LibWeb: Further optimize :hover style invalidation

Previously, we optimized hover style invalidation to mark for style
updates only those elements that were matched by :hover selectors in the
last style calculation.

This change takes it a step further by invalidating only the elements
where the set of selectors that use :hover changes after hovered element
is modified. The implementation is as follows:
1. Collect all elements whose styles might be affected by a change in
   the hovered element.
2. Retrieve a list of all selectors that use :hover.
3. Test each selector against each element and record which selectors
   match.
4. Update m_hovered_node to the newly hovered element.
5. Repeat step 3.
6. For each element, compare the previous and current sets of matched
   selectors. If they differ, mark the element for style recalculation.

Libraries/LibWeb/CSS/Selector.cpp

@@ -57,10 +57,15 @@ Selector::Selector(Vector<CompoundSelector>&& compound_selectors)
                 break;
             }
             if (simple_selector.type == SimpleSelector::Type::PseudoClass) {
+                if (simple_selector.pseudo_class().type == PseudoClass::Hover) {
+                    m_contains_hover_pseudo_class = true;
+                }
                 for (auto const& child_selector : simple_selector.pseudo_class().argument_selector_list) {
                     if (child_selector->contains_the_nesting_selector()) {
                         m_contains_the_nesting_selector = true;
-                        break;
+                    }
+                    if (child_selector->contains_hover_pseudo_class()) {
+                        m_contains_hover_pseudo_class = true;
                     }
                 }
                 if (m_contains_the_nesting_selector)

Libraries/LibWeb/CSS/Selector.h

@@ -261,6 +261,7 @@ public:
     Optional<PseudoElement> const& pseudo_element() const { return m_pseudo_element; }
     NonnullRefPtr<Selector> relative_to(SimpleSelector const&) const;
     bool contains_the_nesting_selector() const { return m_contains_the_nesting_selector; }
+    bool contains_hover_pseudo_class() const { return m_contains_hover_pseudo_class; }
     RefPtr<Selector> absolutized(SimpleSelector const& selector_for_nesting) const;
     u32 specificity() const;
     String serialize() const;
@@ -274,6 +275,7 @@ private:
     mutable Optional<u32> m_specificity;
     Optional<Selector::PseudoElement> m_pseudo_element;
     bool m_contains_the_nesting_selector { false };
+    bool m_contains_hover_pseudo_class { false };
 
     void collect_ancestor_hashes();
 

Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -421,6 +421,11 @@ bool StyleComputer::should_reject_with_ancestor_filter(Selector const& selector)
     }
     return false;
 }
+Vector<MatchingRule> const& StyleComputer::get_hover_rules() const
+{
+    build_rule_cache_if_needed();
+    return m_hover_rules;
+}
 
 Vector<MatchingRule> StyleComputer::collect_matching_rules(DOM::Element const& element, CascadeOrigin cascade_origin, Optional<CSS::Selector::PseudoElement::Type> pseudo_element, bool& did_match_any_hover_rules, FlyString const& qualified_layer_name) const
 {
@@ -2540,7 +2545,7 @@ void StyleComputer::collect_selector_insights(Selector const& selector, Selector
     }
 }
 
-NonnullOwnPtr<StyleComputer::RuleCache> StyleComputer::make_rule_cache_for_cascade_origin(CascadeOrigin cascade_origin, SelectorInsights& insights)
+NonnullOwnPtr<StyleComputer::RuleCache> StyleComputer::make_rule_cache_for_cascade_origin(CascadeOrigin cascade_origin, SelectorInsights& insights, Vector<MatchingRule>& hover_rules)
 {
     auto rule_cache = make<RuleCache>();
 
@@ -2623,6 +2628,10 @@ NonnullOwnPtr<StyleComputer::RuleCache> StyleComputer::make_rule_cache_for_casca
                     }
                 }
 
+                if (selector.contains_hover_pseudo_class()) {
+                    hover_rules.append(matching_rule);
+                }
+
                 // NOTE: We traverse the simple selectors in reverse order to make sure that class/ID buckets are preferred over tag buckets
                 //       in the common case of div.foo or div#foo selectors.
                 bool added_to_bucket = false;
@@ -2834,9 +2843,9 @@ void StyleComputer::build_rule_cache()
 
     build_qualified_layer_names_cache();
 
