LibWeb: Use invalidation sets for :has() invalidation

Prior to this change, we invalidated all elements in the document if it
used any selectors with :has(). This change aims to improve that by
applying a combination of techniques:
- Collect metadata for each element if it was matched against a selector
  with :has() in the subject position. This is needed to invalidate all
  elements that could be affected by selectors like `div:has(.a:empty)`
  because they are not covered by the invalidation sets.
- Use invalidation sets to invalidate elements that are affected by
  selectors with :has() in a non-subject position.

Selectors like `.a:has(.b) + .c` still cause whole-document invalidation
because invalidation sets cover only descendants, not siblings. As a
result, there is no performance improvement on github.com due to this
limitation. However, youtube.com and discord.com benefit from this
change.

Libraries/LibWeb/CSS/SelectorEngine.cpp

@@ -549,6 +549,9 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
         // :has() cannot be nested in a :has()
         if (selector_kind == SelectorKind::Relative)
             return false;
+        if (context.collect_per_element_selector_involvement_metadata && &element == context.subject) {
+            const_cast<DOM::Element&>(element).set_affected_by_has_pseudo_class_in_subject_position(true);
+        }
         // These selectors should be relative selectors (https://drafts.csswg.org/selectors-4/#relative-selector)
         for (auto& selector : pseudo_class.argument_selector_list) {
             if (matches_has_pseudo_class(selector, element, shadow_host, context))

Libraries/LibWeb/CSS/SelectorEngine.h

@@ -18,6 +18,8 @@ enum class SelectorKind {
 
 struct MatchContext {
     GC::Ptr<CSS::CSSStyleSheet const> style_sheet_for_rule {};
+    GC::Ptr<DOM::Element const> subject {};
+    bool collect_per_element_selector_involvement_metadata { false };
     bool did_match_any_hover_rules { false };
 };
 

Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -586,7 +586,11 @@ Vector<MatchingRule const*> StyleComputer::collect_matching_rules(DOM::Element c
 
         auto const& selector = rule_to_run.selector;
 
-        SelectorEngine::MatchContext context { .style_sheet_for_rule = *rule_to_run.sheet };
+        SelectorEngine::MatchContext context {
+            .style_sheet_for_rule = *rule_to_run.sheet,
+            .subject = element,
+            .collect_per_element_selector_involvement_metadata = true,
+        };
         ScopeGuard guard = [&] {
             if (context.did_match_any_hover_rules)
                 did_match_any_hover_rules = true;

Libraries/LibWeb/CSS/StyleInvalidationData.cpp

@@ -125,6 +125,7 @@ static void build_invalidation_sets_for_simple_selector(Selector::SimpleSelector
         case PseudoClass::Disabled:
         case PseudoClass::PlaceholderShown:
         case PseudoClass::Checked:
+        case PseudoClass::Has:
             invalidation_set.set_needs_invalidate_pseudo_class(pseudo_class.type);
             break;
         default:

Libraries/LibWeb/DOM/Element.cpp

@@ -1166,6 +1166,8 @@ bool Element::includes_properties_from_invalidation_set(CSS::InvalidationSet con
         }
         case CSS::InvalidationSet::Property::Type::PseudoClass: {
             switch (property.value.get<CSS::PseudoClass>()) {
+            case CSS::PseudoClass::Has:
+                return true;
             case CSS::PseudoClass::Enabled: {
                 return (is<HTML::HTMLButtonElement>(*this) || is<HTML::HTMLInputElement>(*this) || is<HTML::HTMLSelectElement>(*this) || is<HTML::HTMLTextAreaElement>(*this) || is<HTML::HTMLOptGroupElement>(*this) || is<HTML::HTMLOptionElement>(*this) || is<HTML::HTMLFieldSetElement>(*this))
                     && !is_actually_disabled();

Libraries/LibWeb/DOM/Element.h

@@ -411,6 +411,9 @@ public:
     bool has_style_containment() const;
     bool has_paint_containment() const;
 
+    bool affected_by_has_pseudo_class_in_subject_position() const { return m_affected_by_has_pseudo_class_in_subject_position; }
+    void set_affected_by_has_pseudo_class_in_subject_position(bool value) { m_affected_by_has_pseudo_class_in_subject_position = value; }
+
 protected:
     Element(Document&, DOM::QualifiedName);
     virtual void initialize(JS::Realm&) override;
@@ -500,6 +503,7 @@ private:
 
     bool m_in_top_layer { false };
     bool m_style_uses_css_custom_properties { false };
+    bool m_affected_by_has_pseudo_class_in_subject_position { false };
 
