Read the marks rather than the queue to decide the walk is done

Review of the two commits above. The queue was the walk's only record of
what was left, so a mark that reached `needs_redraw` without going through
`mark` -- an `on_undraw` handler is the reachable one -- would have waited
for the next frame. The set is read again once the queue drains, which is
what the scan it replaced did for free. `pop_last` takes the deepest entry
in one step rather than reading and then removing it.

The rest is comments: nine lines shorter, and the arm that takes an
ordinary ask said only what it does for a declared length.

Unchanged by all of it: 109 suite and 20 core tests, the four fuzzer runs
(100 seeds, 400 trees at depth 5, 1000 at depth 6, 2000 at depth 4), the
five reference renders and the resized `tabs`, and every counter on the
diagnostics rig.

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

core/src/ui/painter.rs

@@ -185,12 +185,10 @@ impl<'a> Painter<'a> {
     /// resolves declared lengths and reports against that frame, then places
     /// its drawing by its own alignment.
     ///
-    /// `DrawRegion::Extent` gives a part of where this widget's own drawing
-    /// sits instead, which is what a container whose children belong inside
-    /// its drawing rather than inside the box it was offered wants. The
-    /// child's box then follows the extent without this widget's drawing
-    /// depending on where that extent is, so moving it re-places the child
-    /// rather than drawing this widget again.
+    /// `DrawRegion::Extent` gives a part of where this widget's drawing sits
+    /// instead, for a container whose children belong inside that rather than
+    /// inside the box it was offered. The part is what is kept, so moving the
+    /// extent re-places the child rather than drawing this widget again.
     pub fn widget_within<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
@@ -322,20 +320,18 @@ impl<'a> Painter<'a> {
                             result.placement = Some(self.placement);
                         }
                     }
-                    // Its box is a part of this widget's extent, so both what
-                    // it was given and what it took of that are ranges on the
-                    // extent and none of them a range on the frame. That is
-                    // what lets this widget's drawing move without being made
-                    // again: only the part's length reaches the child, and
-                    // where the part sits is re-placed rather than redrawn.
+                    // Its box is a part of this widget's extent, so what it
+                    // holds for is a range on that extent and none of it a
+                    // range on the frame. Only the part's length reaches it,
+                    // which is what lets the extent move without a redraw.
                     Some(ExtentPlacement::Within(part)) if declared[n].is_none() => {
                         result.extent[n] = holds.frame[n]
                             .and(holds.extent[n].through(chosen))
                             .through(part.axis(axis).len());
                     }
-                    // A declared length is a length of this widget's frame
-                    // wherever the box it sits in came from, so what the child
-                    // holds for is a range on the frame either way.
+                    // Its box is a length of this widget's frame: an
+                    // ordinary ask, or a declared length, which is that
+                    // length wherever the box it sits in came from.
                     _ => {
                         result.frame[n] = holds.frame[n].through(local.axis(axis).len()).and(
                             holds.extent[n]
@@ -349,12 +345,10 @@ impl<'a> Painter<'a> {
         };
         self.under = self.under.and(in_parent(holds));
         let mut answer_holds = in_parent(answer_holds);
-        // What it reports is a fraction of the box it was given, and that box
-        // is a part of this widget's extent -- so the same fraction is a
-        // different length once the extent is. Only the report: where the
-        // extent moved without changing what it holds, the drawing under it
-        // is re-placed rather than made again, which is what the extent ask
-        // is for. Pixels come up unchanged and say nothing.
+        // What it reports is a fraction of the box it was given, which is a
+        // part of this widget's extent -- so the same fraction is a different
+        // length once that extent is, and pixels are not. The answer only:
+        // the drawing this holds is re-placed rather than made again.
         if matches!(extent, Some(ExtentPlacement::Within(_)))
             && AXES.into_iter().any(|axis| {
                 declared[axis as usize].is_none() && size.axis(axis).rel != crate::Rel::ZERO