It's set to "Reverse Scrolling" in Cinnamon. Turning that on and off does't change a thing. It's set to "Natural Scroll" in Gnome Classic. Likewise.

I've now installed ScrollAnywhere in Firefox; the options include the scroll behavior I'd like to see elsewhere. But it was most urgent there.

As libinput output is not consistent with the desktop environments you listed, I would recommend writing a bug report for the desktop(s) concerned.

Well, if it's a desktop issue, what other desktops do I need to test?

I tried Plasma, but it had too much pain/animation, and it makes it too hard to disable all the effects, so I just got a migraine for my trouble and uninstalled Plasma.

That's up to you to test/choose. I've been a KDE user for all my time with Linux (over 20 years). I don't use (or even like) reverse scrolling.

You could try the advice given here, where a user claimed that they got natural scrolling working via an Xorg config AND enabling in the desktop as well:

https://forums.linuxmint.com/viewtopic.php?t=392727

I wouldn't recommend editing the distro-supplied config files in /usr/share/X11/xorg.conf.d/ as it could get overwritten by a future update. Instead, copy to the /etc/X11/xorg.conf.d/ directory, and edit there. Not sure why both the system-wide Xorg configuration and the desktop user configuration need to be applied, but give it a go. You'll need to restart the X-session before the Xorg config will be applied of course.

Thanks, but that still didn't work. I copied the file, added the line, moved it to the right location, logged out, logged in, no luck, shut down, restarted, still no luck.

I don't have anything further to offer, except to submit a bug report. Your other thread on this seems to be equally without resolution.

I'm not sure *where* to file the report. Not with Cinnamon, because Gnome has the same bug. Not with Gnome, because Cinnamon does. Not with Xorg, because Cinnamon on Wayland (experimental) has the same bug. Not with Wayland, because Gnome Classic on Xorg (compatibility) does. Maybe with libinput.

It doesn't help that libinput seems to have special bug reporting requirements, and the bug reporting documentation pages trigger my migraines.

migraine warning for the links:

https://wayland.freedesktop.org/libi...ing-bugs-mouse

https://wayland.freedesktop.org/libi...ibinput-record

P.S. I couldn't meet all the requirements, but reported what I could: https://gitlab.freedesktop.org/libin...t/-/issues/985