Bug 1501382 - Build display items within opacity:0 if we need them for APZ hit testing. r=kats

Differential Revision: https://phabricator.services.mozilla.com/D14230

--HG--
extra : moz-landing-system : lando

layout/generic/nsFrame.cpp

@@ -2690,11 +2690,14 @@ void nsIFrame::BuildDisplayListForStackingContext(
   // 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 needHitTestInfo = aBuilder->BuildCompositorHitTestInfo() &&
+                         StyleUI()->GetEffectivePointerEvents(this) !=
+                             NS_STYLE_POINTER_EVENTS_NONE;
   bool opacityItemForEventsAndPluginsOnly = false;
   if (effects->mOpacity == 0.0 && aBuilder->IsForPainting() &&
       !(disp->mWillChangeBitField & NS_STYLE_WILL_CHANGE_OPACITY) &&
       !nsLayoutUtils::HasAnimationOfProperty(effectSet, eCSSProperty_opacity)) {
-    if (aBuilder->WillComputePluginGeometry()) {
+    if (needHitTestInfo || aBuilder->WillComputePluginGeometry()) {
       opacityItemForEventsAndPluginsOnly = true;
     } else {
       return;