Bug 1130982 - Ensure the clip rect on a multi-metrics layer belongs to the bottommost layer. r=botond

gfx/layers/LayerMetricsWrapper.h

@@ -335,7 +335,11 @@ public:
   {
     MOZ_ASSERT(IsValid());
 
-    return mLayer->GetClipRect();
+    if (AtBottomLayer()) {
+      return mLayer->GetClipRect();
+    }
+
+    return nullptr;
   }
 
   bool GetForceDispatchToContentRegion() const {

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -227,7 +227,8 @@ ComputeClipRegion(GeckoContentController* aController,
     clipRegion = nsIntRegion(*aLayer.GetClipRect());
   } else {
     // if there is no clip on this layer (which should only happen for the
-    // root scrollable layer in a process) fall back to using the comp
+    // root scrollable layer in a process, or for some of the LayerMetrics
+    // expansions of a multi-metrics layer), fall back to using the comp
     // bounds which should be equivalent.
     clipRegion = nsIntRegion(ParentLayerIntRect::ToUntyped(
         RoundedToInt(aLayer.Metrics().mCompositionBounds)));