Bug 906427 - When a touch event has multiple touch points, send it to the root APZC for the layer subtree. r=BenWa

gfx/layers/composite/APZCTreeManager.cpp

@@ -222,6 +222,10 @@ APZCTreeManager::ReceiveInputEvent(const InputData& aEvent)
           nsRefPtr<AsyncPanZoomController> apzc2 = GetTargetAPZC(ScreenPoint(multiTouchInput.mTouches[i].mScreenPoint));
           mApzcForInputBlock = CommonAncestor(mApzcForInputBlock.get(), apzc2.get());
           APZC_LOG("Using APZC %p as the common ancestor\n", mApzcForInputBlock.get());
+          // For now, we only ever want to do pinching on the root APZC for a given layers id. So
+          // when we find the common ancestor of multiple points, also walk up to the root APZC.
+          mApzcForInputBlock = RootAPZCForLayersId(mApzcForInputBlock);
+          APZC_LOG("Using APZC %p as the root APZC for multi-touch\n", mApzcForInputBlock.get());
         }
       } else if (mApzcForInputBlock) {
         APZC_LOG("Re-using APZC %p as continuation of event block\n", mApzcForInputBlock.get());
@@ -289,6 +293,10 @@ APZCTreeManager::ReceiveInputEvent(const nsInputEvent& aEvent,
             GetTargetAPZC(ScreenPoint(point.x, point.y));
           mApzcForInputBlock = CommonAncestor(mApzcForInputBlock.get(), apzc2.get());
           APZC_LOG("Using APZC %p as the common ancestor\n", mApzcForInputBlock.get());
+          // For now, we only ever want to do pinching on the root APZC for a given layers id. So
+          // when we find the common ancestor of multiple points, also walk up to the root APZC.
+          mApzcForInputBlock = RootAPZCForLayersId(mApzcForInputBlock);
+          APZC_LOG("Using APZC %p as the root APZC for multi-touch\n", mApzcForInputBlock.get());
         }
       } else if (mApzcForInputBlock) {
         APZC_LOG("Re-using APZC %p as continuation of event block\n", mApzcForInputBlock.get());
@@ -659,5 +667,14 @@ APZCTreeManager::CommonAncestor(AsyncPanZoomController* aApzc1, AsyncPanZoomCont
   return nullptr;
 }
 
+AsyncPanZoomController*
+APZCTreeManager::RootAPZCForLayersId(AsyncPanZoomController* aApzc)
+{
+  while (aApzc && !aApzc->IsRootForLayersId()) {
+    aApzc = aApzc->GetParent();
+  }
+  return aApzc;
+}
+
 }
 }

gfx/layers/composite/APZCTreeManager.h

@@ -258,6 +258,7 @@ private:
   AsyncPanZoomController* FindTargetAPZC(AsyncPanZoomController* aApzc, const ScrollableLayerGuid& aGuid);
   AsyncPanZoomController* GetAPZCAtPoint(AsyncPanZoomController* aApzc, const gfxPoint& aHitTestPoint);
   AsyncPanZoomController* CommonAncestor(AsyncPanZoomController* aApzc1, AsyncPanZoomController* aApzc2);
+  AsyncPanZoomController* RootAPZCForLayersId(AsyncPanZoomController* aApzc);
 
   /**
    * Recursive helper function to build the APZC tree. The tree of APZC instances has

gfx/layers/ipc/AsyncPanZoomController.h

@@ -633,6 +633,14 @@ public:
   AsyncPanZoomController* GetLastChild() const { return mLastChild; }
   AsyncPanZoomController* GetPrevSibling() const { return mPrevSibling; }
   AsyncPanZoomController* GetParent() const { return mParent; }
+
+  /* Returns true if there is no APZC higher in the tree with the same
+   * layers id.
+   */
+  bool IsRootForLayersId() const {
+    return !mParent || (mParent->mLayersId != mLayersId);
+  }
+
 private:
   nsRefPtr<AsyncPanZoomController> mLastChild;
   nsRefPtr<AsyncPanZoomController> mPrevSibling;