LibWeb: Resolve paintable containing blocks eagerly

Resolve every paintable containing block after constructing the paint
tree instead of populating an optional weak cache on first access.

Store the resolved relationship as a non-owning pointer and make the hot
pointer accessor an inline load. Clear the pointer during relayout and
detachment, and invalidate any absolute geometry cached before commit
finishes resolving the relationship.

Author: awesomekling

Libraries/LibWeb/Layout/LayoutState.cpp

@@ -252,6 +252,19 @@ static void build_paint_tree(Node& node, Painting::Paintable* parent_paintable =
         build_paint_tree(*child, paintable_for_children);
 }
 
+void LayoutState::resolve_paintable_containing_blocks(Node& root)
+{
+    root.for_each_in_inclusive_subtree([](Node& node) {
+        auto* paintable = node.paintable_ptr();
+        if (!paintable)
+            return TraversalDecision::Continue;
+
+        auto* containing_block = node.containing_block();
+        paintable->set_containing_block(containing_block ? containing_block->paintable_ptr() : nullptr);
+        return TraversalDecision::Continue;
+    });
+}
+
 void LayoutState::commit(Box& root)
 {
     if (!root.is_viewport()) {
@@ -469,6 +482,7 @@ void LayoutState::commit_used_values_and_build_paint_tree(Box& root, RefPtr<Pain
     });
 
     build_paint_tree(root, parent_paintable.ptr(), insert_before_paintable.ptr());
+    resolve_paintable_containing_blocks(root);
 
     resolve_relative_positions();
 

Libraries/LibWeb/Layout/LayoutState.h

@@ -403,6 +403,7 @@ struct LayoutState {
     [[nodiscard]] Optional<ContainedAbsposChild> take_next_contained_abspos_child(Box const& target);
 
 private:
+    void resolve_paintable_containing_blocks(Node& root);
     void resolve_relative_positions();
 
     PagedStore<UsedValues> m_used_values_store;

Libraries/LibWeb/Painting/Paintable.cpp

@@ -84,27 +84,15 @@ DOM::Document& Paintable::document()
 
 RefPtr<Paintable> Paintable::containing_block() const
 {
-    return const_cast<Paintable*>(containing_block_ptr());
+    return m_containing_block;
 }
 
-Paintable const* Paintable::containing_block_ptr() const
+void Paintable::set_containing_block(Paintable* containing_block)
 {
-    if (m_containing_block.has_value()) {
-        if (auto* containing_block = m_containing_block->ptr())
-            return containing_block;
-    }
-
-    auto* containing_block = [&] -> Paintable const* {
-        auto containing_layout_box = layout_node().containing_block();
-        if (!containing_layout_box)
-            return nullptr;
-        auto* paintable_box = containing_layout_box->paintable_ptr();
-        if (!paintable_box)
-            return nullptr;
-        return paintable_box;
-    }();
+    if (m_containing_block == containing_block)
+        return;
     m_containing_block = containing_block;
-    return containing_block;
+    invalidate_absolute_geometry_cache(InvalidateDescendantGeometry::No);
 }
 
 CSS::ImmutableComputedValues const& Paintable::computed_values() const
@@ -979,7 +967,7 @@ Paintable::~Paintable()
 
 void Paintable::detach_from_layout_node(Badge<Layout::Node>)
 {
-    m_containing_block.clear();
+    m_containing_block = nullptr;
     m_layout_node.clear();
     detach_chrome_widgets();
 }
@@ -1064,7 +1052,7 @@ void Paintable::reset_for_relayout()
     while (first_child())
         first_child()->remove();
 
-    m_containing_block = {};
+    m_containing_block = nullptr;
 
     m_offset = {};
     m_content_size = {};

Libraries/LibWeb/Painting/Paintable.h

@@ -103,7 +103,7 @@ class WEB_API Paintable
     GC::Ptr<HTML::LocalNavigable> navigable() const;
 
     RefPtr<Paintable> containing_block() const;
-    Paintable const* containing_block_ptr() const;
+    Paintable const* containing_block_ptr() const { return m_containing_block; }
 
     template<typename T>
     bool fast_is() const = delete;
@@ -456,9 +456,9 @@ class WEB_API Paintable
     CSSPixels available_scrollbar_length(ScrollDirection direction, ChromeMetrics const& chrome_metrics) const;
     Optional<CSSPixelRect> absolute_resizer_rect(ChromeMetrics const& chrome_metrics) const;
 
-    Optional<WeakPtr<Paintable>> mutable m_containing_block;
-
 private:
+    friend struct Layout::LayoutState;
+
     struct CachedPaintData;
     enum class InvalidateDescendantGeometry {
         No,
@@ -467,6 +467,7 @@ class WEB_API Paintable
 
     void detach_from_layout_node(Badge<Layout::Node>);
     void detach_chrome_widgets();
+    void set_containing_block(Paintable* containing_block);
 
     void paint_middle_button_scroll_indicator(DisplayListRecordingContext&) const;
     void acquire_cache_references_for_cached_commands(ReadonlyBytes) const;
@@ -475,6 +476,7 @@ class WEB_API Paintable
 
     GC::Weak<DOM::Node> m_dom_node;
     WeakPtr<Layout::NodeWithStyleAndBoxModelMetrics const> m_layout_node;
+    Paintable* m_containing_block { nullptr };
 
     SelectionState m_selection_state { SelectionState::None };