Further update on my experiments with VMs on Virtualbox running on Debian Stable host with Debian Stable guest (mainly for purposes of building LFS):
The following give me best results:
Switch Display controller from VBoxVGA to VMSVGA
(this is because Virtualbox reports support for VBoxVGA will be discontinued from the next version upward and any VMs saved with that option will then stop working, I think)
Install latest version of Guest Additions
(as this seems to have the code for mouse integration)
Switch Pointing Device from USB Tablet to PS/2 Mouse
(I have a touchscreen and I think the drivers for that sometimes don’t work properly, both on Linux and Windows, neither USB Tablet nor USB multi-touch tablet worked properly on my system)
Enable Mouse Integration
(When I tried disabling it and switching manually, it went completely haywire on Debian 10 + Gnome + Wayland, though it may be useful on other systems)
Disable 3D Acceleration
(I do believe this is an experimental feature and I’m having more success with it disabled.)
Run the guest in a maximised Virtualbox window on the host. Do not run the guest in full-screen mode
(this reduces chance of erratic mouse behaviour because the pointer won’t be moving between host and guest windows constantly. When I switch the guest to full-screen mode that’s not in a Virtualbox window on the host, I often end-up losing the mouse completely in the guest, then the mouse might not work properly on other applications on the host DE either, until the VM is closed.)
Disable Shared clipboard and Disable Drag’n’drop
(Many people on the web have previously reported this helps reduce erratic mouse behaviour and it certainly helps me, though it would be useful if it worked.)