LibWeb: Add stacking contexts to Node::for_each_child_in_paint_order

The existing implementation, which is used by Node::hit_test() and
sub-classes, does not include stacking contexts which prevents hit
testing from returning elements contained by those stacking contexts
in some situations.

This is quite rough and definitely not optimal. The stacking contexts
are not retrieved in the correct order. They should be sorted by
z-index then tree order.

This change makes DuckDuckGo technically usable with all the absolute
and relative positioning they use.

Userland/Libraries/LibWeb/Layout/Node.h

@@ -158,18 +158,67 @@ public:
     template<typename Callback>
     void for_each_child_in_paint_order(Callback callback) const
     {
+        // Element traversal using the order defined in https://www.w3.org/TR/CSS2/zindex.html#painting-order.
+        // Note: Some steps are skipped because they are not relevant to node traversal.
+
+        // 3. Stacking contexts formed by positioned descendants with negative z-indices (excluding 0) in z-index order
+        //    (most negative first) then tree order.
+        // FIXME: This does not retrieve elements in the z-index order.
+        for_each_child([&](auto& child) {
+            if (!child.is_positioned() || !is<Box>(child))
+                return;
+
+            auto& box_child = verify_cast<Box>(child);
+            auto* stacking_context = box_child.stacking_context();
+            if (stacking_context && box_child.computed_values().z_index().has_value() && box_child.computed_values().z_index().value() < 0)
+                callback(child);
+        });
+
+        // 4. For all its in-flow, non-positioned, block-level descendants in tree order: If the element is a block, list-item,
+        //    or other block equivalent:
         for_each_child([&](auto& child) {
             if (is<Box>(child) && verify_cast<Box>(child).stacking_context())
                 return;
             if (!child.is_positioned())
                 callback(child);
         });
+
+        // 5. All non-positioned floating descendants, in tree order. For each one of these, treat the element as if it created
+        //    a new stacking context, but any positioned descendants and descendants which actually create a new stacking context
+        //    should be considered part of the parent stacking context, not this new one.
         for_each_child([&](auto& child) {
             if (is<Box>(child) && verify_cast<Box>(child).stacking_context())
                 return;
             if (child.is_positioned())
                 callback(child);
         });
+
+        // 8. All positioned descendants with 'z-index: auto' or 'z-index: 0', in tree order. For those with 'z-index: auto', treat
+        //    the element as if it created a new stacking context, but any positioned descendants and descendants which actually
+        //    create a new stacking context should be considered part of the parent stacking context, not this new one. For those
+        //    with 'z-index: 0', treat the stacking context generated atomically.
+        for_each_child([&](auto& child) {
+            if (!child.is_positioned() || !is<Box>(child))
+                return;
+
+            auto& box_child = verify_cast<Box>(child);
+            auto* stacking_context = box_child.stacking_context();
+            if (stacking_context && box_child.computed_values().z_index().has_value() && box_child.computed_values().z_index().value() == 0)
+                callback(child);
+        });
+
+        // 9. Stacking contexts formed by positioned descendants with z-indices greater than or equal to 1 in z-index order
+        //    (smallest first) then tree order.
+        // FIXME: This does not retrieve elements in the z-index order.
+        for_each_child([&](auto& child) {
+            if (!child.is_positioned() || !is<Box>(child))
+                return;
+
+            auto& box_child = verify_cast<Box>(child);
+            auto* stacking_context = box_child.stacking_context();
+            if (stacking_context && box_child.computed_values().z_index().has_value() && box_child.computed_values().z_index().value() > 0)
+                callback(child);
+        });
     }
 
 protected: