SUMMARY I think that Okular should not start in fullscreen mode when it was closed in it previously. It should start always as a normal window. (From https://bugs.kde.org/show_bug.cgi?id=319856) STEPS TO REPRODUCE 1. Open a document 2. Switch to fullscreen 3. Exit okular using Alt+F4 4. Start okular again. OBSERVED RESULT Okular starts in the fullscreen mode. EXPECTED RESULT Okular should start as normal window. SOFTWARE/OS VERSIONS Linux/KDE Plasma: 5.20.5 (Debian)
Gwenview does this too. On one hand, it's nice to remember the user's last-used view mode. On the other hand, full screen mode might be an exception to this rule, and also if you don't know how to get out of full screen mode, it can be scary--especially since Okular has no visible "Exit full screen mode" button in the top-right corner the way Gwenview and Kate do. See Bug 419377.
I'm setting this to confirmed as this is definitely bad UX. I think Okular should definitely have an exit fullscreen button, and optionally but strongly think Okular should not start in fullscreen if last session was fullscreen.