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/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) {