> > (setq mouse-wheel-tilt-scroll t)
>
> I tried this, but still it doesn't scroll horizontally
> with the mouse wheel. It seems this feature expects the
> mouse events mouse-6 and mouse-7, but scrolling the mouse wheel
> produces mouse-4 and mouse-5.
>
> FWIW, emacs uses the "original" (I think they started with XFree86?)
> bindings that treat up/down mouse wheel as mouse buttons 4/5, while
> left/right are 6&7, so this is working as (at least at one point in the
> past) intended. There might be a communication issue around "by wheel"
> here: are you using a mouse with a wheel that tilts to the left/right? I
> believe that's what the OP had in mind.
Do you mean a mouse with many additional buttons? I heard such monsters
exist, but never seen them.
On one hand, if you believe that mice should have exactly 3 buttons in a manner perhaps similar to the Spanish Inquisition, then yes.
On the other, for mice with a physical wheel, on X11 and derivatives (but neither windows nor mac), a physically rolling the wheel appears to the window system (and thus to emacs) as mouse buttons numbered 4 and 5. If the wheel also registers a tilt to the left/right, these appear as buttons 6 & 7. The former is very common; the latter seems to be uncommon these days. Neither of these are the so-called "MMO mouse" (apparently now sometimes "MOBA mouse") with 6-15 (yes, really) extra buttons that are *relatively* common amongst hardcore PC gamers. (For example:
https://www.howtogeek.com/403685/how-to-use-an-mmo-or-moba-mouse-for-productivity/) I've only tried one of these once, and at the time the extra buttons weren't supported by X11, so I can't tell you if the mouse equivalent of the space cadet keyboard is emacs nirvana, instant thumb-sprain, or just a waste of plastic.
These days, trackpads/touchpads are very common, and most of them have a scrolling feature that uses gestures to fake those 4 extra buttons. Emacs master currently does the expected scrolling things for up/down but not left/right nor shift-up/down.
Hope that helps,
~Chad