The logic in evdev.c (https://github.com/wayland-project/libinput/blob/688142c8a22c00cfe7b9dec6a71e935b899d8b65/src/evdev.c#L167-L169) remembers how many times a key is pressed simultaneously and only emits KEY_UP events when this counter is incremented to 1 and KEY_DOWN when it is decremented to 0.

This is problematic as key release events might get dropped, for example if the keyboard is exclusively grabbed by evdev's EVIOCGRAB while a key is pressed.

This issue is easily reproducible by launching one instance of libinput-debug-events (or any libinput-based application that reads keyboard input) and then running this in another terminal:
libinput-debug-events --device /path/to/keyboard --grab & (sleep 1 ; killall libinput-debug-events)

Because you submit the command by pressing the Enter key and libinput-debug-events launches fast enough, the release event for your Enter key happens after the device is grabbed, so the first instance of libinput-debug-events will not receive it. Because of this bug, the Enter key is now permanently stuck there as it is always pressed either 1 or 2 times, so libinput will never send any key events for it again.

(Please note that running this test inside any libinput-based desktop environment (xf86-input-libinput or Wayland) obviously triggers the same bug and may force you to restart your window manager.)



In other implementations (xf86-input-evdev, Microsoft Windows), missing release events are handled gracefully: As pressing an already held key indicates a missing release event, they just ignore this, so a subsequent key release event actually releases the key.