Hello,
I think almost everything is in the title. It's not 100% occurrence but very close.
I often save my guest instead of shutting down. But in recent Virtualbox versions, 5.26 and current 5.28, when I wakeup that saved guest, I have no more mouse pointer.
It is just invisible, but I can click and when the invisible pointer hovers a sensitive area, the area react.
I have tried disabling and re-enabling the input/mouse integration option, minimizing, restoring the VM window, but no way, the pointer remains invisible.
The initial session works fine, mouse is there as expected...
Recovery: I don't know except restarting the guest
Version 5.2.8 r121009 (Qt5.6.3)
Guest: Ubuntu 17.10 / X11 / Gnome
Nota: I use a W10 guest the same way too, and do not face any trouble with the pointer here. Seems Linux/GA specific. Maybe this post should be moved to Linux guest instead...