Steps to Reproduce

Open zkfiddle with the default example, wrap the window's content with a <vlayout vflex="1">

<zk> 
<window border="normal" title="hello" apply="pkg$.TestComposer"> 
<vlayout vflex="1"> 
<div>Welcome to ZK Fiddle , run it right now!</div> 
<button id="btn" label="Click Me To Evalute Event Binding With Composer" />   
</vlayout>   
</window>
</zk>

Current Result

: the content does not show up.

Expected Result

The content should be visible

Debug Information

Workaround

add <custom-attributes org.zkoss.zul.css.flex="false"/>