LibWeb: Cache the used CSS 'position' value on LayoutNodeWithStyle

This avoids having to query the StyleProperties hash map whenever we
need to know if an element is absolutely positioned. This was extremely
hot in interactive window resize profiles.

Libraries/LibWeb/Layout/LayoutBlock.cpp

@@ -642,7 +642,7 @@ void LayoutBlock::place_block_level_non_replaced_element_in_normal_flow(LayoutBl
 
     auto* relevant_sibling = block.previous_sibling();
     while (relevant_sibling != nullptr) {
-        if (relevant_sibling->style().position() != CSS::Position::Absolute)
+        if (relevant_sibling->position() != CSS::Position::Absolute)
             break;
         relevant_sibling = relevant_sibling->previous_sibling();
     }

Libraries/LibWeb/Layout/LayoutBox.cpp

@@ -323,7 +323,7 @@ bool LayoutBox::establishes_stacking_context() const
         return false;
     if (node() == document().root())
         return true;
-    auto position = style().position();
+    auto position = this->position();
     auto z_index = style().z_index();
     if (position == CSS::Position::Absolute || position == CSS::Position::Relative) {
         if (z_index.has_value())

Libraries/LibWeb/Layout/LayoutNode.cpp

@@ -57,7 +57,7 @@ void LayoutNode::layout(LayoutMode layout_mode)
 
 bool LayoutNode::can_contain_boxes_with_position_absolute() const
 {
-    return style().position() != CSS::Position::Static || is_root();
+    return position() != CSS::Position::Static || is_root();
 }
 
 const LayoutBlock* LayoutNode::containing_block() const
@@ -72,7 +72,7 @@ const LayoutBlock* LayoutNode::containing_block() const
     if (is_text())
         return nearest_block_ancestor();
 
-    auto position = style().position();
+    auto position = this->position();
 
     if (position == CSS::Position::Absolute) {
         auto* ancestor = parent();
@@ -199,7 +199,7 @@ bool LayoutNode::is_absolutely_positioned() const
 {
     if (!has_style())
         return false;
-    auto position = style().position();
+    auto position = this->position();
     return position == CSS::Position::Absolute || position == CSS::Position::Fixed;
 }
 
@@ -207,7 +207,16 @@ bool LayoutNode::is_fixed_position() const
 {
     if (!has_style())
         return false;
-    return style().position() == CSS::Position::Fixed;
+    auto position = this->position();
+    return position == CSS::Position::Fixed;
+}
+
+LayoutNodeWithStyle::LayoutNodeWithStyle(const Node* node, NonnullRefPtr<StyleProperties> style)
+    : LayoutNode(node)
+    , m_style(move(style))
+{
+    m_has_style = true;
+    m_position = m_style->position();
 }
 
 }

Libraries/LibWeb/Layout/LayoutNode.h

@@ -190,6 +190,7 @@ public:
     virtual LayoutNode& inline_wrapper() { return *this; }
 
     const StyleProperties& style() const;
+    CSS::Position position() const;
 
     LayoutNodeWithStyle* parent();
     const LayoutNodeWithStyle* parent() const;
@@ -257,16 +258,14 @@ public:
     const StyleProperties& style() const { return m_style; }
     void set_style(const StyleProperties& style) { m_style = style; }
 
+    CSS::Position position() const { return m_position; }
+
 protected:
-    explicit LayoutNodeWithStyle(const Node* node, NonnullRefPtr<StyleProperties> style)
+    explicit LayoutNodeWithStyle(const Node*, NonnullRefPtr<StyleProperties>);
-        : LayoutNode(node)
-        , m_style(move(style))
-    {
-        m_has_style = true;
-    }
 
 private:
     NonnullRefPtr<StyleProperties> m_style;
+    CSS::Position m_position;
 };
 
 class LayoutNodeWithStyleAndBoxModelMetrics : public LayoutNodeWithStyle {
@@ -291,6 +290,13 @@ inline const StyleProperties& LayoutNode::style() const
     return parent()->style();
 }
 
+inline CSS::Position LayoutNode::position() const
+{
+    if (m_has_style)
+        return static_cast<const LayoutNodeWithStyle*>(this)->position();
+    return parent()->position();
+}
+
 inline const LayoutNodeWithStyle* LayoutNode::parent() const
 {
     return static_cast<const LayoutNodeWithStyle*>(TreeNode<LayoutNode>::parent());