Bug 1574100 - For RefLayers with multiple metrics, only expose the event regions override for the bottom one. r=tnikkel

We only want the bottom LayerMetricsWrapper / WebRenderScrollDataWrapper to
be treated as a ref layer, and only ref layers have event regions overrides.

Differential Revision: https://phabricator.services.mozilla.com/D42339

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

gfx/layers/LayerMetricsWrapper.h

@@ -373,8 +373,8 @@ class MOZ_STACK_CLASS LayerMetricsWrapper final {
   EventRegionsOverride GetEventRegionsOverride() const {
     MOZ_ASSERT(IsValid());
 
-    if (mLayer->AsRefLayer()) {
-      return mLayer->AsRefLayer()->GetEventRegionsOverride();
+    if (AsRefLayer()) {
+      return AsRefLayer()->GetEventRegionsOverride();
     }
     return EventRegionsOverride::NoOverride;
   }

gfx/layers/wr/WebRenderScrollDataWrapper.h

@@ -326,7 +326,11 @@ class MOZ_STACK_CLASS WebRenderScrollDataWrapper final {
 
   EventRegionsOverride GetEventRegionsOverride() const {
     MOZ_ASSERT(IsValid());
-    return mLayer->GetEventRegionsOverride();
+    // Only ref layers can have an event regions override.
+    if (GetReferentId()) {
+      return mLayer->GetEventRegionsOverride();
+    }
+    return EventRegionsOverride::NoOverride;
   }
 
   const ScrollbarData& GetScrollbarData() const {