The reason why this "bug" is still alive and kicking is that sandervl73 was right 8 years ago: it's a limitation. Not of VirtualBox, it's a limitation of X11. It can't handle XOR cursors at all. That's why there's so little progress. The proper fix would be to convince the X.org people to add the necessary functionality.
Trying to work around the issue in VirtualBox is rather complex: we'd need to introduce a new mode for handling mouse pointer drawing, "semi-integrated", where the mouse position handling is like with current mouse integration, but the mouse pointer is actually drawn by the guest OS in software (and not as it currently is done by the host using hardware). A LOT of effort for such a tiny issue.
