LibWeb: Paint positioned elements after non-positioned ones

Within the same stacking context, positioned elements must be painted
after non-positioned ones.

I added a Layout::Node::for_each_child_in_paint_order() to help with
this since it's also needed for hit testing.

Libraries/LibWeb/Layout/Box.cpp

@@ -106,9 +106,7 @@ HitTestResult Box::hit_test(const Gfx::IntPoint& position, HitTestType type) con
     //        parts of the layout tree, but currently we can't just check
     //        m_rect.contains() since inline text rects can't be trusted..
     HitTestResult result { absolute_rect().contains(position.x(), position.y()) ? this : nullptr };
-    for_each_child([&](auto& child) {
+    for_each_child_in_paint_order([&](auto& child) {
-        if (is<Box>(child) && downcast<Box>(child).stacking_context())
-            return;
         auto child_result = child.hit_test(position, type);
         if (child_result.layout_node)
             result = child_result;

Libraries/LibWeb/Layout/Node.cpp

@@ -93,9 +93,7 @@ void Node::paint(PaintContext& context, PaintPhase phase)
 
     before_children_paint(context, phase);
 
-    for_each_child([&](auto& child) {
+    for_each_child_in_paint_order([&](auto& child) {
-        if (child.is_box() && downcast<Box>(child).stacking_context())
-            return;
         child.paint(context, phase);
     });
 
@@ -105,11 +103,7 @@ void Node::paint(PaintContext& context, PaintPhase phase)
 HitTestResult Node::hit_test(const Gfx::IntPoint& position, HitTestType type) const
 {
     HitTestResult result;
-    for_each_child([&](auto& child) {
+    for_each_child_in_paint_order([&](auto& child) {
-        // Skip over children that establish their own stacking context.
-        // The outer loop who called us will take care of those.
-        if (is<Box>(child) && downcast<Box>(child).stacking_context())
-            return;
         auto child_result = child.hit_test(position, type);
         if (child_result.layout_node)
             result = child_result;

Libraries/LibWeb/Layout/Node.h

@@ -177,6 +177,23 @@ public:
     SelectionState selection_state() const { return m_selection_state; }
     void set_selection_state(SelectionState state) { m_selection_state = state; }
 
+    template<typename Callback>
+    void for_each_child_in_paint_order(Callback callback) const
+    {
+        for_each_child([&](auto& child) {
+            if (is<Box>(child) && downcast<Box>(child).stacking_context())
+                return;
+            if (!child.is_positioned())
+                callback(child);
+        });
+        for_each_child([&](auto& child) {
+            if (is<Box>(child) && downcast<Box>(child).stacking_context())
+                return;
+            if (child.is_positioned())
+                callback(child);
+        });
+    }
+
 protected:
     Node(DOM::Document&, DOM::Node*);