Make alignment a widget property

src/widget/position/stack.rs

@@ -13,18 +13,20 @@ impl Widget for Stack {
             StackSize::Default => None,
             StackSize::Child(i) => Some(i),
         };
-        let mut size = Size::default();
+        // Whichever child sizes the stack decides the box every child gets.
+        // The stack reports that size, so a child given a longer box would
+        // draw outside what the stack says it occupies.
+        let size = match sizing.and_then(|i| self.children.get(i)) {
+            Some(child) => painter.widget(child).size(),
+            None => Size::LEFTOVER,
+        };
+        let region = painter.box_of(size);
         for (i, child) in self.children.iter().enumerate() {
             match i {
                 0 => painter.child_layer(),
                 _ => painter.next_layer(),
             }
-            let drawn = painter.widget(child);
-            // Only the child that sizes the stack is read, so the others
-            // changing size does not redraw it.
-            if sizing == Some(i) {
-                size = drawn.size();
-            }
+            painter.widget_aligned(child, region, RegionAlign::NEAR);
         }
         size
     }