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LibWeb: Make StackingContext sorting a lot faster

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Stacking contexts are sorted after building a tree of them. They are
sorted by z-index first, DOM tree order second.

Sorting was previously *very* slow on pages with many stacking contexts.
That was because the sort() function used Node::is_before() in the
quick_sort comparator to see if one StackingContext was before another.
is_before() does tree traversal and can take quite a long time per call.

This patch avoids all that by letting StackingContext know its index
among all StackingContexts within the same document in tree order.
There's a noticeable snappiness increase on the CSS-FLEXBOX-1 spec page,
for instance. :^)
This commit is contained in:
Andreas Kling 2023-06-02 12:01:14 +02:00
parent 500b7b08d6
commit d6c3cbd958
Notes: sideshowbarker 2024-07-16 23:52:22 +09:00 
Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/d6c3cbd958
Pull-request: https://github.com/SerenityOS/serenity/pull/19221
3 changed files with 8 additions and 5 deletions
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5
Userland/Libraries/LibWeb/Layout/Viewport.cpp
View file

@ -33,8 +33,9 @@ void Viewport::build_stacking_context_tree_if_needed()
void Viewport::build_stacking_context_tree()
{
const_cast<Painting::PaintableBox*>(paintable_box())->set_stacking_context(make<Painting::StackingContext>(*this, nullptr));
const_cast<Painting::PaintableBox*>(paintable_box())->set_stacking_context(make<Painting::StackingContext>(*this, nullptr, 0));
size_t index_in_tree_order = 1;
for_each_in_subtree_of_type<Box>([&](Box& box) {
if (!box.paintable_box())
return IterationDecision::Continue;
@ -45,7 +46,7 @@ void Viewport::build_stacking_context_tree()
}
auto* parent_context = const_cast<Painting::PaintableBox*>(box.paintable_box())->enclosing_stacking_context();
VERIFY(parent_context);
const_cast<Painting::PaintableBox*>(box.paintable_box())->set_stacking_context(make<Painting::StackingContext>(box, parent_context));
const_cast<Painting::PaintableBox*>(box.paintable_box())->set_stacking_context(make<Painting::StackingContext>(box, parent_context, index_in_tree_order++));
return IterationDecision::Continue;
});

5
Userland/Libraries/LibWeb/Painting/StackingContext.cpp
View file

@ -28,11 +28,12 @@ static void paint_node(Layout::Node const& layout_node, PaintContext& context, P
paintable->paint(context, phase);
}
StackingContext::StackingContext(Layout::Box& box, StackingContext* parent)
StackingContext::StackingContext(Layout::Box& box, StackingContext* parent, size_t index_in_tree_order)
: m_box(box)
, m_transform(combine_transformations(m_box->computed_values().transformations()))
, m_transform_origin(compute_transform_origin())
, m_parent(parent)
, m_index_in_tree_order(index_in_tree_order)
{
VERIFY(m_parent != this);
if (m_parent)
@ -45,7 +46,7 @@ void StackingContext::sort()
auto a_z_index = a->m_box->computed_values().z_index().value_or(0);
auto b_z_index = b->m_box->computed_values().z_index().value_or(0);
if (a_z_index == b_z_index)
return a->m_box->is_before(b->m_box);
return a->m_index_in_tree_order < b->m_index_in_tree_order;
return a_z_index < b_z_index;
});

3
Userland/Libraries/LibWeb/Painting/StackingContext.h
View file

@ -15,7 +15,7 @@ namespace Web::Painting {
class StackingContext {
public:
StackingContext(Layout::Box&, StackingContext* parent);
StackingContext(Layout::Box&, StackingContext* parent, size_t index_in_tree_order);
StackingContext* parent() { return m_parent; }
StackingContext const* parent() const { return m_parent; }
@ -47,6 +47,7 @@ private:
Gfx::FloatPoint m_transform_origin;
StackingContext* const m_parent { nullptr };
Vector<StackingContext*> m_children;
size_t m_index_in_tree_order { 0 };
void paint_internal(PaintContext&) const;
Gfx::FloatMatrix4x4 get_transformation_matrix(CSS::Transformation const& transformation) const;