The chrome feature "Disallow sync XHR in page dismissal" (currently (2019-01-31) under development) is planning to prevent sync ajax requests during beforeunload.
This will prevent ZK's current mechanism to clean up desktops when reloading / navigating pages / closing the browser.
https://github.com/zkoss/zk/blob/v8.6.0.1/zk/src/archive/web/js/zk/au.js#L488

This will affect almost all current and past ZK versions.

currently reproduced with chromium version:

• "Version 74.0.3690.0 (Developer Build) (64-bit)"
• "Version 74.0.3705.0 (Developer Build) (64-bit)"

recent builds available here: https://download-chromium.appspot.com/