I am using popup with data loaded on ON_OPEN event.
It is quite quick, so I don't expect Progressbox to be shown.
But it is together with ZK-4237 - Cannot style progressbox for popup it's a problem.

Only way I can avoid wrong z-apply-loading DIV displaying is to override Popup animation to disable mask on open.

Popup.js: https://github.com/zkoss/zk/blob/4d2bce3fadb6239d2b6ef39db6b39ad244b5efb3/zul/src/archive/web/js/zul/wgt/Popup.js#L148
effect.js (Mask itself): https://github.com/zkoss/zk/blob/1e82bea329ea6d627aafb435ed192e1afb85c7e5/zk/src/archive/web/js/zk/effect.js#L254