LibWeb: Let paintables cache their containing block and absolute rect

The absolute rect of a paintable is somewhat expensive to compute. This
is because all coordinates are relative to the nearest containing block,
so we have to traverse the containing block chain and apply each offset
to get the final rect.

Paintables will never move between containing blocks, nor will their
absolute rect change. If anything changes, we'll simpl make a new
paintable and replace the old one.

Take advantage of this by caching the containing block and absolute rect
after first access.

Userland/Libraries/LibWeb/Painting/InlinePaintable.cpp

@@ -38,7 +38,7 @@ void InlinePaintable::paint(PaintContext& context, Painting::PaintPhase phase) c
         auto top_right_border_radius = computed_values().border_top_right_radius();
         auto bottom_right_border_radius = computed_values().border_bottom_right_radius();
         auto bottom_left_border_radius = computed_values().border_bottom_left_radius();
-        auto containing_block_position_in_absolute_coordinates = layout_node().containing_block()->paint_box()->absolute_position();
+        auto containing_block_position_in_absolute_coordinates = containing_block()->paint_box()->absolute_position();
 
         for_each_fragment([&](auto const& fragment, bool is_first_fragment, bool is_last_fragment) {
             Gfx::FloatRect absolute_fragment_rect { containing_block_position_in_absolute_coordinates.translated(fragment.offset()), fragment.size() };
@@ -89,7 +89,7 @@ void InlinePaintable::paint(PaintContext& context, Painting::PaintPhase phase) c
             .left = computed_values().border_left(),
         };
 
-        auto containing_block_position_in_absolute_coordinates = layout_node().containing_block()->paint_box()->absolute_position();
+        auto containing_block_position_in_absolute_coordinates = containing_block()->paint_box()->absolute_position();
 
         for_each_fragment([&](auto const& fragment, bool is_first_fragment, bool is_last_fragment) {
             Gfx::FloatRect absolute_fragment_rect { containing_block_position_in_absolute_coordinates.translated(fragment.offset()), fragment.size() };
@@ -132,7 +132,7 @@ void InlinePaintable::for_each_fragment(Callback callback) const
 {
     // FIXME: This will be slow if the containing block has a lot of fragments!
     Vector<Layout::LineBoxFragment const&> fragments;
-    layout_node().containing_block()->paint_box()->for_each_fragment([&](auto& fragment) {
+    containing_block()->paint_box()->for_each_fragment([&](auto& fragment) {
         if (layout_node().is_inclusive_ancestor_of(fragment.layout_node()))
             fragments.append(fragment);
         return IterationDecision::Continue;

Userland/Libraries/LibWeb/Painting/Paintable.cpp

@@ -24,7 +24,7 @@ void Paintable::handle_mousemove(Badge<EventHandler>, Gfx::IntPoint const&, unsi
 
 bool Paintable::handle_mousewheel(Badge<EventHandler>, Gfx::IntPoint const&, unsigned, unsigned, int wheel_delta_x, int wheel_delta_y)
 {
-    if (auto* containing_block = layout_node().containing_block()) {
+    if (auto* containing_block = this->containing_block()) {
         if (!containing_block->is_scrollable())
             return false;
         auto new_offset = containing_block->scroll_offset();

Userland/Libraries/LibWeb/Painting/Paintable.h

@@ -68,6 +68,13 @@ public:
 
     void set_needs_display() const { const_cast<Layout::Node&>(m_layout_node).set_needs_display(); }
 
+    Layout::BlockContainer const* containing_block() const
+    {
+        if (!m_containing_block.has_value())
+            m_containing_block = const_cast<Layout::Node&>(m_layout_node).containing_block();
+        return *m_containing_block;
+    }
+
 protected:
     explicit Paintable(Layout::Node const& layout_node)
         : m_layout_node(layout_node)
@@ -76,6 +83,7 @@ protected:
 
 private:
     Layout::Node const& m_layout_node;
+    Optional<Layout::BlockContainer*> mutable m_containing_block;
 };
 
 }

Userland/Libraries/LibWeb/Painting/PaintableBox.cpp

@@ -58,7 +58,7 @@ void PaintableBox::set_content_size(Gfx::FloatSize const& size)
 Gfx::FloatPoint PaintableBox::effective_offset() const
 {
     if (m_containing_line_box_fragment.has_value()) {
-        auto const& fragment = layout_box().containing_block()->paint_box()->line_boxes()[m_containing_line_box_fragment->line_box_index].fragments()[m_containing_line_box_fragment->fragment_index];
+        auto const& fragment = containing_block()->paint_box()->line_boxes()[m_containing_line_box_fragment->line_box_index].fragments()[m_containing_line_box_fragment->fragment_index];
         return fragment.offset();
     }
     return m_offset;
@@ -66,10 +66,13 @@ Gfx::FloatPoint PaintableBox::effective_offset() const
 
 Gfx::FloatRect PaintableBox::absolute_rect() const
 {
-    Gfx::FloatRect rect { effective_offset(), content_size() };
-    for (auto* block = layout_box().containing_block(); block; block = block->containing_block())
-        rect.translate_by(block->paint_box()->effective_offset());
-    return rect;
+    if (!m_absolute_rect.has_value()) {
+        Gfx::FloatRect rect { effective_offset(), content_size() };
+        for (auto const* block = containing_block(); block && block->paintable(); block = block->paintable()->containing_block())
+            rect.translate_by(block->paint_box()->effective_offset());
+        m_absolute_rect = rect;
+    }
+    return *m_absolute_rect;
 }
 
 void PaintableBox::set_containing_line_box_fragment(Optional<Layout::LineBoxFragmentCoordinate> fragment_coordinate)

Userland/Libraries/LibWeb/Painting/PaintableBox.h

@@ -131,6 +131,8 @@ private:
     Painting::BorderRadiusData normalized_border_radius_data() const;
 
     OwnPtr<Painting::StackingContext> m_stacking_context;
+
+    Optional<Gfx::FloatRect> mutable m_absolute_rect;
 };
 
 class PaintableWithLines : public PaintableBox {