Answer "have I asked this child?" in one read

A container's draw asked it once per child by searching the list of children
it had added so far, and four other per-child steps searched a list too, so
one draw cost the square of its children: 70% of a 1,600-child redraw was
those searches. A draw takes a DrawId and leaves it on every widget it asks
about; one note per widget is enough because the handle a container holds a
child by cannot be cloned. tests/children_cost.rs is the rig that shows it,
and it is the only one here that varies width: 3.680 ms to 0.811 ms at 1,600
children, and flat per child.

Beside it, the rest of the fourteenth sweep of #19: a mask's rectangle
resolved once per fragment instead of once per instance, which takes the
storage buffers out of the fragment stage and is 8.8x on a screenful of
deeply nested clips; TextBuffer::shape copying its attrs before the check
that would not need them, which allocated once per named-family text per
frame; a should_panic test on a debug assertion that made cargo test
--release fail; Fixed::div, reached only by its own test; Moves::remove
re-uploading an array it cannot have changed; and two comments the branch
itself falsified.

docs/LAYOUT_LOG.md has all eight with their measurements, the five things
looked at and left, and what was verified.

tests/cases/retained.rs

@@ -1563,3 +1563,30 @@ fn a_contract_this_window_is_outside_is_not_kept() {
         "the leaf settled once and its parent kept what it settled"
     );
 }
+
+/// A container that measures a child by drawing it, takes that drawing back,
+/// and then places the child, which `place_at` answers by asking again: taking
+/// a child back takes it out of the child list, and the note on the child
+/// saying it is in there has to go with it, or the placement re-expresses a
+/// drawing that no longer exists.
+#[test]
+fn a_child_taken_back_and_placed_again_is_asked_again() {
+    struct Retake(StrongWidget);
+
+    impl Widget for Retake {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter.widget(&self.0);
+            painter.undraw(&self.0);
+            painter.place_at(&self.0, PlaceDesc::WHOLE).size()
+        }
+    }
+
+    let mut h = Harness::new((600, 200));
+    let child = rect(Color::RED).add_strong(&mut h.rsc);
+    let id = child.id();
+    let root = Retake(child).add(&mut h.rsc);
+    h.set_root(root);
+    h.frame();
+
+    assert_corners!(h, id, (0, 0), (600, 200));
+}