Steps to Reproduce

<zk>
  <button id="btn"/>
</zk>

execute in the console

var b = zk.$('$btn'); 
b.detach();
b.fire('onCustomEvent', null, {toServer: true});

Current Result

browser tab gets stuck at 100% CPU in an infinite loop

Expected Result

fail with a (meaningful) error instead due to incorrect usage

Debug Information

the infinite loop happens here:
https://github.com/zkoss/zk/blob/v9.6.0/zk/src/archive/web/js/zk/au.ts#L139-L142

In the latest 9.6.1-FL-2021-09-28 the infinite loop code is not reached, instead NOTHING happens, no indication of an error (maybe also not the best outcome)

the potential infinite loop code is still there and needs to fail if called incorrectly

Workaround

don't fire events on detached components.
e.g. check manually whether the current widget is detached before firing the event

var b = zk.$('$btn'); 
b.detach(); 
if(b.parent && b.desktop) {
   b.fire('onCustomEvent', null, {toServer: true});
}