Replace placement calls with region nodes

core/src/ui/active.rs

@@ -32,14 +32,14 @@ pub struct ActiveData {
     pub children: Vec<WidgetId>,
     /// The children whose size this widget read while drawing.
     pub size_deps: Vec<WidgetId>,
-    /// The slot its primitives are positioned through: its own if its parent
-    /// placed it, otherwise the nearest ancestor that has one.
+    /// The movable region its primitives are positioned through: its own when
+    /// opted in, otherwise the nearest ancestor's.
     pub move_idx: MoveIdx,
     /// The declared lengths whoever drew this widget resolved into its box.
     /// A change to one moves a box this widget cannot fix by drawing again,
     /// and comparing them is what says so.
     pub declared: [Option<Len>; 2],
-    /// The slot `region` is given in, which is whatever its parent drew in.
+    /// The movable region whose coordinates `region` uses.
     pub parent_move: MoveIdx,
     pub mask: MaskIdx,
     pub layer: LayerId,