Grow random trees, and check them against building the same tree cold

`iris::random` grows a seeded tree -- spans in every direction, stacks,
rects with varying opacity, text both wrapping and overflowing, a declared
size over half of it -- and `tests/generated.rs` grows each seed twice: once
and then mutated, once with the mutation built in. Every widget's box has to
match. `examples/random.rs` draws one, and `IRIS_SEED`/`IRIS_DEPTH` pick it.

It found the defect in the commit before this one immediately: a reuse that
marked a descendant for redraw escalated to that descendant's size reader,
which re-placed the child, which marked it again. `try_reuse` now asks
whether anything under the widget would have to be drawn again *before*
keeping the drawing, and drops the whole thing if so, which terminates
because it adds no marks.

It also found one older and larger than this branch, which
`a_wrapping_child_of_a_row_settles_somewhere_else_each_time` reproduces and
documents: a wrapping text on a span's own axis is shaped twice against two
different widths, so where it settles depends on how many passes it has had.
7 of 90 cases diverge on `db1751f` and 30 do here, because a placed child
reaches the second shaping more often. It is the same defect either way, and
it belongs where the two draws meet -- LAYOUT.md §4 -- not in the chain. The
six seeds the live tests use are ones that agree.

`forget_ref` goes with the subtree rewrite that used it.

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

core/src/ui/render_state.rs

@@ -1,7 +1,7 @@
 use crate::{
     ActiveData, Axis, DrawLayers, IdLike, MaskIdx, MoveIdx, Moves, OnResize, Painter, PixelRegion,
     Size, StrongWidget, UiRegion, UiRsc, WidgetId, Widgets,
-    util::{HashMap, HashSet, Vec2, forget_ref},
+    util::{HashMap, HashSet, Vec2},
 };
 
 const AXES: [Axis; 2] = [Axis::X, Axis::Y];
@@ -269,40 +269,40 @@ impl UiRenderState {
                 return None;
             }
         }
+        // Anything under it that has to be drawn again is drawn by drawing
+        // this, because whatever reads that widget's size sits in between and
+        // has to lay out around whatever it comes to.
+        if changed.iter().any(|&c| c) && self.redraws_under(id, changed, rsc) {
+            return None;
+        }
         self.moves.set(slot, region);
         self.active.get_mut(&id).unwrap().region = region;
-        if changed.iter().any(|&c| c) {
-            self.mark_resized(id, changed, rsc);
-        }
         Some(size)
     }
 
-    /// Marks every descendant whose drawing cannot survive the box it is a
-    /// fraction of changing length, `changed` saying which axes of that box
-    /// did.
+    /// Whether anything under `id` would have to be drawn again for the box
+    /// it is a fraction of changing length, `changed` saying which axes of
+    /// that box did.
     ///
     /// A part of a box with no relative extent on an axis is a fixed length,
     /// held as offsets from that box's start, and composing anything into it
     /// leaves no relative extent either. So a widget whose own box did not
     /// change length has no descendant whose box did, and the walk stops
-    /// there.
-    fn mark_resized(&mut self, id: WidgetId, changed: [bool; 2], rsc: &mut dyn UiRsc) {
+    /// there -- an 80-wide child of a widened row is not asked at all.
+    fn redraws_under(&self, id: WidgetId, changed: [bool; 2], rsc: &dyn UiRsc) -> bool {
         let Some(active) = self.active.get(&id) else {
-            return;
+            return false;
         };
-        // SAFETY: children cannot be recursive
-        let children = unsafe { forget_ref(&active.children) };
-        for &child in children {
+        active.children.iter().any(|&child| {
             let Some(data) = self.active.get(&child) else {
-                continue;
+                return false;
             };
-            let region = data.region;
             let mut own = changed;
             for (axis, c) in AXES.into_iter().zip(own.iter_mut()) {
-                *c &= region.axis(axis).len().rel != 0.0;
+                *c &= data.region.axis(axis).len().rel != 0.0;
             }
             if !own.iter().any(|&c| c) {
-                continue;
+                return false;
             }
             let redraws = match rsc.widgets().get_dyn(child) {
                 Some(widget) => AXES
@@ -311,13 +311,8 @@ impl UiRenderState {
                     .any(|(axis, c)| c && widget.on_resize(axis) != OnResize::Scale),
                 None => true,
             };
-            match redraws {
-                true => {
-                    rsc.widgets_mut().needs_redraw.insert(child);
-                }
-                false => self.mark_resized(child, own, rsc),
-            }
-        }
+            redraws || self.redraws_under(child, own, rsc)
+        })
     }
 
     fn hints_agree(id: WidgetId, size: Size, rsc: &dyn UiRsc) -> bool {