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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
3 changed files with 8 additions and 5 deletions
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5
Userland/Libraries/LibWeb/Layout/Viewport.cpp
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@ -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;
});