Strange video behavior in VB

[MR ZenWiz]

I did some experimentation and found a few really strange oddities.

Full disclosure (shoulda done this before): I'm running these VMs in
VirtualBox 6.1.14, which I believe is the latest from Oracle.  The
host is Xubuntu 20.04.1.

The Mint VM runs ok, just slow.  It has the VMSVGA driver and no 3D
acceleration (and it complains about that, but I couldn't get it to
work with that option enabled, so no).  It runs in full screen view
just fine.  I probably won't use it because I still don't like the
Mint GUI or the mouse menu "start" button) structure.  It doesn't map
well for me.

I fooled around with the Ubuntu VM to play with display sizes.  I
could get it up to 2048x1560 (I think), but anything wider and it went
black.  If I then shrank it, the GUI came back.  Other than that, it
works fine.  I don't like the GNOME window manager at all - that's one
of the reasons I switched to Xubuntu back in '12 or '14, I forget
which.  My host screen, panel and keyboard shortcuts are configured
the way I like them and I see no reason to go back to a weighty GUI/wm
like GNOME, or Cinnamon.

Then I checked the display settings in my Xubuntu 20.04.1 VM (I used
that to play with it before upgrading).  The other two had 16Mb of
memory for display, but this one had 32Mb.  The lights went on, so I
upped the Mint and Ubuntu VMs to 32Mb, and voila - no more display
problems.  Both run fine in full screen mode now.

However, I then noticed that my Xubuntu 20.04.1 VM would not use the
whole full screen, and the display settings won't adjust to my 2K
dimensions.  It does allow 2560x1600, but that falls off the bottom of
the 1440 pixel screen, and the closest I can get it to a useful size
is 1920x1200, which looks silly with all that black around it.  I
don't need it, so I probably won't use it, but I'll hang onto it for
the heck of it.

So, VM display problem partially solved, enough that i can proceed.

Many thanks to all who responded, especially Liam, who challenged me
to look around at many things.

I'll shut up now.  :-)

Mark