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LibWeb: Avoid duplicate work when computing style
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Browse source 
Before this change we were running the CSS cascade machinery twice per
element:

- First, to compute the "logical alias mapping context" based on
  writing-mode and pals.

- Then, to compute all properties.

This patch factors out the heaviest work from the cascade machinery
to a separate step that can be run only once. This step will:

- Collect all the matching rules for the element
- Resolve custom properties for the element

We still perform the per-element cascade twice, but now this is hogging
less than 1% of CPU time when typing on Discord (compared to 9% before.)
This commit is contained in:
Andreas Kling 2025-07-20 15:00:32 +02:00 • committed by Alexander Kalenik
commit 128b66e30d
Notes: github-actions[bot] 2025-07-20 21:23:25 +00:00
2 changed files with 47 additions and 40 deletions
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28
Libraries/LibWeb/CSS/StyleComputer.h
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@ -203,9 +203,22 @@ private:
struct MatchingFontCandidate;
LogicalAliasMappingContext compute_logical_alias_mapping_context(DOM::Element&, Optional<CSS::PseudoElement>, ComputeStyleMode) const;
struct LayerMatchingRules {
FlyString qualified_layer_name;
Vector<MatchingRule const*> rules;
};
struct MatchingRuleSet {
Vector<MatchingRule const*> user_agent_rules;
Vector<MatchingRule const*> user_rules;
Vector<LayerMatchingRules> author_rules;
};
[[nodiscard]] MatchingRuleSet build_matching_rule_set(DOM::Element const&, Optional<PseudoElement>, PseudoClassBitmap& attempted_pseudo_class_matches, bool& did_match_any_pseudo_element_rules, ComputeStyleMode) const;
LogicalAliasMappingContext compute_logical_alias_mapping_context(DOM::Element&, Optional<CSS::PseudoElement>, ComputeStyleMode, MatchingRuleSet const&) const;
[[nodiscard]] GC::Ptr<ComputedProperties> compute_style_impl(DOM::Element&, Optional<CSS::PseudoElement>, ComputeStyleMode) const;
[[nodiscard]] GC::Ref<CascadedProperties> compute_cascaded_values(DOM::Element&, Optional<CSS::PseudoElement>, bool& did_match_any_pseudo_element_rules, PseudoClassBitmap& attempted_pseudo_class_matches, ComputeStyleMode, Optional<LogicalAliasMappingContext>) const;
[[nodiscard]] GC::Ref<CascadedProperties> compute_cascaded_values(DOM::Element&, Optional<CSS::PseudoElement>, bool did_match_any_pseudo_element_rules, ComputeStyleMode, MatchingRuleSet const&, Optional<LogicalAliasMappingContext>) const;
static RefPtr<Gfx::FontCascadeList const> find_matching_font_weight_ascending(Vector<MatchingFontCandidate> const& candidates, int target_weight, float font_size_in_pt, bool inclusive);
static RefPtr<Gfx::FontCascadeList const> find_matching_font_weight_descending(Vector<MatchingFontCandidate> const& candidates, int target_weight, float font_size_in_pt, bool inclusive);
RefPtr<Gfx::FontCascadeList const> font_matching_algorithm(FlyString const& family_name, int weight, int slope, float font_size_in_pt) const;
@ -228,17 +241,6 @@ private:
Vector<FlyString> m_qualified_layer_names_in_order;
void build_qualified_layer_names_cache();
struct LayerMatchingRules {
FlyString qualified_layer_name;
Vector<MatchingRule const*> rules;
};
struct MatchingRuleSet {
Vector<MatchingRule const*> user_agent_rules;
Vector<MatchingRule const*> user_rules;
Vector<LayerMatchingRules> author_rules;
};
void cascade_declarations(
CascadedProperties&,
DOM::Element&,