Bug 1211654 - Force opacity layers that were only created for APZ hit-testing information to always be inactive. r=mstange

layout/base/nsDisplayList.cpp

@@ -3821,9 +3821,12 @@ nsresult nsDisplayWrapper::WrapListsInPlace(nsDisplayListBuilder* aBuilder,
 }
 
 nsDisplayOpacity::nsDisplayOpacity(nsDisplayListBuilder* aBuilder,
-                                   nsIFrame* aFrame, nsDisplayList* aList)
+                                   nsIFrame* aFrame, nsDisplayList* aList,
+                                   bool aForEventsOnly)
     : nsDisplayWrapList(aBuilder, aFrame, aList)
-    , mOpacity(aFrame->StyleDisplay()->mOpacity) {
+    , mOpacity(aFrame->StyleDisplay()->mOpacity)
+    , mForEventsOnly(aForEventsOnly)
+{
   MOZ_COUNT_CTOR(nsDisplayOpacity);
 }
 
@@ -3954,6 +3957,14 @@ nsDisplayItem::LayerState
 nsDisplayOpacity::GetLayerState(nsDisplayListBuilder* aBuilder,
                                 LayerManager* aManager,
                                 const ContainerLayerParameters& aParameters) {
+  // If we only created this item so that we'd get correct nsDisplayEventRegions for child
+  // frames, then force us to inactive to avoid unnecessary layerization changes for content
+  // that won't ever be painted.
+  if (mForEventsOnly) {
+    MOZ_ASSERT(mOpacity == 0);
+    return LAYER_INACTIVE;
+  }
+
   if (NeedsActiveLayer(aBuilder))
     return LAYER_ACTIVE;
 

layout/base/nsDisplayList.h

@@ -3167,7 +3167,7 @@ protected:
 class nsDisplayOpacity : public nsDisplayWrapList {
 public:
   nsDisplayOpacity(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
-                   nsDisplayList* aList);
+                   nsDisplayList* aList, bool aForEventsOnly);
 #ifdef NS_BUILD_REFCNT_LOGGING
   virtual ~nsDisplayOpacity();
 #endif
@@ -3202,6 +3202,7 @@ public:
 
 private:
   float mOpacity;
+  bool mForEventsOnly;
 };
 
 class nsDisplayMixBlendMode : public nsDisplayWrapList {

layout/generic/nsFrame.cpp

@@ -1920,13 +1920,17 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
   // need to have display items built for them.
   bool needEventRegions = aBuilder->IsBuildingLayerEventRegions() &&
       StyleVisibility()->GetEffectivePointerEvents(this) != NS_STYLE_POINTER_EVENTS_NONE;
+  bool opacityItemForEventsOnly = false;
   if (disp->mOpacity == 0.0 && aBuilder->IsForPainting() &&
       !aBuilder->WillComputePluginGeometry() &&
       !(disp->mWillChangeBitField & NS_STYLE_WILL_CHANGE_OPACITY) &&
       !nsLayoutUtils::HasCurrentAnimationOfProperty(this,
-                                                    eCSSProperty_opacity) &&
-      !needEventRegions) {
-    return;
+                                                    eCSSProperty_opacity)) {
+    if (needEventRegions) {
+      opacityItemForEventsOnly = true;
+    } else {
+      return;
+    }
   }
 
   if (disp->mWillChangeBitField != 0) {
@@ -2135,7 +2139,7 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
     DisplayListClipState::AutoSaveRestore opacityClipState(aBuilder);
     opacityClipState.Clear();
     resultList.AppendNewToTop(
-        new (aBuilder) nsDisplayOpacity(aBuilder, this, &resultList));
+        new (aBuilder) nsDisplayOpacity(aBuilder, this, &resultList, opacityItemForEventsOnly));
   }
   /* If we have sticky positioning, wrap it in a sticky position item.
    */