Steps to Reproduce

<zk>
	<style>
		iframe {
			opacity: 1!important;
			outline: 2px solid red;
		}
	</style>

	<div>
		<div height="200px"/>
		label shifting it to the button to the right: <button popup="pop,position=after_end" label="click for popup"/>
		<popup id="pop" width="200px" height="150px">Dummy Content</popup>
	</div>
</zk>

click the button to trigger a popup 2x

Current Result

first: the stackup iframe (styled to show a red outline) renders in the wrong position (0, 0)
second: the stackup iframe is rendered below the popup (as expected)

Expected Result

stackup iframe is always positioned below the popup (if still needed)

Evaluate if this feature is still needed for modern browsers in future versions.

Workaround

disable stackup (related docs)

<?script content="zk.useStackup=false;"?>