This has happened numerous times in the past, most recently I noticed it with 5.5->5.6 and now 5.6->5.7. Whenver Qt sees a major upgrade the overlay dialogs for logout and friends does not actually load. The blur&shade effects are applied but then the dialog never gets loaded. I forgot to take a look at the logs but I am guessing this is because the qml files fail to load due to incompatible qtquick c++ plugins that would be dragged in. This essentially makes it impossible for a user to get out of the session. To prevent this the logout/reboot/shutdown/... components should be lazy loaded after startup and then held in-memory so we can always display the dialogs to actually end the currently running session. Reproducible: Always
It's on my todo to have ksmserver load the shutdown qml stuff on login and then kept to prevent this exact situation.
wouldn't using offline updates be the better solution to the problem?
Yes. I don't see those happening in distributions in any feasible time frame though. I also have the feeling that last time I discussed that with someone raised reasonable concerns against them, alas, I can't remember what they were ^^. Supposedly it was that when the user needs to be informed about something or actively needs to take action (configuration changes, license aggrement, whatever) it makes the entire affair fairly faffy. Also if things break they break hard if reliable rollbacks aren't and option, and TBH the only reliable rollback is a brfs snapshot rollback. Anyway, we need some solution to the problem at hand, in addition to that we could poll distros for thoughts on out-of-session upgrades.
The logout dialog is a separate binary now which shouldn't exhibit this problem.