https://sourceforge.net/tracker/?func=detail&aid=3316222&group_id=152762&atid=785191

page1.zul
<window id="w2" title="w2" border="normal" width="300px">
<div style="padding: 10px">
<include src="page2.zul" mode="defer"/> <!-- remove mode to see difference -->
</div>
<button id="b1" label="b1" onClick='alert(desktop.getPage("pg2"))'/>
</window>

page2.zul
<?page id="pg2" title="Include page"?>
<window id="w1" title="w1" border="normal">
<button id="b2" label="b2"/>
</window>