Bug 1433336 - Collect only documents which are observerd by IntersectionObserver in nsRefreshDriver::Tick(). r=smaug

We don't need to collect whole descendants documents.

This patch intentionally leaves animation events handling which is another
caller of CollectDocuments in nsRefreshDriver since the animation part will
be fixed in a different way in bug 1415780.

MozReview-Commit-ID: INAJm1NHLuI

--HG--
extra : rebase_source : 4e255cef604d4455d6efa5dc2ebddd5dc649ee65

dom/base/nsDocument.h

@@ -620,6 +620,10 @@ public:
   virtual void UpdateIntersectionObservations() override;
   virtual void ScheduleIntersectionObserverNotification() override;
   virtual void NotifyIntersectionObservers() override;
+  virtual bool HasIntersectionObservers() const override
+  {
+    return !mIntersectionObservers.IsEmpty();
+  }
 
   virtual void NotifyLayerManagerRecreated() override;
 

dom/base/nsIDocument.h

@@ -3120,6 +3120,7 @@ public:
   virtual void UpdateIntersectionObservations() = 0;
   virtual void ScheduleIntersectionObserverNotification() = 0;
   virtual void NotifyIntersectionObservers() = 0;
+  virtual bool HasIntersectionObservers() const = 0;
 
   // Dispatch a runnable related to the document.
   virtual nsresult Dispatch(mozilla::TaskCategory aCategory,

layout/base/nsRefreshDriver.cpp

@@ -1587,6 +1587,28 @@ CollectDocuments(nsIDocument* aDocument, void* aDocArray)
   return true;
 }
 
+void
+nsRefreshDriver::UpdateIntersectionObservations()
+{
+  AutoTArray<nsCOMPtr<nsIDocument>, 32> documents;
+
+  if (mPresContext->Document()->HasIntersectionObservers()) {
+    documents.AppendElement(mPresContext->Document());
+  }
+
+  mPresContext->Document()->CollectDescendantDocuments(
+    documents,
+    [](const nsIDocument* document) -> bool {
+      return document->HasIntersectionObservers();
+    });
+
+  for (uint32_t i = 0; i < documents.Length(); ++i) {
+    nsIDocument* doc = documents[i];
+    doc->UpdateIntersectionObservations();
+    doc->ScheduleIntersectionObserverNotification();
+  }
+}
+
 void
 nsRefreshDriver::DispatchAnimationEvents()
 {
@@ -1955,13 +1977,7 @@ nsRefreshDriver::Tick(int64_t aNowEpoch, TimeStamp aNowTime)
   }
 #endif
 
-  AutoTArray<nsCOMPtr<nsIDocument>, 32> documents;
+  UpdateIntersectionObservations();
-  CollectDocuments(mPresContext->Document(), &documents);
-  for (uint32_t i = 0; i < documents.Length(); ++i) {
-    nsIDocument* doc = documents[i];
-    doc->UpdateIntersectionObservations();
-    doc->ScheduleIntersectionObserverNotification();
-  }
 
   /*
    * Perform notification to imgIRequests subscribed to listen

layout/base/nsRefreshDriver.h

@@ -385,6 +385,7 @@ private:
   void DispatchPendingEvents();
   void DispatchAnimationEvents();
   void RunFrameRequestCallbacks(mozilla::TimeStamp aNowTime);
+  void UpdateIntersectionObservations();
   void Tick(int64_t aNowEpoch, mozilla::TimeStamp aNowTime);
 
   enum EnsureTimerStartedFlags {