When I go to Session and Startup, in the General tab, and uncheck "Prompt at Logout", then rather than any logout dialogue that asks if I want to Shutdown, Restart, Suspend, etc., it just logs out. Re-checking "Prompt at Logout" fixes the problem.
Uh, how is this a bug? You told it not to prompt, and it didn't.
The semantics are unclear: it gives the impression that unchecking that means that one is not prompted about saving sessions, rather than whether I should shutdown/restart/logout etc. (Indeed, that seems to be how it worked in 4.4, but whether that was the correct behaviour, I don't know.)
Not really... I just read the dialog again, and it's in a section with a heading "Logout Settings". The fact that it's underneath the "Automatically save sesion on logout" is pretty unrelated and irrelevant. Hell, I'd even say the two options would be mutually exclusive if the "prompt" one had to do with saving the session. If you want to automatically save your session, why would you want to prompt?