Looks like you're missing a verticalAlign="middle" in your HGroup.
<s:HGroup verticalAlign="middle">
<s:Label id="l_url" text="url"/>
<s:TextInput id="url" invalid="url.getFocus()"/>
</s:HGroup>
Also I don't usually use form containers, but since you're only doing a mobile UI that might be a good suggestion to look into here if you don't mind the limitations.