[I mentioned most of this in my post on virtualbox v. suspend-wake, but it’s become clear that really it’s own set of issues…]
Virtualbox 5.2.6 on TrueOS 18.03:
In virtualbox guests in fullscreen (but not windowed display) keyboard input is largely lost. “Largely” because even though everything including the host-key doesn’t do anything, ctrl-alt-F? does switch to consoles (and the keyboard works fine there). Also “largely” because the keyboard did start working at one point in a linux-4.16 guest in fullscreen after not working, but was lost on return to fullscreen. Note that linux-4.16 attempts integrating at least much of the virtualbox guest-additions functionality into the kernel…fullscreen at native resolution does work…
Also in fullscreen, the popup virtualbox menu at the bottom (optionally top) of the screen doesn’t come up at all (never seen it).
(Note that the same machine with EL7 as the host-OS had none of these problems…)