Scrollbar stops working when the mouse is moved

Click scrollbar's square (at the ends) and move mouse out (without releasing the mouse),
then move it back in, the scrollbar's square doesn't work anymore.
<zk>
<grid height="500px" width="300px">
<custom-attributes org.zkoss.zul.grid.rod="true" />
<rows>
<zscript>
Object o = new Object[100];
</zscript>
<row forEach="${o}">
<label value="x"></label>
y
</row>
</rows>
</grid>
</zk>

note1: when we move the mouse out, even it is loading data (ROD), it should still work after we move the mouse back in.

note2: This issue is inconsistent with different browsers.
Please reproduce it with Chrome and test against all browsers after it's fixed as it seems to be related to browser.