LibWeb: Skip :has() invalidation in update_style when nothing is pending

Add a document-level boolean flag that tracks whether any :has()
invalidations have been scheduled. This avoids iterating over all
shadow roots just to check is_empty() on each style scope when no
:has() invalidations are pending, which is the common case during
scrolling on complex pages like Reddit.

Results in ~10% reduction of is_empty() calls in profiles when
scrolling on Reddit.

Libraries/LibWeb/CSS/StyleScope.cpp

@@ -468,6 +468,12 @@ void StyleScope::for_each_active_css_style_sheet(Function<void(CSS::CSSStyleShee
     }
 }
 
+void StyleScope::schedule_ancestors_style_invalidation_due_to_presence_of_has(DOM::Node& node)
+{
+    m_pending_nodes_for_style_invalidation_due_to_presence_of_has.set(node);
+    document().set_needs_invalidation_of_elements_affected_by_has();
+}
+
 void StyleScope::invalidate_style_of_elements_affected_by_has()
 {
     if (m_pending_nodes_for_style_invalidation_due_to_presence_of_has.is_empty()) {

Libraries/LibWeb/CSS/StyleScope.h

@@ -107,7 +107,7 @@ public:
 
     void invalidate_style_of_elements_affected_by_has();
 
-    void schedule_ancestors_style_invalidation_due_to_presence_of_has(DOM::Node& node) { m_pending_nodes_for_style_invalidation_due_to_presence_of_has.set(node); }
+    void schedule_ancestors_style_invalidation_due_to_presence_of_has(DOM::Node& node);
 
     void visit_edges(GC::Cell::Visitor&);
 

Libraries/LibWeb/DOM/Document.cpp

@@ -1690,10 +1690,13 @@ void Document::update_style()
     // style change event. [CSS-Transitions-2]
     m_transition_generation++;
 
-    style_scope().invalidate_style_of_elements_affected_by_has();
-    for_each_shadow_root([&](auto& shadow_root) {
-        shadow_root.style_scope().invalidate_style_of_elements_affected_by_has();
-    });
+    if (m_needs_invalidation_of_elements_affected_by_has) {
+        m_needs_invalidation_of_elements_affected_by_has = false;
+        style_scope().invalidate_style_of_elements_affected_by_has();
+        for_each_shadow_root([&](auto& shadow_root) {
+            shadow_root.style_scope().invalidate_style_of_elements_affected_by_has();
+        });
+    }
 
     if (!m_style_invalidator->has_pending_invalidations() && !needs_full_style_update() && !needs_style_update() && !child_needs_style_update())
         return;

Libraries/LibWeb/DOM/Document.h

@@ -813,6 +813,8 @@ public:
     void set_needs_to_resolve_paint_only_properties() { m_needs_to_resolve_paint_only_properties = true; }
     void set_needs_animated_style_update() { m_needs_animated_style_update = true; }
 
+    void set_needs_invalidation_of_elements_affected_by_has() { m_needs_invalidation_of_elements_affected_by_has = true; }
+
     void set_needs_accumulated_visual_contexts_update(bool value) { m_needs_accumulated_visual_contexts_update = value; }
     bool needs_accumulated_visual_contexts_update() const { return m_needs_accumulated_visual_contexts_update; }
 
@@ -1308,6 +1310,7 @@ private:
 
     bool m_needs_to_resolve_paint_only_properties { true };
     bool m_needs_accumulated_visual_contexts_update { false };
+    bool m_needs_invalidation_of_elements_affected_by_has { false };
 
     mutable GC::Ptr<WebIDL::ObservableArray> m_adopted_style_sheets;