clem -- I think I may have found a "fix" !!!
Just adding 2 lines to /etc/modules :-
This seems to allow vboxvga and vboxsvga with-and-without 3d-accel enabled, to work without blackscreening, on my 19.3 cinnamon install where it used to fail a lot!.
The question is, does loading those modules cause any conflict or issue when not inside virtualbox...
I would reccomend creating an image with that change and test... At very least you could test this and add to release notes etc.
The file.deb test is only there to check gdebi's association with Firefox, it's a dummy (probably broken package).
I can reproduce the issue with nvidia-prime. We'll need to check that out post-release. It looks upstream from us (nvidia-prime package).
The issue with VBoxVGA happened since we upgraded the stack (kernel + xorg) but it doesn't happen "always". It seems to happen to some people and only on some ISOs. Here, with two ISOs built the exact same way, it'd happen on one but not the other... it's not something we control and we don't know why it "always" fails, but it fails consistently enough to say using VBoxVGA isn't a solution and only VMSVGA works reliably. This is either on Oracle or upstream to fix, Ubuntu is also affected. In the meantime, the release notes were updates to mention using VMSVGA as a workaround.
Actually, in some sense "scratch that" -- once system installed and updated, kernel 5.0.0-37 etc. -- issue recurs inside virtualbox, blackscreen occurs! Ctrl+Alt+Backspack needed. Goes back to the point about race-condition with vboxdrv ... Hope that helps.
This image seems better in terms of virtualbox graphics (see comments in MATE image etc.).
So far, this has consistently succeeded in not black-screening, and in both VBoxVGA/VBoxSVGA, supports auto-resize to virtualbox machine window size, albeit not in VMSVGA mode.
file.deb - matedesktop, error: dependency is not satisfiable: python-mate
Works fine with apt-get (doesn't even need to install python-mate)
nvidia prime: switching TO intel, logging out/in works (you end up with modesetting)
switching TO nvidia, logging out/in you still have modesetting, need to reboot instead, you end up with nvidia + modesetting, using nvidia renderer