Bug 1166500 - Part 7: Throttle paint-only animations if the presShell is not active. r=dbaron

dom/animation/KeyframeEffect.cpp

@@ -19,6 +19,7 @@
 #include "nsCSSProps.h" // For nsCSSProps::PropHasFlags
 #include "nsCSSPseudoElements.h" // For CSSPseudoElementType
 #include "nsDOMMutationObserver.h" // For nsAutoAnimationMutationBatch
+#include "nsIPresShell.h" // For nsIPresShell
 
 namespace mozilla {
 
@@ -1066,9 +1067,13 @@ KeyframeEffectReadOnly::CanThrottle() const
   }
 
   // We can throttle the animation if the animation is paint only and
-  // the target frame is out of view.
-  if (CanIgnoreIfNotVisible() && frame->IsScrolledOutOfView()) {
-    return true;
+  // the target frame is out of view or the document is in background tabs.
+  if (CanIgnoreIfNotVisible()) {
+    nsIPresShell* presShell = GetPresShell();
+    if ((presShell && !presShell->IsActive()) ||
+        frame->IsScrolledOutOfView()) {
+      return true;
+    }
   }
 
   // First we need to check layer generation and transform overflow
@@ -1201,14 +1206,20 @@ KeyframeEffectReadOnly::GetRenderedDocument() const
   return mTarget->mElement->GetComposedDoc();
 }
 
-nsPresContext*
-KeyframeEffectReadOnly::GetPresContext() const
+nsIPresShell*
+KeyframeEffectReadOnly::GetPresShell() const
 {
   nsIDocument* doc = GetRenderedDocument();
   if (!doc) {
     return nullptr;
   }
-  nsIPresShell* shell = doc->GetShell();
+  return doc->GetShell();
+}
+
+nsPresContext*
+KeyframeEffectReadOnly::GetPresContext() const
+{
+  nsIPresShell* shell = GetPresShell();
   if (!shell) {
     return nullptr;
   }

dom/animation/KeyframeEffect.h

@@ -37,6 +37,7 @@ class nsCSSPropertySet;
 class nsIContent;
 class nsIDocument;
 class nsIFrame;
+class nsIPresShell;
 class nsPresContext;
 
 namespace mozilla {
@@ -332,6 +333,7 @@ public:
 
   nsIDocument* GetRenderedDocument() const;
   nsPresContext* GetPresContext() const;
+  nsIPresShell* GetPresShell() const;
 
   // Associates a warning with the animated property on the specified frame
   // indicating why, for example, the property could not be animated on the