Layout: Use containing block to compute scrollable overflow

Instead of using the border boxes of all descendants to compute
scrollable overflow, we use only descendants which have the box as its
containing block.

Userland/Libraries/LibWeb/Layout/LayoutState.cpp

@@ -100,9 +100,13 @@ static CSSPixelRect measure_scrollable_overflow(Box const& box)
     //   and whose border boxes are positioned not wholly in the negative scrollable overflow region,
     //   FIXME: accounting for transforms by projecting each box onto the plane of the element that establishes its 3D rendering context. [CSS3-TRANSFORMS]
     if (!box.children_are_inline()) {
-        box.for_each_child_of_type<Box>([&box, &scrollable_overflow_rect, &content_overflow_rect](Box const& child) {
+        box.for_each_in_subtree_of_type<Box>([&box, &scrollable_overflow_rect, &content_overflow_rect](Box const& child) {
             if (!child.paintable_box())
-                return IterationDecision::Continue;
+                return TraversalDecision::Continue;
+
+            if (child.containing_block() != &box) {
+                return TraversalDecision::Continue;
+            }
 
             auto child_border_box = child.paintable_box()->absolute_border_box_rect();
             // NOTE: Here we check that the child is not wholly in the negative scrollable overflow region.
@@ -123,7 +127,7 @@ static CSSPixelRect measure_scrollable_overflow(Box const& box)
                     scrollable_overflow_rect.unite_vertically(child_scrollable_overflow);
             }
 
-            return IterationDecision::Continue;
+            return TraversalDecision::Continue;
         });
     } else {
         box.for_each_child([&scrollable_overflow_rect, &content_overflow_rect](Node const& child) {