https://bugs.winehq.org/show_bug.cgi?id=49786
--- Comment #7 from Marcin Zajaczkowski mszpak@wp.pl --- Good idea with the rebinding. It was months ago, so I checked that those keys aren't defined at all - they seem to work (up to 5.7) out of the box. I tried to remap it, however, XCOM 2 and with 5.7 "Thumb Mouse Button 1/2" is recognized (which is bad in general, but was working in that game) - back/forward buttons. With 5.9 those keys are no longer detected at all.
As the horizontal scrolling worked fine in Firefox when you tested it, I would suspect that XCOM 2 is not able to properly recognize horizontal scrolling. As a result, I'm willing to closing this issue. Sorry for bothering you, Murray and thanks for your explanations.
Two more extra things: 1. Do you know any way to display (for debug purpose, live with xev) pressed keys in Wine? I tried with some popular Windows app, but they didn't want to run (or didn't work) - probably due to using some low level Windows mechanisms. 2. Is it possible to remap keys globally in Wine?