Bug 476491

I hope this is the correct place to report this. If not, I'd appreciate it if you could point me to the right direction.

scrollOnButtonDown only works for buttons of the same device. 
My scenario is this: I use input-remapper to remap buttons on my Elecom Huge trackball. I currently do on-button scrolling by holding the middle click while moving the trackball. I enabled it with these commands:

$ qdbus org.kde.KWin /org/kde/KWin/InputDevice/event31 org.kde.KWin.InputDevice.scrollOnButtonDown true
$ qdbus org.kde.KWin /org/kde/KWin/InputDevice/event31 org.kde.KWin.InputDevice.scrollButton 274 

274 corresponds to the middle click button code.
Instead of the middle click, I'd like to use another button. The problem is that, in my case, mouse cursor movements belong to event31 but input-remapper assigns remapped buttons to e.g. event28. I verify this by doing 

$ libinput debug-events

When I try to assign the scrollButton to one of the other buttons that I remapped with input-remapper, it doesn't work. In this case, I try to do it with a button that I remapped to right click (button code 273).

$ qdbus org.kde.KWin /org/kde/KWin/InputDevice/event31 org.kde.KWin.InputDevice.scrollOnButtonDown true
$ qdbus org.kde.KWin /org/kde/KWin/InputDevice/event28 org.kde.KWin.InputDevice.scrollOnButtonDown true
$ qdbus org.kde.KWin /org/kde/KWin/InputDevice/event28 org.kde.KWin.InputDevice.scrollButton 273 

These commands have no effect and don't throw an error. Turns out that this feature only works when the mouse cursor movements and the scrollButton come from the same device. I tested it by plugging in another mouse and setting one of its buttons as the scrollButton with commands like shown above. When I hold that button on the second mouse while moving my trackball, it doesn't work. If I move the cursor with my second mouse instead of my trackball, it works. In my case, it works with the middle click because I haven't remapped it in input-remapper so it belongs to the same device as the mouse cursor movements (event31).

STEPS TO REPRODUCE
1. Plug in two mice, mouse A and mouse B 
2. $ libinput debug-events
3. Press the desired scrollButton on mouse A and move the cursor with mouse B
4. Note which eventXX those events belong to, as well as which button code corresponds to the desired button
5. Enable scrollOnButtonDown on mouse B with

$ qdbus org.kde.KWin /org/kde/KWin/InputDevice/<eventBB> org.kde.KWin.InputDevice.scrollOnButtonDown true

6. Set the desired button on mouse A as the scrollButton with

$ qdbus org.kde.KWin /org/kde/KWin/InputDevice/<eventAA> org.kde.KWin.InputDevice.scrollButton <button code>

OBSERVED RESULT
Holding down the desired scrollButton on mouse A and moving the cursor with mouse B only moves the cursor and doesn't scroll.

EXPECTED RESULT
Holding down the desired scrollButton on mouse A and moving the cursor with mouse B should scroll.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch
KDE Plasma Version: 5.27.8
KDE Frameworks Version: 5.110.0
Qt Version: 5.15.10
Kernel Version: 6.5.5-arch1-1 (64-bit)
Graphics Platform: Wayland

ADDITIONAL INFORMATION
I realize that my use case is very niche since I use a trackball and input-remapper but I think enabling this feature to work on two separate devices would be nice for other people as well who would like to e.g. hold down the space button and move the cursor to scroll.