zkdemo animation unexpected behavior

To reproduce
1. Go to zkdemo - animation
2. Keep Clicking on "Toggle Visible"
3. The Window sometimes pops up from top-right corner

Code:
<window title="Animation Effects" border="normal" xmlns:h="http://www.w3.org/1999/xhtml">
<button label="Toggle Visible" onClick="win.visible = !win.visible"/>
<window id="win" border="normal" width="200px" position="center" mode="overlapped"
action="onshow:anima.moveDown(#{self});onhide:anima.fade(#{self})" visible="false">
<caption image="/img/inet.png" label="Hi there!"/>
<checkbox label="Hello, Effects!"/>
</window>
</window>
Note:
If following code is removed, it works fine.
onhide:anima.fade(#{self})