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If you are on Wayland, the blessed way is to [use the configuration for your desktop environment][libinput-de] (DE) to map mouse keys and other input devices, except for a couple well-known options (eg [left-handed mode][lefty] and [middle mouse emulation][middle-mouse]).

If your DE _doesn't have_ configuration for mouse key bindings, you may be able to mess with the `hwdb` like [this person on Reddit][reddit]. Wayland's version of `libinput` says [not to do this anymore][no-hwdb], but if you're stuck you're stuck.

### Get the device ID

> ```sh
> $ lsusb | grep Kens
> Bus 001 Device 008: ID 047d:8018 Kensington
> ```

### Get the input events with `evtest`

> ```
> Event: time 1584228876.372060, -------------- SYN_REPORT ------------
> Event: time 1584228877.572302, type 4 (EV_MSC), code 4 (MSC_SCAN), value 90003
> Event: time 1584228877.572302, type 1 (EV_KEY), code 275 (BTN_SIDE), value 1
> Event: time 1584228877.572302, -------------- SYN_REPORT ------------
> ```
> I was lucky because I really just wanted to swap two buttons, and `evtest` dumped the supported events when you select your device. You may be able to figure out the key codes to emulate. I did get hung up on `evtest` reporting `BTN_SIDE`, but having to use lower-case `btn_side` in the `hwdb` entry.

### Add changes to the `hwdb`

> I'm on Fedora 31, and have been using [hwdb] entry for a while, because I didn't like the default button configuration on my trackball -- they're frankly backwards for a right-handed person, and the Windows software recognized the need to swap them. (note: Indents are important iirc)
>
> [The] rule in my `hwdb` entry (`evdev:input:b*v047Dp8018*`) is just the USB Vendor + Product
>
> ```sh
> $ cat 90-trackball-buttons.hwdb 
> # Kensington Expert Mouse Trackball:
> # * Rebind top-left from BTN_MIDDLE to BTN_SIDE
> # * Rebind top-right from BTN_SIDE to BTN_MIDDLE
> evdev:input:b*v047Dp8018*
>  KEYBOARD_KEY_90003=btn_side
>  KEYBOARD_KEY_90004=btn_middle
>```

### [Reload your `hwdb`][hwdb-reload]

```sh
$ sudo systemd-hwdb update
$ sudo udevadm trigger /sys/class/input/eventX
```

[libinput-de]: https://wayland.freedesktop.org/libinput/doc/latest/faqs.html#how-do-i-configure-my-device-on-wayland
[lefty]: https://wayland.freedesktop.org/libinput/doc/latest/configuration.html#left-handed-mode
[middle-mouse]: https://wayland.freedesktop.org/libinput/doc/latest/configuration.html#middle-button-emulation
[reddit]: https://www.reddit.com/r/linux4noobs/comments/fih5aw/comment/fkiuyui/
[no-hwdb]: https://wayland.freedesktop.org/libinput/doc/latest/device-configuration-via-udev.html#modifying-the-hwdb
[hwdb]: https://wayland.freedesktop.org/libinput/doc/1.11.3/udev_config.html#hwdb
[hwdb-reload]: https://wayland.freedesktop.org/libinput/doc/latest/device-configuration-via-udev.html#reloading-the-hwdb