See Bug 475704 for the investigation that led here. Pen buttons are re-bindable. However nowhere is it shown what they are bound to by default, so you don't know. Apparently there are pens with buttons that do different things; some buttons emit different kinds of clicks when pressed, while others toggle the pen between modes (e.g. drawing mode and eraser mode). However nowhere is this shown in the UI, so the user has to guess at what the buttons do, figure it out through trial and error, or be a KWin developer and scrutinize WAYLAND_DEBUG logs to figure it out. It would be much better if the buttons' current functions were shown in the UI, and the UI made it clear that binding actions to those buttons would amount to overriding the default actions.
The default button functionality is entirely application-defined, so there's nothing we can really show here
(In reply to Nicolas Fella from comment #1) > The default button functionality is entirely application-defined, so there's > nothing we can really show here Correct, so I'm closing this. To add onto the confusion, this is under the assumption the pen buttons are doing what they're supposed to be doing correctly, and that said information is relayed to libwacom/libinput. I took a look at the bug report, and their pen side buttons are sending... pen contact and pen eraser contact events(???) which isn't great. Ideally their kernel driver sends well-defined button events, and the hardware would have a definition in libwacom - so everyone up and down the stack knows how to translate them properly into something usable. In the correct scenario, all pen buttons are application-defined (this is also our default) and currently there's no way for Plasma/KWin/anyone to know what the application does with them. So it's unactionable for us until that's resolved somehow.