[IntersectionObserver] Refactor to avoid ancestor frame tree walk

Previously, the dirty flags to IntersectionObservation::Compute were
computed by doing a walk up the ancestor tree. This is silly, since
the IntersectionObserver algorithm runs in a top-down tree walk. This
CL propagates ancestor frame flags down to child frames to avoid the
tree walk.

R=chrishtr@chromium.org

Change-Id: I09f95696ce4d22c15ee76bc0a0ecc4e12778284c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1501492
Commit-Queue: Stefan Zager <szager@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637829}

third_party/blink/renderer/core/frame/frame_view.h

@@ -15,7 +15,11 @@ struct IntrinsicSizingInfo;
 class CORE_EXPORT FrameView : public EmbeddedContentView {
  public:
   ~FrameView() override = default;
-  virtual void UpdateViewportIntersectionsForSubtree() = 0;
+
+  // parent_flags is the result of calling GetIntersectionObservationFlags on
+  // the LocalFrameView parent of this FrameView (if any). It contains dirty
+  // bits based on whether geometry may have changed in the parent frame.
+  virtual void UpdateViewportIntersectionsForSubtree(unsigned parent_flags) = 0;
 
   virtual bool GetIntrinsicSizingInfo(IntrinsicSizingInfo&) const = 0;
   virtual bool HasIntrinsicSizingInfo() const = 0;

third_party/blink/renderer/core/frame/local_frame_view.cc

@@ -1068,7 +1068,7 @@ void LocalFrameView::RunIntersectionObserverSteps() {
                "LocalFrameView::UpdateViewportIntersectionsForSubtree");
   SCOPED_UMA_AND_UKM_TIMER(EnsureUkmAggregator(),
                            LocalFrameUkmAggregator::kIntersectionObservation);
-  UpdateViewportIntersectionsForSubtree();
+  UpdateViewportIntersectionsForSubtree(0);
 #if DCHECK_IS_ON()
   DCHECK(was_dirty || !NeedsLayout());
 #endif
@@ -3869,7 +3869,8 @@ void LocalFrameView::CollectAnnotatedRegions(
     CollectAnnotatedRegions(*curr, regions);
 }
 
-void LocalFrameView::UpdateViewportIntersectionsForSubtree() {
+void LocalFrameView::UpdateViewportIntersectionsForSubtree(
+    unsigned parent_flags) {
   // TODO(dcheng): Since LocalFrameView tree updates are deferred, FrameViews
   // might still be in the LocalFrameView hierarchy even though the associated
   // Document is already detached. Investigate if this check and a similar check
@@ -3878,28 +3879,31 @@ void LocalFrameView::UpdateViewportIntersectionsForSubtree() {
   if (!GetFrame().GetDocument()->IsActive())
     return;
 
+  unsigned flags = GetIntersectionObservationFlags(parent_flags);
+
   if (!NeedsLayout()) {
     // Notify javascript IntersectionObservers
     if (GetFrame().GetDocument()->GetIntersectionObserverController()) {
       GetFrame()
           .GetDocument()
           ->GetIntersectionObserverController()
-          ->ComputeTrackedIntersectionObservations();
+          ->ComputeTrackedIntersectionObservations(flags);
     }
+    intersection_observation_state_ = kNotNeeded;
   }
 
   for (Frame* child = frame_->Tree().FirstChild(); child;
        child = child->Tree().NextSibling()) {
-    child->View()->UpdateViewportIntersectionsForSubtree();
+    child->View()->UpdateViewportIntersectionsForSubtree(flags);
   }
 
   for (HTMLPortalElement* portal :
        DocumentPortals::From(*frame_->GetDocument()).GetPortals()) {
-    if (portal->ContentFrame())
-      portal->ContentFrame()->View()->UpdateViewportIntersectionsForSubtree();
+    if (portal->ContentFrame()) {
+      portal->ContentFrame()->View()->UpdateViewportIntersectionsForSubtree(
+          flags);
+    }
   }
-
-  intersection_observation_state_ = kNotNeeded;
 }
 
 void LocalFrameView::UpdateThrottlingStatusForSubtree() {
@@ -4044,7 +4048,8 @@ void LocalFrameView::SetPaintArtifactCompositorNeedsUpdate() const {
     root->paint_artifact_compositor_->SetNeedsUpdate();
 }
 
-unsigned LocalFrameView::GetIntersectionObservationFlags() const {
+unsigned LocalFrameView::GetIntersectionObservationFlags(
+    unsigned parent_flags) const {
   unsigned flags = 0;
 
   const LocalFrame& target_frame = GetFrame();
@@ -4057,21 +4062,15 @@ unsigned LocalFrameView::GetIntersectionObservationFlags() const {
 
