I have Lubuntu installed in a VirtualBox. The login screen (and Lubuntu splash screen) defaulted to 640x480 resolution. The resolution of the Lubuntu desktop is 1680x1050, which is the largest resolution I could use in a VirtualBox without losing the LXQt Panel.
Is there a way of changing the resolution of the login screen to the same 1680x1050? I looked at the manual page for SDDM Configuration and this is not covered.
I don’t think this is a Lubuntu specific thing. I personally made the case in the forum for Virtualbox about some 2 years ago now but that didn’t lead me to anything… I’ve reinstalled Virtualbox so many times—each time reviewing everything I did—but to this day I still get this on any VM I run now (be it Windows, Linux, etc.).
Apparently not enough people had the issue I had at the time to warrant a bug so I have no clue if that ever got recorded somewhere other than my lousy post.
If you’re not able to get full-screen after logging in despite putting Virtualbox in full screen mode, my guess would be to make sure you have Guest Additions installed in the VM.