Bug 934420 - Try to preserve the right APZC in the face of layer tree changes. r=botond

gfx/layers/composite/APZCTreeManager.cpp

@@ -121,6 +121,22 @@ APZCTreeManager::UpdatePanZoomControllerTree(CompositorParent* aCompositor,
 
         apzc = container->GetAsyncPanZoomController();
 
+        // If the container doesn't have an APZC already, try to find one of our
+        // pre-existing ones that matches. In particular, if we find an APZC whose
+        // ScrollableLayerGuid is the same, then we know what happened is that the
+        // layout of the page changed causing the layer tree to be rebuilt, but the
+        // underlying content for which the APZC was originally created is still
+        // there. So it makes sense to pick up that APZC instance again and use it here.
+        if (apzc == nullptr) {
+          ScrollableLayerGuid target(aLayersId, container->GetFrameMetrics());
+          for (size_t i = 0; i < aApzcsToDestroy->Length(); i++) {
+            if (aApzcsToDestroy->ElementAt(i)->Matches(target)) {
+              apzc = aApzcsToDestroy->ElementAt(i);
+              break;
+            }
+          }
+        }
+
         // The APZC we get off the layer may have been destroyed previously if the layer was inactive
         // or omitted from the layer tree for whatever reason from a layers update. If it later comes
         // back it will have a reference to a destroyed APZC and so we need to throw that out and make

gfx/layers/ipc/AsyncPanZoomController.cpp

@@ -1535,9 +1535,7 @@ void AsyncPanZoomController::UpdateScrollOffset(const CSSPoint& aScrollOffset)
 
 bool AsyncPanZoomController::Matches(const ScrollableLayerGuid& aGuid)
 {
-  // TODO: also check the presShellId, once that is fully propagated
-  // everywhere in RenderFrameParent and AndroidJNI.
-  return aGuid.mLayersId == mLayersId && aGuid.mScrollId == mFrameMetrics.mScrollId;
+  return aGuid == ScrollableLayerGuid(mLayersId, mFrameMetrics);
 }
 
 void AsyncPanZoomController::GetGuid(ScrollableLayerGuid* aGuidOut)