Bug 1247964 - Allow InputBlockState::SetScrolledApzc to accept new APZC when it is an ancestor of the current APZC r=kats

gfx/layers/apz/src/InputBlockState.cpp

@@ -87,11 +87,33 @@ InputBlockState::IsTargetConfirmed() const
   return mTargetConfirmed;
 }
 
+bool
+InputBlockState::IsAncestorOf(AsyncPanZoomController* aA, AsyncPanZoomController* aB)
+{
+  if (aA == aB) {
+    return true;
+  }
+
+  bool seenA = false;
+  for (size_t i = 0; i < mOverscrollHandoffChain->Length(); ++i) {
+    AsyncPanZoomController* apzc = mOverscrollHandoffChain->GetApzcAtIndex(i);
+    if (apzc == aB) {
+      return seenA;
+    }
+    if (apzc == aA) {
+      seenA = true;
+    }
+  }
+  return false;
+}
+
+
 void
 InputBlockState::SetScrolledApzc(AsyncPanZoomController* aApzc)
 {
-  // An input block should only have one scrolled APZC.
-  MOZ_ASSERT(!mScrolledApzc || mScrolledApzc == aApzc);
+  // With immediate handoff disabled, the scrolled APZC cannot move down the handoff chain
+  // but it can move up the handoff chain if we change scrolling directions.
+  MOZ_ASSERT(!mScrolledApzc || IsAncestorOf(aApzc, mScrolledApzc));
 
   mScrolledApzc = aApzc;
 }

gfx/layers/apz/src/InputBlockState.h

@@ -56,6 +56,11 @@ public:
 protected:
   virtual void UpdateTargetApzc(const RefPtr<AsyncPanZoomController>& aTargetApzc);
 
+private:
+  // Checks whether |aA| is an ancestor of |aB| (or the same as |aB|) in
+  // |mOverscrollHandoffChain|.
+  bool IsAncestorOf(AsyncPanZoomController* aA, AsyncPanZoomController* aB);
+
 private:
   RefPtr<AsyncPanZoomController> mTargetApzc;
   bool mTargetConfirmed;