Note: The document is from 7.1.9. It states "You cannot prohibit or detect this in your application. Closing the browser window is not detectable by a JavaScript event or function. Microsoft — possibly by design — provides no event for this; users must be trained to click the Log off link.".

The Pega Infinity version of this is here. https://community.pega.com/sites/default/files/help_v81/procomhelpmain.htm#howto/howto2/howtocustomizelogout.htm#Modifying_log_off_processing

It also states the same - "In contrast, users who exit by closing the browser window do not cause this activity to run. Locks and clipboard are retained until the session times out; this can interfere with the ability of other users to update work items, assignments, or rules locked by that user."