Reproducing Steps:

1. run the zul below:

<zk>
	<vbox vflex="1">
		<button>My button</button>
		<div vflex="1" style="background: yellow" />
	</vbox>
</zk>

Expected:
No scroll bar appears, because we set vflex="1". The height of vbox should only fit exactly the same size as the height of a page.

Actual:
The height of vbox with vflex="1" exceeds a page's max height, so unexpected scroll bar appears on a browser.