SUMMARY There should be a way to force all notifications to close after a given interval. STEPS TO REPRODUCE 1. Allow notifications for a website on Chromium 2. Get a notification 3. Wait for the popup to close. OBSERVED RESULT Notification doesn't close after the interval defined in KCM (5 seconds default). The problem is compounded by the fact that there's no shortcut to dismiss notifications, nor are the popups grouped. You have to manually dismiss it by clicking a the close button on each popup, which is unusable. Theoretically, these apps could be fixed upstream to support KDE's notification system properly but realistically they aren't going to do that. Thus there needs to be a way to force their notifications to disappear.
Chrome requests an interval of 25 seconds for whatever reason. That said, from 5.22 on we announce persistency capability which promots chrome to use 5 seconds instead.