Bug 1462497 - Part 3: Don't condition useOpacity on values that might change silently between paints (HasAnimationsForCompositor, and the will-change budget). r=hiro

MozReview-Commit-ID: Kry5YIAIAHt

--HG--
extra : rebase_source : ccc1085ad534da882a9f5ef03a5b7675bc39ed08

layout/generic/nsFrame.cpp

@@ -2954,9 +2954,12 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
   // layer (for async animations), see
   // nsSVGIntegrationsUtils::PaintMaskAndClipPath or
   // nsSVGIntegrationsUtils::PaintFilter.
+  // Use MayNeedActiveLayer to decide, since we don't want to condition the wrapping
+  // display item on values that might change silently between paints (opacity activity
+  // can depend on the will-change budget).
   bool useOpacity = HasVisualOpacity(effectSet) &&
                     !nsSVGUtils::CanOptimizeOpacity(this) &&
-                    (!usingSVGEffects || nsDisplayOpacity::NeedsActiveLayer(aBuilder, this));
+                    (!usingSVGEffects || nsDisplayOpacity::MayNeedActiveLayer(this));
   bool useBlendMode = effects->mMixBlendMode != NS_STYLE_BLEND_NORMAL;
   bool useStickyPosition = disp->mPosition == NS_STYLE_POSITION_STICKY &&
     IsScrollFrameActive(aBuilder,

layout/painting/nsDisplayList.cpp

@@ -6526,6 +6526,12 @@ nsDisplayOpacity::NeedsActiveLayer(nsDisplayListBuilder* aBuilder, nsIFrame* aFr
   return false;
 }
 
+/* static */ bool
+nsDisplayOpacity::MayNeedActiveLayer(nsIFrame* aFrame)
+{
+  return ActiveLayerTracker::IsStyleMaybeAnimated(aFrame, eCSSProperty_opacity);
+}
+
 void
 nsDisplayOpacity::ApplyOpacity(nsDisplayListBuilder* aBuilder,
                              float aOpacity,

layout/painting/nsDisplayList.h

@@ -5386,6 +5386,7 @@ public:
   bool OpacityAppliedToChildren() const { return mOpacityAppliedToChildren; }
 
   static bool NeedsActiveLayer(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame);
+  static bool MayNeedActiveLayer(nsIFrame* aFrame);
   NS_DISPLAY_DECL_NAME("Opacity", TYPE_OPACITY)
   virtual void WriteDebugInfo(std::stringstream& aStream) override;