While dragging a scroll thumb, a tree doesn't render treerows in time

Steps to Reproduce

1. run attached zul
2. drag the scroll thumb/knob and move down slowly

Current Result

the tree doesn't render any treerows unless you stop scrolling
Tree scroll ZK 9.mp4

Expected Result

the tree keeps rendering treerows while scrolling
Tree scroll ZK 8.mp4

Debug Information

• This issue doesn't happen in 8.5.0.1. It starts to happen since 8.5.1.3
• The reported user thinks the previous behavior provides a better user experience because a user can know the range of items while he is dragging.
• <custom-attributes org.zkoss.zul.tree.initRodSize="-1"/> is not an option since the items are too many.
• I have tried <custom-attributes org.zkoss.zul.tree.preloadSize="5"/>, but it doesn't make a tree render while scrolling.

Workaround

• <tree>
  <custom-attributes org.zkoss.zul.client.rod="false"/>
  </tree>
• <custom-attributes org.zkoss.zul.nativebar="false"/> + tree-onrender.js