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XInput2 introduces XSelectCore function to be used by windows managers. It selects the core pointer, core keyboard, etc.

This could been great to set some keyboard as core for some application. With always on top or no-floating window layout it could increase usability.

Imagine I have small keyboard with only numpads keys. I can run Krunner or some calculator, set it window to transparent, always on top and restrict numpad keyboard only to calculator window. I ddon't have to set window as always on top - if window get core event, it could brink to front.