     OwnPtr<CSS::CountersSet> m_counters_set;
 

Libraries/LibWeb/DOM/Node.cpp

@@ -401,16 +401,47 @@ GC::Ptr<HTML::Navigable> Node::navigable() const
     }
 }
 
+void Node::invalidate_elements_affected_by_has()
+{
+    Vector<CSS::InvalidationSet::Property, 1> changed_properties;
+    changed_properties.append({ .type = CSS::InvalidationSet::Property::Type::PseudoClass, .value = CSS::PseudoClass::Has });
+    auto invalidation_set = document().style_computer().invalidation_set_for_properties(changed_properties);
+    for_each_shadow_including_inclusive_descendant([&](Node& node) {
+        if (!node.is_element())
+            return TraversalDecision::Continue;
+        auto& element = static_cast<Element&>(node);
+        bool needs_style_recalculation = false;
+        // There are two cases in which an element must be invalidated, depending on the position of :has() in a selector:
+        // 1) In the subject position, i.e., ".a:has(.b)". In that case, invalidation sets are not helpful
+        //    for narrowing down the set of elements that need to be invalidated. Instead, we invalidate
+        //    all elements that were tested against selectors with :has() in the subject position during
+        //    selector matching.
+        // 2) In the non-subject position, i.e., ".a:has(.b) > .c". Here, invalidation sets can be used to
+        //    determine that only elements with the "c" class have to be invalidated.
+        if (element.affected_by_has_pseudo_class_in_subject_position()) {
+            needs_style_recalculation = true;
+        } else if (invalidation_set.needs_invalidate_whole_subtree()) {
+            needs_style_recalculation = true;
+        } else if (element.includes_properties_from_invalidation_set(invalidation_set)) {
+            needs_style_recalculation = true;
+        }
+
+        if (needs_style_recalculation) {
+            element.set_needs_style_update(true);
+        } else {
+            element.set_needs_inherited_style_update(true);
+        }
+        return TraversalDecision::Continue;
+    });
+}
+
 void Node::invalidate_style(StyleInvalidationReason reason)
 {
     if (is_character_data())
         return;
 
-    // FIXME: This is very not optimal! We should figure out a smaller set of elements to invalidate,
-    //        but right now the :has() selector means we have to invalidate everything.
-    if (!is_document() && document().style_computer().may_have_has_selectors()) {
-        document().invalidate_style(reason);
-        return;
+    if (document().style_computer().may_have_has_selectors()) {
+        document().invalidate_elements_affected_by_has();
     }
 
     if (!needs_style_update() && !document().needs_full_style_update()) {
@@ -462,7 +493,7 @@ void Node::invalidate_style(StyleInvalidationReason reason)
     document().schedule_style_update();
 }
 
-void Node::invalidate_style(StyleInvalidationReason reason, Vector<CSS::InvalidationSet::Property> const& properties, StyleInvalidationOptions options)
+void Node::invalidate_style(StyleInvalidationReason, Vector<CSS::InvalidationSet::Property> const& properties, StyleInvalidationOptions options)
 {
     if (is_character_data())
         return;
@@ -472,8 +503,7 @@ void Node::invalidate_style(StyleInvalidationReason reason, Vector<CSS::Invalida
         properties_used_in_has_selectors |= document().style_computer().invalidation_property_used_in_has_selector(property);
     }
     if (properties_used_in_has_selectors) {
-        document().invalidate_style(reason);
-        return;
+        document().invalidate_elements_affected_by_has();
     }
 
     auto invalidation_set = document().style_computer().invalidation_set_for_properties(properties);

Libraries/LibWeb/DOM/Node.h

@@ -301,6 +301,7 @@ public:
     [[nodiscard]] bool entire_subtree_needs_style_update() const { return m_entire_subtree_needs_style_update; }
     void set_entire_subtree_needs_style_update(bool b) { m_entire_subtree_needs_style_update = b; }
 
+    void invalidate_elements_affected_by_has();
     void invalidate_style(StyleInvalidationReason);
     struct StyleInvalidationOptions {
         bool invalidate_self { false };