After some inactivity on the host, typically after allowing the screensaver (on the host) to trigger, when I log back in (still on the host), and try to start interacting with my VM in Virtualbox again, I find that the screen fail to redraw.
By failing to redraw I mean that the screen is not just blank, but copies whatever is in the framebuffer from the previous window that was displayed -- i.e. when I switch to Virtualbox, if I had Firefox open, I just get the (static) content of the previous window.
However, sometimes, if I wait some time, such as a few minutes, suddenly Virtualbox will redraw its screen and work as normal from that point. Half the time, no amount of time seems to make it start working again.
What is causing this? How can I mitigate it?
Host: Debian 9.13 Kernel (host): 4.9.0-15-amd64
Virtualbox version: Version 6.1.18 r142142 (Qt5.7.1)
Guest: Debian 10, with kernel 4.19.