-    m_author_rule_cache = make_rule_cache_for_cascade_origin(CascadeOrigin::Author, *m_selector_insights);
-    m_user_rule_cache = make_rule_cache_for_cascade_origin(CascadeOrigin::User, *m_selector_insights);
-    m_user_agent_rule_cache = make_rule_cache_for_cascade_origin(CascadeOrigin::UserAgent, *m_selector_insights);
+    m_author_rule_cache = make_rule_cache_for_cascade_origin(CascadeOrigin::Author, *m_selector_insights, m_hover_rules);
+    m_user_rule_cache = make_rule_cache_for_cascade_origin(CascadeOrigin::User, *m_selector_insights, m_hover_rules);
+    m_user_agent_rule_cache = make_rule_cache_for_cascade_origin(CascadeOrigin::UserAgent, *m_selector_insights, m_hover_rules);
 }
 
 void StyleComputer::invalidate_rule_cache()
@@ -2852,6 +2861,8 @@ void StyleComputer::invalidate_rule_cache()
     // NOTE: It might not be necessary to throw away the UA rule cache.
     //       If we are sure that it's safe, we could keep it as an optimization.
     m_user_agent_rule_cache = nullptr;
+
+    m_hover_rules.clear_with_capacity();
 }
 
 void StyleComputer::did_load_font(FlyString const&)

Libraries/LibWeb/CSS/StyleComputer.h

@@ -146,6 +146,7 @@ public:
     [[nodiscard]] GC::Ref<ComputedProperties> compute_style(DOM::Element&, Optional<CSS::Selector::PseudoElement::Type> = {}) const;
     [[nodiscard]] GC::Ptr<ComputedProperties> compute_pseudo_element_style_if_needed(DOM::Element&, Optional<CSS::Selector::PseudoElement::Type>) const;
 
+    Vector<MatchingRule> const& get_hover_rules() const;
     Vector<MatchingRule> collect_matching_rules(DOM::Element const&, CascadeOrigin, Optional<CSS::Selector::PseudoElement::Type>, bool& did_match_any_hover_rules, FlyString const& qualified_layer_name = {}) const;
 
     void invalidate_rule_cache();
@@ -267,13 +268,14 @@ private:
         HashMap<FlyString, NonnullRefPtr<Animations::KeyframeEffect::KeyFrameSet>> rules_by_animation_keyframes;
     };
 
-    NonnullOwnPtr<RuleCache> make_rule_cache_for_cascade_origin(CascadeOrigin, SelectorInsights&);
+    NonnullOwnPtr<RuleCache> make_rule_cache_for_cascade_origin(CascadeOrigin, SelectorInsights&, Vector<MatchingRule>& hover_rules);
 
     RuleCache const& rule_cache_for_cascade_origin(CascadeOrigin) const;
 
     static void collect_selector_insights(Selector const&, SelectorInsights&);
 
     OwnPtr<SelectorInsights> m_selector_insights;
+    Vector<MatchingRule> m_hover_rules;
     OwnPtr<RuleCache> m_author_rule_cache;
     OwnPtr<RuleCache> m_user_rule_cache;
     OwnPtr<RuleCache> m_user_agent_rule_cache;

Libraries/LibWeb/DOM/Document.cpp

@@ -8,6 +8,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/Bitmap.h>
 #include <AK/CharacterTypes.h>
 #include <AK/Debug.h>
 #include <AK/GenericLexer.h>
@@ -36,6 +37,7 @@
 #include <LibWeb/CSS/FontFaceSet.h>
 #include <LibWeb/CSS/MediaQueryList.h>
 #include <LibWeb/CSS/MediaQueryListEvent.h>
+#include <LibWeb/CSS/SelectorEngine.h>
 #include <LibWeb/CSS/StyleComputer.h>
 #include <LibWeb/CSS/StyleSheetIdentifier.h>
 #include <LibWeb/CSS/SystemColor.h>
@@ -1485,31 +1487,84 @@ static Node* find_common_ancestor(Node* a, Node* b)
     return nullptr;
 }
 
