Replace placement calls with region nodes

tests/retained.rs

@@ -111,6 +111,28 @@ fn a_leaf_that_ignores_its_box_is_not_drawn_again_when_the_box_changes() {
     assert_corners!(h, second, (150, 0), (400, 200));
 }
 
+#[test]
+fn moving_an_ordinary_subtree_remaps_its_mask() {
+    let mut h = Harness::new((400, 200));
+    let (first, _) = counted(&mut h, Size::from((100, 200)), false);
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let masked = inner.masked().add(&mut h.rsc);
+    h.set_root((first, masked).span(Dir::RIGHT));
+
+    h.rsc[first].size = Size::from((150, 200));
+    h.frame();
+
+    let active = &h.render.active[&masked.id()];
+    assert_eq!(
+        h.rsc.ui().masks[active.mask.idx()].region,
+        UiRegion::new(
+            UiSpan::new(UiScalar::px(150.0), UiScalar::rel_max()),
+            UiSpan::FULL,
+        )
+    );
+    assert_corners!(h, inner, (150, 0), (400, 200));
+}
+
 #[test]
 fn a_leaf_that_depends_on_its_box_is_drawn_again_when_the_box_changes() {
     let mut h = Harness::new((400, 200));
@@ -121,7 +143,7 @@ fn a_leaf_that_depends_on_its_box_is_drawn_again_when_the_box_changes() {
     h.frame();
 
     // The preceding fixed child makes the remaining box this child's real
-    // box, so measuring it also draws it in its final place.
+    // box, so measuring it also draws it in its final box.
     assert_eq!(draws.get(), settled + 1);
     assert_corners!(h, second, (150, 0), (400, 200));
 }
@@ -132,7 +154,7 @@ fn a_span_child_that_declares_its_length_is_drawn_once() {
     let (told, told_draws) = counted(&mut h, Size::from((100, 200)), false);
     let (asked, asked_draws) = counted(&mut h, Size::from((100, 200)), true);
     // The span takes one child's length from its hint and has to draw the
-    // other to find out, so only the second is drawn before it is placed.
+    // other to find out, so only the second is drawn before its final box.
     let hinted = told.width(100).add(&mut h.rsc);
     h.set_root((hinted, asked).span(Dir::RIGHT));
 
@@ -140,7 +162,7 @@ fn a_span_child_that_declares_its_length_is_drawn_once() {
     assert_eq!(
         asked_draws.get(),
         2,
-        "drawn to be measured, then again to be placed"
+        "drawn to be measured, then again in its final box"
     );
 }
 
@@ -173,10 +195,10 @@ fn a_repaint_that_keeps_its_size_does_not_relay_out() {
 }
 
 #[test]
-fn a_placed_child_survives_the_next_frame() {
+fn a_span_child_survives_the_next_frame() {
     let mut h = Harness::new((400, 200));
-    // Both children declare a length, so the span places them from their hints
-    // rather than drawing them to find out.
+    // Both children declare a length, so the span chooses their boxes from
+    // hints rather than drawing them to find out.
     let top = rect(Color::RED).height(80).add(&mut h.rsc);
     let bottom = rect(Color::BLUE).height(120).add(&mut h.rsc);
     h.set_root((top, bottom).span(Dir::DOWN));