@leocg So, I googled some more.
And I found a tip I should probably have done from the beginning.

Which is to try another mouse, and I did.

I probably should have done that before I started re-installing my mouse drivers. xD

Anyway, it is an old mouse. Served me well for years.
So it's probably time to get a new one anyway.

Strange it only happens in Opera, but it's probably only going to get worse. 😕

Thanks for your help at least!