simple.zul

<zk>
	<div height="1000px">
		<div vflex="1"></div>
		<div>
			<combobox>
				<comboitem label="United Nations (UN)" />
				<comboitem label="North America (NA)" />
			</combobox>
		</div>
	</div>
</zk>

Steps to reproduce
1. Scroll down to the bottom of the page
2. Open combobox dropdown
3. Select any value (this works as expected)
4. Open dropdown again
Expected:
Document should not scrollback to the top
Actual:
Document scrolls back to the top and user need to scroll back down to the botton to access the combobox component.

Note: In modal window the document scrolls back to the top in step 2 itself i.e. when first time dropdown is opened.
modal.zul

<window mode="modal" width="500px" height="1000px" title="Main"
	border="normal">
	<div vflex="1"></div>
	<div>
		<combobox>
			<comboitem label="United Nations (UN)" />
			<comboitem label="North America (NA)" />
		</combobox>
	</div>
</window>