LibWeb: Don't traverse entire layout tree in every HTML::EventLoop tick

Instead, collect a list of all the elements with content-visibility:auto
after layout.

This way we can skip the tree traversal when updating the rendering.

This was previously eating up ~300 µs of the 60fps frame budget on
our GitHub repo pages (and even more on large pages).

Libraries/LibWeb/DOM/Document.cpp

@@ -1414,6 +1414,18 @@ void Document::update_layout(UpdateLayoutReason reason)
         paintable()->recompute_selection_states(*range);
     }
 
+    // Collect elements with content-visibility: auto. This is used in the HTML event loop to avoid traversing the whole tree every time.
+    Vector<GC::Ref<Painting::PaintableBox>> paintable_boxes_with_auto_content_visibility;
+    paintable()->for_each_in_subtree_of_type<Painting::PaintableBox>([&](auto& paintable_box) {
+        if (paintable_box.dom_node()
+            && paintable_box.dom_node()->is_element()
+            && paintable_box.computed_values().content_visibility() == CSS::ContentVisibility::Auto) {
+            paintable_boxes_with_auto_content_visibility.append(paintable_box);
+        }
+        return TraversalDecision::Continue;
+    });
+    paintable()->set_paintable_boxes_with_auto_content_visibility(move(paintable_boxes_with_auto_content_visibility));
+
     m_layout_root->for_each_in_inclusive_subtree([](auto& node) {
         node.reset_needs_layout_update();
         return TraversalDecision::Continue;

Libraries/LibWeb/HTML/EventLoop/EventLoop.cpp

@@ -23,6 +23,7 @@
 #include <LibWeb/HighResolutionTime/TimeOrigin.h>
 #include <LibWeb/Page/Page.h>
 #include <LibWeb/Painting/PaintableBox.h>
+#include <LibWeb/Painting/ViewportPaintable.h>
 #include <LibWeb/Platform/EventLoopPlugin.h>
 #include <LibWeb/Platform/Timer.h>
 
@@ -408,11 +409,8 @@ void EventLoop::update_the_rendering()
             // 3. For each element element with 'auto' used value of 'content-visibility':
             auto* document_element = document->document_element();
             if (document_element) {
-                document_element->for_each_in_inclusive_subtree_of_type<Web::DOM::Element>([&](auto& element) {
-                    auto const& paintable_box = element.paintable_box();
-                    if (!paintable_box || paintable_box->computed_values().content_visibility() != CSS::ContentVisibility::Auto) {
-                        return TraversalDecision::Continue;
-                    }
+                for (auto& paintable_box : document->paintable()->paintable_boxes_with_auto_content_visibility()) {
+                    auto& element = as<DOM::Element>(*paintable_box->dom_node());
 
                     // 1. Let checkForInitialDetermination be true if element's proximity to the viewport is not determined and it is not relevant to the user. Otherwise, let checkForInitialDetermination be false.
                     bool check_for_initial_determination = element.proximity_to_the_viewport() == Web::DOM::ProximityToTheViewport::NotDetermined && !element.is_relevant_to_the_user();
@@ -424,9 +422,7 @@ void EventLoop::update_the_rendering()
                     if (check_for_initial_determination && element.is_relevant_to_the_user()) {
                         had_initial_visible_content_visibility_determination = true;
                     }
-
-                    return TraversalDecision::Continue;
-                });
+                }
             }
 
             // 4. If hadInitialVisibleContentVisibilityDetermination is true, then continue.

Libraries/LibWeb/Painting/ViewportPaintable.cpp

@@ -395,6 +395,7 @@ void ViewportPaintable::visit_edges(Visitor& visitor)
 {
     Base::visit_edges(visitor);
     visitor.visit(clip_state);
+    visitor.visit(m_paintable_boxes_with_auto_content_visibility);
 }
 
 }

Libraries/LibWeb/Painting/ViewportPaintable.h

@@ -40,6 +40,9 @@ public:
 
     ScrollState const& scroll_state() const { return m_scroll_state; }
 
+    void set_paintable_boxes_with_auto_content_visibility(Vector<GC::Ref<PaintableBox>> paintable_boxes) { m_paintable_boxes_with_auto_content_visibility = move(paintable_boxes); }
+    ReadonlySpan<GC::Ref<PaintableBox>> paintable_boxes_with_auto_content_visibility() const { return m_paintable_boxes_with_auto_content_visibility; }
+
 private:
     void build_stacking_context_tree();
 
@@ -49,6 +52,8 @@ private:
 
     ScrollState m_scroll_state;
     bool m_needs_to_refresh_scroll_state { true };
+
+    Vector<GC::Ref<PaintableBox>> m_paintable_boxes_with_auto_content_visibility;
 };
 
 }