Bug 1450099 - Add a fast-path to FindScrollThumbNode() for when the drag metrics has not been initialized. r=kats

MozReview-Commit-ID: IGhP2KW9WC

--HG--
extra : rebase_source : b3d77febfb9bcbf03ae821147e8301b09e00d8f2

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -2553,6 +2553,12 @@ APZCTreeManager::SetLongTapEnabled(bool aLongTapEnabled)
 RefPtr<HitTestingTreeNode>
 APZCTreeManager::FindScrollThumbNode(const AsyncDragMetrics& aDragMetrics)
 {
+  if (!aDragMetrics.mDirection) {
+    // The AsyncDragMetrics has not been initialized yet - there will be
+    // no matching node, so don't bother searching the tree.
+    return RefPtr<HitTestingTreeNode>();
+  }
+
   RecursiveMutexAutoLock lock(mTreeLock);
 
   return DepthFirstSearch<ReverseIterator>(mRootNode.get(),