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LibWeb: Make StackingContext sorting a lot faster
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. :^)
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3 changed files with 8 additions and 5 deletions
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@ -28,11 +28,12 @@ static void paint_node(Layout::Node const& layout_node, PaintContext& context, P
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paintable->paint(context, phase);
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}
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StackingContext::StackingContext(Layout::Box& box, StackingContext* parent)
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StackingContext::StackingContext(Layout::Box& box, StackingContext* parent, size_t index_in_tree_order)
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: m_box(box)
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, m_transform(combine_transformations(m_box->computed_values().transformations()))
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, m_transform_origin(compute_transform_origin())
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, m_parent(parent)
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, m_index_in_tree_order(index_in_tree_order)
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{
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VERIFY(m_parent != this);
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if (m_parent)
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@ -45,7 +46,7 @@ void StackingContext::sort()
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auto a_z_index = a->m_box->computed_values().z_index().value_or(0);
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auto b_z_index = b->m_box->computed_values().z_index().value_or(0);
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if (a_z_index == b_z_index)
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return a->m_box->is_before(b->m_box);
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return a->m_index_in_tree_order < b->m_index_in_tree_order;
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return a_z_index < b_z_index;
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});
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