Bug 1166500 - Part 3: Add KeyframeEffectReadOnly::CanIgnoreIfNotVisible(). r=dbaron

dom/animation/KeyframeEffect.cpp

@@ -1329,6 +1329,15 @@ KeyframeEffectReadOnly::CalculateCumulativeChangeHint()
   }
 }
 
+bool
+KeyframeEffectReadOnly::CanIgnoreIfNotVisible() const
+{
+  // FIXME: For further sophisticated optimization we need to check
+  // change hint on the segment corresponding to computedTiming.progress.
+  return NS_IsHintSubset(
+    mCumulativeChangeHint, nsChangeHint_Hints_CanIgnoreIfNotVisible);
+}
+
 //---------------------------------------------------------------------
 //
 // KeyframeEffect

dom/animation/KeyframeEffect.h

@@ -345,6 +345,12 @@ public:
   // This is used for deciding the animation is paint-only.
   void CalculateCumulativeChangeHint();
 
+  // Returns true if all of animation properties' change hints
+  // can ignore painting if the animation is not visible.
+  // See nsChangeHint_Hints_CanIgnoreIfNotVisible in nsChangeHint.h
+  // in detail which change hint can be ignored.
+  bool CanIgnoreIfNotVisible() const;
+
 protected:
   KeyframeEffectReadOnly(nsIDocument* aDocument,
                          const Maybe<OwningAnimationTarget>& aTarget,

layout/base/nsChangeHint.h

@@ -359,6 +359,13 @@ inline nsChangeHint NS_HintsNotHandledForDescendantsIn(nsChangeHint aChangeHint)
 #define NS_STYLE_HINT_FRAMECHANGE \
   nsChangeHint(NS_STYLE_HINT_REFLOW | nsChangeHint_ReconstructFrame)
 
+#define nsChangeHint_Hints_CanIgnoreIfNotVisible   \
+  nsChangeHint(NS_STYLE_HINT_VISUAL |              \
+               nsChangeHint_NeutralChange |        \
+               nsChangeHint_UpdateOpacityLayer |   \
+               nsChangeHint_UpdateTransformLayer | \
+               nsChangeHint_UpdateUsesOpacity)
+
 /**
  * |nsRestyleHint| is a bitfield for the result of
  * |HasStateDependentStyle| and |HasAttributeDependentStyle|.  When no