+void Document::invalidate_style_for_elements_affected_by_hover_change(GC::Ptr<Node> old_new_hovered_common_ancestor, GC::Ptr<Node> hovered_node)
+{
+    auto const& hover_rules = style_computer().get_hover_rules();
+    if (hover_rules.is_empty())
+        return;
+
+    auto& invalidation_root = [&] -> Node& {
+        if (style_computer().has_has_selectors())
+            return *this;
+        return old_new_hovered_common_ancestor ? *old_new_hovered_common_ancestor : *this;
+    }();
+
+    Vector<Element&> elements;
+    invalidation_root.for_each_shadow_including_inclusive_descendant([&](Node& node) {
+        if (!node.is_element())
+            return TraversalDecision::Continue;
+        auto& element = static_cast<Element&>(node);
+        if (element.affected_by_hover())
+            elements.append(element);
+        return TraversalDecision::Continue;
+    });
+
+    auto compute_hover_selectors_match_state = [&] {
+        Vector<AK::Bitmap> state;
+        state.resize(elements.size());
+        for (size_t element_index = 0; element_index < elements.size(); ++element_index) {
+            auto const& element = elements[element_index];
+            state[element_index] = MUST(AK::Bitmap::create(hover_rules.size(), 0));
+            for (size_t rule_index = 0; rule_index < hover_rules.size(); ++rule_index) {
+                auto const& rule = hover_rules[rule_index];
+                auto const& selector = rule.absolutized_selectors()[rule.selector_index];
+
+                SelectorEngine::MatchContext context;
+                bool selector_matched = false;
+                if (rule.can_use_fast_matches) {
+                    if (SelectorEngine::fast_matches(selector, element, {}, context))
+                        selector_matched = true;
+                } else {
+                    if (SelectorEngine::matches(selector, element, {}, context, {}))
+                        selector_matched = true;
+                }
+                if (element.has_pseudo_elements()) {
+                    if (SelectorEngine::matches(selector, element, {}, context, CSS::Selector::PseudoElement::Type::Before))
+                        selector_matched = true;
+                    if (SelectorEngine::matches(selector, element, {}, context, CSS::Selector::PseudoElement::Type::After))
+                        selector_matched = true;
+                }
+                if (selector_matched)
+                    state[element_index].set(rule_index, true);
+            }
+        }
+        return state;
+    };
+
+    auto previous_hover_selectors_match_state = compute_hover_selectors_match_state();
+    m_hovered_node = hovered_node;
+    auto new_hover_selectors_match_state = compute_hover_selectors_match_state();
+
+    for (size_t element_index = 0; element_index < elements.size(); ++element_index) {
+        if (previous_hover_selectors_match_state[element_index].view() == new_hover_selectors_match_state[element_index].view())
+            continue;
+
+        elements[element_index].set_needs_style_update(true);
+        elements[element_index].for_each_in_subtree_of_type<Element>([](auto& element) {
+            element.set_needs_inherited_style_update(true);
+            return TraversalDecision::Continue;
+        });
+    }
+}
+
 void Document::set_hovered_node(Node* node)
 {
     if (m_hovered_node.ptr() == node)
         return;
 
     GC::Ptr<Node> old_hovered_node = move(m_hovered_node);
-    m_hovered_node = node;
-
-    auto* common_ancestor = find_common_ancestor(old_hovered_node, m_hovered_node);
-    if (!style_computer().has_has_selectors()) {
-        Node& invalidation_root = common_ancestor ? *common_ancestor : document();
-        invalidation_root.for_each_in_inclusive_subtree([&](Node& node) {
-            if (!node.is_element())
-                return TraversalDecision::Continue;
-            auto& element = static_cast<Element&>(node);
-            if (element.affected_by_hover()) {
-                element.set_needs_style_update(true);
-            } else {
-                element.set_needs_inherited_style_update(true);
-            }
-            return TraversalDecision::Continue;
-        });
-    } else {
-        invalidate_style(StyleInvalidationReason::Hover);
-    }
+    auto* common_ancestor = find_common_ancestor(old_hovered_node, node);
+    invalidate_style_for_elements_affected_by_hover_change(common_ancestor, node);
 
     // https://w3c.github.io/uievents/#mouseout
     if (old_hovered_node && old_hovered_node != m_hovered_node) {

Libraries/LibWeb/DOM/Document.h

@@ -174,6 +174,7 @@ public:
 
     virtual FlyString node_name() const override { return "#document"_fly_string; }
 
+    void invalidate_style_for_elements_affected_by_hover_change(GC::Ptr<Node> old_new_hovered_common_ancestor, GC::Ptr<Node> hovered_node);
     void set_hovered_node(Node*);
     Node* hovered_node() { return m_hovered_node.ptr(); }
     Node const* hovered_node() const { return m_hovered_node.ptr(); }