     pub asked: PlaceDesc,
     /// Whether the parent already asked about this widget in this draw.
     pub re_asked: bool,
-    /// The rel base in pixels, resolved once against the window.
-    pub px: PxVec2,
 }
 
 /// What one draw of a widget came to: the answer it gave, and the boxes and
@@ -48,7 +46,6 @@ pub(super) struct Placing {
     pub id: WidgetId,
     pub region: UiRegion,
     pub rel_base: UiVec2,
-    pub window: PxVec2,
     pub depth: usize,
     pub move_idx: MoveIdx,
     pub mask: MaskIdx,
@@ -130,7 +127,6 @@ impl UiRenderState {
     /// The root is asked about in the output. Its own rules narrow both its
     /// rel base and box; nothing above it chose a different one.
     fn root_info(&self, rel_base: UiVec2, region: UiRegion) -> DrawInfo {
-        let px = rel_base.to_px(self.output_size);
         DrawInfo {
             layer: 0,
             parent: None,
@@ -143,7 +139,6 @@ impl UiRenderState {
             placed: PlaceDesc::WHOLE,
             asked: PlaceDesc::WHOLE,
             re_asked: false,
-            px,
         }
     }
 
@@ -222,7 +217,13 @@ impl UiRenderState {
         #[cfg(feature = "layout-diagnostics")]
         {
             diag::bump(Counter::DrawRequests);
-            diag::draw_request(id, info.parent, region, info.px, info.region_node);
+            diag::draw_request(
+                id,
+                info.parent,
+                region,
+                region.to_px(self.output_size).size(),
+                info.region_node,
+            );
         }
         let align = rsc.widgets().alignment(id);
         let declared = rsc.widgets().declared_lens(id);
@@ -312,8 +313,6 @@ impl UiRenderState {
             .and_then(|old| old.mask_region.map(|_| old.mask));
         let old_children = old.map_or_else(Vec::new, |old| old.children);
         rsc.widgets_mut().needs_redraw.remove(&id);
-        let px = info.px;
-
         let window = self.output_size;
         let mut painter = Painter {
             state: self,
@@ -409,8 +408,9 @@ impl UiRenderState {
                     self.output_size,
                     axis
                 )),
-            "'{}' ({id:?}) clips to {px:?} and reports {size}",
+            "'{}' ({id:?}) clips to {} and reports {size}",
             rsc.widgets().label(id),
+            region.to_px(window),
         );
         for c in &old_children {
             if !children.contains(c) {
@@ -440,8 +440,9 @@ impl UiRenderState {
             .fold(answer_holds, |holds, (_, child)| holds.and(child));
         debug_assert!(
             holds.contains(self.output_size, info.rel_base, region),
-            "'{}' ({id:?}) drew in {px:?}, outside the ranges it reported: {holds:?}",
+            "'{}' ({id:?}) drew in {}, outside the ranges it reported: {holds:?}",
             rsc.widgets().label(id),
+            region.to_px(window),
         );
         // What it asked about and did not draw is still something it asked,
         // and a change there has to reach it. Asking answered whatever mark
@@ -462,7 +463,6 @@ impl UiRenderState {
                         placed: PlaceDesc::WHOLE,
                         asked: PlaceDesc::WHOLE,
                         re_asked: false,
-                        px,
                     },
                     rsc,
                 );
@@ -612,31 +612,7 @@ impl UiRenderState {
             .contains(self.output_size, info.rel_base, region)
         {
             #[cfg(feature = "layout-diagnostics")]
-            {
+            diag::outside(id, active.holds, region, info.rel_base, self.output_size);
-                // Which of the three said no, so a rel base that redraws more
-                // than it should says where to look. They overlap: a drawing
-                // can be outside two of them at once.
-                for axis in Axis::BOTH {
-                    let holds = active.holds[axis];
-                    let len = region[axis].len();
-                    let window = self.output_size[axis];
-                    if holds.region_len.is_some_and(|pinned| pinned != len) {
-                        diag::bump(Counter::OutsidePinnedLen);
-                    }
-                    if !holds.window.contains(window)
-                        || holds
-                            .rel_base
-                            .is_some_and(|pinned| pinned != info.rel_base[axis])
-                    {
-                        diag::bump(Counter::OutsideRelBase);
-                    }
-                    if !holds.region.contains(len.to_px(window)) {
-                        diag::bump(Counter::OutsideRegion);
-                    }
-                }
-                diag::bump(Counter::ReuseOutside);
-                diag::reuse(id, ReuseOutcome::Outside);
-            }
             return false;
         }
         self.relocate(id, placed, info, rsc);
@@ -718,7 +694,6 @@ impl UiRenderState {
             placed: place,
             asked: active.asked,
             re_asked: active.re_asked,
-            px: rel_base.to_px(at.window),
         };
         self.relocate(child, placed, info, rsc);
     }
@@ -749,7 +724,6 @@ impl UiRenderState {
             id,
             region: placed,
             rel_base: info.rel_base,
-            window: self.output_size,
             depth: info.depth,
             move_idx: active.move_idx,
             mask: active.mask,
@@ -1116,7 +1090,6 @@ impl UiRenderState {
             placed: active.asked,
             asked: active.asked,
             re_asked: false,
-            px: rel_base.to_px(self.output_size),
         };
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::LocalRedraws);
@@ -1165,7 +1138,6 @@ impl UiRenderState {
             id,
             region,
             rel_base: active.rel_base,
-            window: self.output_size,
             depth: active.depth,
             move_idx: active.move_idx,
             mask: active.mask,

tests/trace_unsettled.rs

@@ -42,12 +42,12 @@ fn dump(label: &str, report: &diag::Report, text: WidgetId) {
             TraceEvent::DrawRequest {
                 id,
                 region,
-                pixel_size,
+                region_px,
                 ..
             } if *id == text => {
                 println!(
                     "  draw in {:.2}x{:.2}  region {region:?}",
-                    pixel_size.x, pixel_size.y
+                    region_px.x, region_px.y
                 )
             }
             TraceEvent::SizeReported { id, size } if *id == text => {