Replace placement calls with region nodes

tests/layout.rs

@@ -119,8 +119,8 @@ fn a_resize_lands_where_a_cold_start_would() {
 #[test]
 fn a_fixed_box_is_drawn_again_rather_than_stretched() {
     let mut h = Harness::new((400, 400));
-    // The panel fills a stack sized by its sibling, so it is drawn in the
-    // whole box and then placed in the shorter one. Reusing it in that fixed
+    // The panel fills a stack sized by its sibling, so it is first asked in
+    // the whole box and then given the shorter one. Reusing it in that fixed
     // box afterwards would leave it whatever height it happened to have.
     let panel = rect(Color::BLUE).add(&mut h.rsc);
     let leaf = rect(Color::RED).height(100).add(&mut h.rsc);
@@ -142,15 +142,15 @@ fn a_moved_subtree_takes_its_children_with_it() {
     let mut h = Harness::new((400, 400));
     let first = rect(Color::RED).height(40).add(&mut h.rsc);
     let inner = rect(Color::BLUE).add(&mut h.rsc);
-    let row = inner.pad(10).height(40).add(&mut h.rsc);
+    let row = inner.pad(10).height(40).region_node().add(&mut h.rsc);
     h.set_root((first, row).span(Dir::DOWN));
     assert_corners!(h, inner, (10, 50), (390, 70));
 
     h.rsc[first].y = Some(Len::px(80));
     h.frame();
 
-    // The row is the same shape somewhere else, so one slot moved it and
-    // `inner`'s own region was never rewritten.
+    // The row opted into one movable region, so its descendants follow one
+    // entry rather than having their primitive regions rewritten.
     assert_corners!(h, inner, (10, 90), (390, 110));
 }
 
@@ -202,21 +202,29 @@ fn a_box_with_a_fixed_length_can_be_stretched_on_its_other_axis() {
 }
 
 #[test]
-fn only_a_container_that_places_its_children_lengthens_the_chain() {
+fn only_a_region_node_lengthens_the_chain_and_it_can_be_removed() {
     let mut h = Harness::new((400, 200));
     let leaf = rect(Color::BLUE).add(&mut h.rsc);
-    // Four widgets between the span and the leaf, none of which places what
-    // it draws, so all of them share the span's slot.
     let buried = leaf.pad(4).pad(4).pad(4).pad(4).add(&mut h.rsc);
     let bar = rect(Color::RED).width(100).add(&mut h.rsc);
     h.set_root((bar, buried).span(Dir::RIGHT));
 
-    let slot = h.render.active[&leaf.id()].parent_move;
+    let move_idx = h.render.active[&leaf.id()].parent_move;
+    assert_eq!(h.render.moves.depth(move_idx), 1, "only the root region");
+
+    h.rsc.widgets_mut().set_region_node(buried, true);
+    h.frame();
+    let move_idx = h.render.active[&leaf.id()].parent_move;
     assert_eq!(
-        h.render.moves.depth(slot),
+        h.render.moves.depth(move_idx),
         2,
-        "the span above the leaf, and the root the window is held in"
+        "the opted-in widget's region and the root region"
     );
+
+    h.rsc.widgets_mut().set_region_node(buried, false);
+    h.frame();
+    let move_idx = h.render.active[&leaf.id()].parent_move;
+    assert_eq!(h.render.moves.depth(move_idx), 1);
 }
 
 /// A span that sizes from its children passes their `leftover` weight up
@@ -412,3 +420,21 @@ fn only_a_pure_leftover_child_disappears_when_nothing_is_left() {
     // is leftover is omitted.
     assert_corners!(h, mixed, (100, 0), (120, 20));
 }
+
+#[test]
+fn leftover_children_disappear_at_the_exact_fixed_content_boundary() {
+    let mut h = Harness::new((100, 100));
+    let first = rect(Color::RED).height(90).add(&mut h.rsc);
+    let a = rect(Color::GREEN).add(&mut h.rsc);
+    let b = rect(Color::BLUE).add(&mut h.rsc);
+    let inner = (a, b).span(Dir::DOWN).gap(4).add(&mut h.rsc);
+    h.set_root((first, inner).span(Dir::DOWN));
+    assert!(h.region(&a).is_some());
+    assert!(h.region(&b).is_some());
+
+    h.rsc[first].y = Some(Len::px(96.0));
+    h.frame();
+
+    assert!(h.region(&a).is_none());
+    assert!(h.region(&b).is_none());
+}