LibWeb: Build stacking context tree lazily

There's no actual need to build the stacking context tree before
performing layout. Instead, make it lazy and build the tree when it's
actually needed for something.

This avoids a bunch of work in situations where multiple synchronous
layouts are forced (typically by JavaScript) without painting or hit
testing taking place in between.

It also opens up for style invalidations that only target the stacking
context tree.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -585,8 +585,6 @@ void Document::update_layout()
     root_formatting_context.run(*m_layout_root, Layout::LayoutMode::Normal);
     formatting_state.commit();
 
-    m_layout_root->build_stacking_context_tree();
-
     browsing_context()->set_needs_display();
 
     if (browsing_context()->is_top_level()) {

Userland/Libraries/LibWeb/Layout/InitialContainingBlock.cpp

@@ -18,13 +18,19 @@ InitialContainingBlock::InitialContainingBlock(DOM::Document& document, NonnullR
 
 InitialContainingBlock::~InitialContainingBlock() = default;
 
+void InitialContainingBlock::build_stacking_context_tree_if_needed()
+{
+    if (paint_box()->stacking_context())
+        return;
+    build_stacking_context_tree();
+}
+
 void InitialContainingBlock::build_stacking_context_tree()
 {
     const_cast<Painting::PaintableWithLines*>(paint_box())->set_stacking_context(make<Painting::StackingContext>(*this, nullptr));
 
-    for_each_in_inclusive_subtree_of_type<Box>([&](Box& box) {
-        if (&box == this)
-            return IterationDecision::Continue;
+    for_each_in_subtree_of_type<Box>([&](Box& box) {
+        const_cast<Painting::PaintableBox*>(box.paint_box())->invalidate_stacking_context();
         if (!box.establishes_stacking_context()) {
             VERIFY(!box.paint_box()->stacking_context());
             return IterationDecision::Continue;
@@ -40,6 +46,7 @@ void InitialContainingBlock::build_stacking_context_tree()
 
 void InitialContainingBlock::paint_all_phases(PaintContext& context)
 {
+    build_stacking_context_tree_if_needed();
     context.painter().fill_rect(enclosing_int_rect(paint_box()->absolute_rect()), context.palette().base());
     context.painter().translate(-context.viewport_rect().location());
     paint_box()->stacking_context()->paint(context);

Userland/Libraries/LibWeb/Layout/InitialContainingBlock.h

@@ -24,11 +24,11 @@ public:
     void set_selection(const LayoutRange&);
     void set_selection_end(const LayoutPosition&);
 
-    void build_stacking_context_tree();
-
+    void build_stacking_context_tree_if_needed();
     void recompute_selection_states();
 
 private:
+    void build_stacking_context_tree();
     virtual bool is_initial_containing_block_box() const override { return true; }
 
     LayoutRange m_selection;

Userland/Libraries/LibWeb/Painting/PaintableBox.cpp

@@ -7,6 +7,7 @@
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/HTML/HTMLHtmlElement.h>
 #include <LibWeb/Layout/BlockContainer.h>
+#include <LibWeb/Layout/InitialContainingBlock.h>
 #include <LibWeb/Painting/BackgroundPainting.h>
 #include <LibWeb/Painting/PaintableBox.h>
 #include <LibWeb/Painting/ShadowPainting.h>
@@ -28,6 +29,11 @@ PaintableBox::~PaintableBox()
 {
 }
 
+void PaintableBox::invalidate_stacking_context()
+{
+    m_stacking_context = nullptr;
+}
+
 PaintableWithLines::PaintableWithLines(Layout::BlockContainer const& layout_box)
     : PaintableBox(layout_box)
 {
@@ -524,8 +530,10 @@ void PaintableBox::for_each_child_in_paint_order(Callback callback) const
 
 HitTestResult PaintableBox::hit_test(Gfx::FloatPoint const& position, HitTestType type) const
 {
-    if (layout_box().is_initial_containing_block_box())
+    if (layout_box().is_initial_containing_block_box()) {
+        const_cast<Layout::InitialContainingBlock&>(static_cast<Layout::InitialContainingBlock const&>(layout_box())).build_stacking_context_tree_if_needed();
         return stacking_context()->hit_test(position, type);
+    }
 
     HitTestResult result { absolute_border_box_rect().contains(position.x(), position.y()) ? this : nullptr };
     for_each_child_in_paint_order([&](auto& child) {

Userland/Libraries/LibWeb/Painting/PaintableBox.h

@@ -117,6 +117,8 @@ public:
 
     virtual HitTestResult hit_test(Gfx::FloatPoint const&, HitTestType) const override;
 
+    void invalidate_stacking_context();
+
 protected:
     explicit PaintableBox(Layout::Box const&);