Current state

Following http://tracker.zkoss.org/browse/ZK-3241: if an error message is trigger on a non-visible component, it will be ignored instead.
If the component becomes visible afterward, the error message will not be displayed.

Possible improvement

If possible, setting this._errbox even when non-visible to the proper value would allow most inputWidgets to display the error message when they become visible again. (most widgets such as textbox already hold a onShow listener able to show currently set errbox)