An extra mouse button worked in one application but not another, which sent me directly to the KDE control center. I changed mappings until ordinary buttons became surprising too. This was useful only as a demonstration that random adjustments scale badly.

I started over at the bottom of the stack. The kernel input layer exposes event devices and reports key, button, and relative motion events. X.Org opens an input device through its configured driver, translates those events into the X input model, and sends them to clients. KDE and its applications then apply shortcuts or application behavior.

That sequence gives each test a specific question. Kernel event diagnostics answer whether hardware input reaches the kernel. The X.Org log answers which device and driver the server opened. xev answers which X events a client receives. An application test answers what that client does with them.

The button appeared at the kernel layer but was mapped unexpectedly by X.Org. I had copied a configuration stanza from a different mouse, including a button mapping whose numbers reflected that device. Removing the borrowed assumption and mapping the observed buttons deliberately fixed the problem.

The exercise also exposed a trap in logs. A warning near the end of Xorg.0.log looked relevant because it mentioned input, but the earlier device initialization lines showed that it belonged to another configured device. Timestamps, identifiers, and full initialization sections matter more than a conveniently alarming word.

I saved a small checklist with the configuration:

1. Does the kernel report the event?
2. Which device does X.Org open?
3. What does xev receive?
4. How does the application interpret it?

This is not sophisticated debugging, and that is why it works. Each layer has a narrow contract. The first failed contract determines where to investigate; layers above it cannot repair missing information.

Desktop Linux can feel complicated because these boundaries are visible. I prefer visible complexity to one giant mystery, but only when I respect the boundaries. Starting at the application and changing every layer on the way down is not diagnosis. It is redecorating the staircase while looking for a leak.