When the window is mapped, some ICCCM WM_HINTS are set.
The input field is set to true and state is set to NormalState.
To quote the spec, "The input field is used to communicate to the window
manager the input focus model used by the client" and "[c]lients with
the Passive and Locally Active models should set the input flag to True".
mpv falls under the Passive Input model, since it expects keyboard input,
but only listens for key events on its single, top-level window instead
of subordinate windows (Locally Active) or the root window (Globally
Active).
From the end users prospective, all EWMH/ICCCM compliant WMs (especially
the minimalistic ones) will allow the user to focus mpv, which will allow
mpv to receive key events. If the input field is not set, WMs are
allowed to assume that mpv doesn't require focus.
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