--- a/gfx/layers/apz/src/AsyncPanZoomController.cpp
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -2196,16 +2196,17 @@ bool AsyncPanZoomController::UpdateAnima
     }
     UpdateSharedCompositorFrameMetrics();
     return true;
   }
   return false;
 }
 
 Matrix4x4 AsyncPanZoomController::GetOverscrollTransform() const {
+  ReentrantMonitorAutoEnter lock(mMonitor);
   if (!IsOverscrolled()) {
     return Matrix4x4();
   }
 
   // The overscroll effect is a uniform stretch along the overscrolled axis,
   // with the edge of the content where we have reached the end of the
   // scrollable area pinned into place.