LibWeb: Don't paint children stacking contexts inside the current one

Author: https://github.com/Cleverking2003
Commit: 495f69b76b
Pull-request: https://github.com/SerenityOS/serenity/pull/8956
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/alimpfard

Userland/Libraries/LibWeb/Painting/StackingContext.cpp

@@ -32,6 +32,8 @@ StackingContext::StackingContext(Box& box, StackingContext* parent)
 void StackingContext::paint_descendants(PaintContext& context, Node& box, StackingContextPaintPhase phase)
 {
     box.for_each_child([&](auto& child) {
+        if (child.establishes_stacking_context())
+            return;
         switch (phase) {
         case StackingContextPaintPhase::BackgroundAndBorders:
             if (!child.is_floating() && !child.is_positioned()) {