A call to showBusy on a component that is not visible is ignored. The problem with this is when a component that may be invisible at the time of the call (say on an inactive tab) later becomes visible (when the parent tab is selected). Try the following zk snippet. It will attempt to perform a showBusy on a div on the second tab when the button is clicked. You can see that unless tab 2 is visible at the time the button is clicked, showBusy will have no effect.

<zk>
<tabbox height="200px">
<tabs>
<tab label="tab 1" />
<tab label="tab 2" />
</tabs>
<tabpanels>
<tabpanel />
<tabpanel>
<div id="div" height="100%" width="100%" />
</tabpanel>
</tabpanels>
</tabbox>
<button label="Show Busy" onClick='Clients.showBusy(div, "busy");' />
</zk>