ZK-355 regressions in mobile

<zk>
1. Please scroll the scrollbar to the middle of the list, and click the "click me" button.
<separator/>
2. You should see the scrollbar is placed at the same area.

	<zscript>
		ListModel strset = new ListModelList(Locale.getAvailableLocales());
	</zscript>
	<listbox id="list" width="200px" rows="10" model="&#36;{strset}">
		<listhead>
			<listheader label="Load on Demend" sort="auto"/>
		</listhead>
	</listbox>
<button label="click me" onClick="list.invalidate()"/>
</zk>