LibWeb: Defer entire-subtree style invalidations

Instead of traversing the entire DOM subtrees and marking nodes for
style update, this patch adds a new mechanism where we can mark a
subtree root as "entire subtree needs style update".

A new pass in Document::update_style() then takes care of coalescing
all these invalidations in a single traversal of the DOM.

This shaves *minutes* of loading time off of https://wpt.fyi/ subpages.

Libraries/LibWeb/DOM/Document.cpp

@@ -1382,6 +1382,34 @@ void Document::update_layout()
     return invalidation;
 }
 
+// This function makes a full pass over the entire DOM and converts "entire subtree needs style update"
+// into "needs style update" for each inclusive descendant where it's found.
+static void perform_pending_style_invalidations(Node& node, bool invalidate_entire_subtree)
+{
+    invalidate_entire_subtree |= node.entire_subtree_needs_style_update();
+
+    if (invalidate_entire_subtree) {
+        node.set_needs_style_update_internal(true);
+        if (node.has_child_nodes())
+            node.set_child_needs_style_update(true);
+    }
+
+    for (auto* child = node.first_child(); child; child = child->next_sibling()) {
+        perform_pending_style_invalidations(*child, invalidate_entire_subtree);
+    }
+
+    if (node.is_element()) {
+        auto& element = static_cast<Element&>(node);
+        if (auto shadow_root = element.shadow_root()) {
+            perform_pending_style_invalidations(*shadow_root, invalidate_entire_subtree);
+            if (invalidate_entire_subtree)
+                node.set_child_needs_style_update(true);
+        }
+    }
+
+    node.set_entire_subtree_needs_style_update(false);
+}
+
 void Document::update_style()
 {
     if (!browsing_context())
@@ -1396,6 +1424,8 @@ void Document::update_style()
     if (!needs_full_style_update() && !needs_style_update() && !child_needs_style_update())
         return;
 
+    perform_pending_style_invalidations(*this, false);
+
     // NOTE: If this is a document hosting <template> contents, style update is unnecessary.
     if (m_created_for_appropriate_template_contents)
         return;

Libraries/LibWeb/DOM/Node.cpp

@@ -429,6 +429,12 @@ void Node::invalidate_style(StyleInvalidationReason reason)
         return;
     }
 
+    // If any ancestor is already marked for an entire subtree update, there's no need to do anything here.
+    for (auto* ancestor = this; ancestor; ancestor = ancestor->parent_or_shadow_host()) {
+        if (ancestor->entire_subtree_needs_style_update())
+            return;
+    }
+
     // When invalidating style for a node, we actually invalidate:
     // - the node itself
     // - all of its descendants
@@ -436,37 +442,23 @@ void Node::invalidate_style(StyleInvalidationReason reason)
     // - all of its subsequent siblings and their descendants
     // FIXME: This is a lot of invalidation and we should implement more sophisticated invalidation to do less work!
 
-    auto invalidate_entire_subtree = [&](Node& subtree_root) {
-        subtree_root.for_each_in_inclusive_subtree([&](Node& node) {
-            node.m_needs_style_update = true;
-            if (node.has_children())
-                node.m_child_needs_style_update = true;
-            if (auto shadow_root = node.is_element() ? static_cast<DOM::Element&>(node).shadow_root() : nullptr) {
-                node.m_child_needs_style_update = true;
-                shadow_root->m_needs_style_update = true;
-                if (shadow_root->has_children())
-                    shadow_root->m_child_needs_style_update = true;
-            }
-            return TraversalDecision::Continue;
-        });
-    };
-
-    invalidate_entire_subtree(*this);
+    set_entire_subtree_needs_style_update(true);
 
     if (reason == StyleInvalidationReason::NodeInsertBefore || reason == StyleInvalidationReason::NodeRemove) {
         for (auto* sibling = previous_sibling(); sibling; sibling = sibling->previous_sibling()) {
             if (sibling->is_element())
-                invalidate_entire_subtree(*sibling);
+                sibling->set_entire_subtree_needs_style_update(true);
         }
     }
 
     for (auto* sibling = next_sibling(); sibling; sibling = sibling->next_sibling()) {
         if (sibling->is_element())
-            invalidate_entire_subtree(*sibling);
+            sibling->set_entire_subtree_needs_style_update(true);
     }
 
     for (auto* ancestor = parent_or_shadow_host(); ancestor; ancestor = ancestor->parent_or_shadow_host())
         ancestor->m_child_needs_style_update = true;
+
     document().schedule_style_update();
 }
 

Libraries/LibWeb/DOM/Node.h

@@ -290,6 +290,7 @@ public:
 
     bool needs_style_update() const { return m_needs_style_update; }
     void set_needs_style_update(bool);
+    void set_needs_style_update_internal(bool) { m_needs_style_update = true; }
 
     bool needs_inherited_style_update() const { return m_needs_inherited_style_update; }
     void set_needs_inherited_style_update(bool);
@@ -297,6 +298,9 @@ public:
     bool child_needs_style_update() const { return m_child_needs_style_update; }
     void set_child_needs_style_update(bool b) { m_child_needs_style_update = b; }
 
+    [[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; }
+
     enum class ForceSelfStyleInvalidation : bool {
         Yes,
         No
@@ -807,6 +811,7 @@ protected:
     bool m_needs_style_update { false };
     bool m_needs_inherited_style_update { false };
     bool m_child_needs_style_update { false };
+    bool m_entire_subtree_needs_style_update { false };
 
     UniqueNodeID m_unique_id;