Sounds like a good solution. Note that I didn't say I wanted to downvote the answer but to remove my upvote which I'd find less offensive but rather reflective of the value the answer contributes to any future readers.

@guntbert I didn't update anything. The linked question is by someone else. Anyway the whole issue seems resolved (see my answer there), at least for me. It seems to have been a (now fixed) problem with the Guest Additions.

Thanks. Should I report this bug for Ubuntu or for VirtualBox? And what did you update when it "borked"?

Thanks @JorgeCastro. I also noticed that you took away my edit to make it obvious that it is about Xperia devices, I guess you thought that the original question was clear enough?