Fix for 50691, r=anthonyd

layout/xul/base/src/nsXULTreeOuterGroupFrame.cpp

@@ -286,6 +286,8 @@ nsXULTreeOuterGroupFrame::SetRowHeight(nscoord aRowHeight)
     mTreeLayoutState = eTreeLayoutAbort;
     if (mCurrentIndex > 0)
       VerticalScroll(mCurrentIndex * mRowHeight);
+
+    PostReflowCallback();
   } 
 }