   // Observers with explicit roots only need to be checked on the same frame,
   // since in this case target and root must be in the same document.
-  if (intersection_observation_state_ != kNotNeeded)
-    flags |= IntersectionObservation::kExplicitRootObserversNeedUpdate;
+  if (intersection_observation_state_ != kNotNeeded) {
+    flags |= (IntersectionObservation::kExplicitRootObserversNeedUpdate |
+              IntersectionObservation::kImplicitRootObserversNeedUpdate);
+  }
 
   // For observers with implicit roots, we need to check state on the whole
-  // local frame tree.
-  const LocalFrameView* local_root_view = target_frame.LocalFrameRoot().View();
-  for (const LocalFrameView* view = this; view;
-       view = view->ParentFrameView()) {
-    if (view->intersection_observation_state_ != kNotNeeded) {
-      flags |= IntersectionObservation::kImplicitRootObserversNeedUpdate;
-      break;
-    }
-    if (view == local_root_view)
-      break;
-  }
+  // local frame tree, as passed down from the parent.
+  flags |= (parent_flags &
+            IntersectionObservation::kImplicitRootObserversNeedUpdate);
 
   return flags;
 }

third_party/blink/renderer/core/frame/local_frame_view.h

@@ -218,7 +218,7 @@ class CORE_EXPORT LocalFrameView final
 
   // Get the InstersectionObservation::ComputeFlags for target elements in this
   // view.
-  unsigned GetIntersectionObservationFlags() const;
+  unsigned GetIntersectionObservationFlags(unsigned parent_flags) const;
 
   void SetPaintArtifactCompositorNeedsUpdate() const;
 
@@ -867,7 +867,8 @@ class CORE_EXPORT LocalFrameView final
   template <typename Function>
   void ForAllNonThrottledLocalFrameViews(const Function&);
 
-  void UpdateViewportIntersectionsForSubtree() override;
+  void UpdateViewportIntersectionsForSubtree(unsigned parent_flags) override;
+
   void UpdateThrottlingStatusForSubtree();
 
   void NotifyResizeObservers();

third_party/blink/renderer/core/frame/remote_frame_view.cc

@@ -83,7 +83,13 @@ RemoteFrameView* RemoteFrameView::Create(RemoteFrame* remote_frame) {
   return view;
 }
 
-void RemoteFrameView::UpdateViewportIntersectionsForSubtree() {
+void RemoteFrameView::UpdateViewportIntersectionsForSubtree(
+    unsigned parent_flags) {
+  if (!(parent_flags &
+        IntersectionObservation::kImplicitRootObserversNeedUpdate)) {
+    return;
+  }
+
   LayoutEmbeddedContent* owner = remote_frame_->OwnerLayoutObject();
   if (!owner)
     return;

third_party/blink/renderer/core/frame/remote_frame_view.h

@@ -58,7 +58,7 @@ class RemoteFrameView final : public GarbageCollectedFinalized<RemoteFrameView>,
   void Show() override;
   void SetParentVisible(bool) override;
 
-  void UpdateViewportIntersectionsForSubtree() override;
+  void UpdateViewportIntersectionsForSubtree(unsigned parent_flags) override;
 
   bool GetIntrinsicSizingInfo(IntrinsicSizingInfo&) const override;
 

third_party/blink/renderer/core/intersection_observer/intersection_observer_controller.cc

@@ -63,14 +63,12 @@ void IntersectionObserverController::DeliverIntersectionObservations() {
   intersection_observers_being_invoked_.clear();
 }
 
-void IntersectionObserverController::ComputeTrackedIntersectionObservations() {
+void IntersectionObserverController::ComputeTrackedIntersectionObservations(
+    unsigned flags) {
   if (Document* document = To<Document>(GetExecutionContext())) {
     TRACE_EVENT0("blink",
                  "IntersectionObserverController::"
                  "computeTrackedIntersectionObservations");
-    unsigned flags;
-    if (LocalFrameView* target_view = document->View())
-      flags = target_view->GetIntersectionObservationFlags();
     for (auto& element : tracked_observation_targets_)
       element->ComputeIntersectionObservations(flags);
   }

third_party/blink/renderer/core/intersection_observer/intersection_observer_controller.h

@@ -33,7 +33,11 @@ class IntersectionObserverController
 
   void ScheduleIntersectionObserverForDelivery(IntersectionObserver&);
   void DeliverIntersectionObservations();
-  void ComputeTrackedIntersectionObservations();
+
+  // The flags argument is composed of values from
+  // IntersectionObservation::ComputeFlags. They are dirty bits that control
+  // whether an IntersectionObserver needs to do any work.
+  void ComputeTrackedIntersectionObservations(unsigned flags);
   void AddTrackedTarget(Element&);
   void RemoveTrackedTarget(Element&);