Bug 1245925 - Don't allow expiring the displayport on root scrollframes. r=tnikkel

MozReview-Commit-ID: 8LGsk9uKyL5

--HG--
extra : rebase_source : 9edf7c6a4ce795da670f1ad06c8f86f9f05e4850

layout/generic/nsGfxScrollFrame.cpp

@@ -2353,7 +2353,7 @@ RemoveDisplayPortCallback(nsITimer* aTimer, void* aClosure)
   MOZ_ASSERT(helper->mDisplayPortExpiryTimer);
   helper->mDisplayPortExpiryTimer = nullptr;
 
-  if (helper->IsAlwaysActive() || helper->mIsScrollParent) {
+  if (!helper->AllowDisplayPortExpiration() || helper->mIsScrollParent) {
     // If this is a scroll parent for some other scrollable frame, don't
     // expire the displayport because it would break scroll handoff. Once the
     // descendant scrollframes have their displayports expired, they will
@@ -2417,9 +2417,20 @@ void ScrollFrameHelper::ResetDisplayPortExpiryTimer()
   }
 }
 
-void ScrollFrameHelper::TriggerDisplayPortExpiration()
+bool ScrollFrameHelper::AllowDisplayPortExpiration()
 {
   if (IsAlwaysActive()) {
+    return false;
+  }
+  if (mIsRoot && mOuter->PresContext()->IsRoot()) {
+    return false;
+  }
+  return true;
+}
+
+void ScrollFrameHelper::TriggerDisplayPortExpiration()
+{
+  if (!AllowDisplayPortExpiration()) {
     return;
   }
 

layout/generic/nsGfxScrollFrame.h

@@ -370,6 +370,7 @@ public:
   void NotifyImageVisibilityUpdate();
   bool GetDisplayPortAtLastImageVisibilityUpdate(nsRect* aDisplayPort);
 
+  bool AllowDisplayPortExpiration();
   void TriggerDisplayPortExpiration();
   void ResetDisplayPortExpiryTimer();