Strange video behavior in VB

[MR ZenWiz]

I'm not sure where to post this, but I thought I;d try here first.

I just installed Linux Mint 20 in a VB VM on my Xubuntu 20.04.1
desktop, and it seems to work more or less well - until I tell it to
go into full screen mode.  The video goes all black and this persists
through one or two reboots before it clears up.  I can't use the full
screen mode at all.

I do get a warning that I don't have hardware acceleration enabled for
lack of a driver, but the drier search turned up nothing - not really
a surprise since the VB video driver isn't really hardware.

I have the guest additions installed, and I can resize the
screen/window, but in full screen mode, it goes blank and does not
appear to recover.

Any clues?

Thanks.

Mark