Bug 1116588 - Don't optimize away opacity:0 items if we're building event regions, unless the frame also has pointer-events:none. r=tn

layout/base/nsDisplayList.cpp

@@ -3735,10 +3735,6 @@ already_AddRefed<Layer>
 nsDisplayOpacity::BuildLayer(nsDisplayListBuilder* aBuilder,
                              LayerManager* aManager,
                              const ContainerLayerParameters& aContainerParameters) {
-  if (mOpacity == 0 && mFrame->GetContent() &&
-      !nsLayoutUtils::HasAnimations(mFrame->GetContent(), eCSSProperty_opacity)) {
-    return nullptr;
-  }
   nsRefPtr<Layer> container = aManager->GetLayerBuilder()->
     BuildContainerLayerFor(aBuilder, aManager, mFrame, this, &mList,
                            aContainerParameters, nullptr);

layout/generic/nsFrame.cpp

@@ -1952,10 +1952,13 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
   // we're painting, and we're not animating opacity. Don't do this
   // if we're going to compute plugin geometry, since opacity-0 plugins
   // need to have display items built for them.
+  bool needEventRegions = aBuilder->IsBuildingLayerEventRegions() &&
+      StyleVisibility()->GetEffectivePointerEvents(this) != NS_STYLE_POINTER_EVENTS_NONE;
   if (disp->mOpacity == 0.0 && aBuilder->IsForPainting() &&
       !aBuilder->WillComputePluginGeometry() &&
       !(disp->mWillChangeBitField & NS_STYLE_WILL_CHANGE_OPACITY) &&
-      !nsLayoutUtils::HasAnimations(mContent, eCSSProperty_opacity)) {
+      !nsLayoutUtils::HasAnimations(mContent, eCSSProperty_opacity) &&
+      !needEventRegions) {
     return;
   }