--- a/widget/nsBaseWidget.cpp
+++ b/widget/nsBaseWidget.cpp
@@ -1873,17 +1873,17 @@ void
 nsBaseWidget::StartAsyncScrollbarDrag(const AsyncDragMetrics& aDragMetrics)
 {
   if (!AsyncPanZoomEnabled()) {
     return;
   }
 
   MOZ_ASSERT(XRE_IsParentProcess() && mCompositorSession);
 
-  int layersId = mCompositorSession->RootLayerTreeId();;
+  uint64_t layersId = mCompositorSession->RootLayerTreeId();
   ScrollableLayerGuid guid(layersId, aDragMetrics.mPresShellId, aDragMetrics.mViewId);
 
   APZThreadUtils::RunOnControllerThread(NewRunnableMethod
     <ScrollableLayerGuid, AsyncDragMetrics>(mAPZC,
                                             &IAPZCTreeManager::StartScrollbarDrag,
                                             guid, aDragMetrics));
 }