This property is the only way to move fullscreen windows across monitors with KWin, it needs to be set to avoid having to mess with _NET_WM_STATE workarounds.
Both KWin and Mutter support cases with invalid indices and treat them mostly the same as clearing the property.
For Mutter, see: * meta_x11_display_xinerama_index_to_logical_monitor[1] * meta_window_update_fullscreen_monitors[2] * meta_window_clear_fullscreen_monitors[3]
For KWin, see: * NETFullscreenMonitors structure from KWindowSystem[4], * xineramaIndexToOutput[5] * fullscreenMonitorArea[6]
Mostly reverts 70c9239cb2eef696eea109f3f8e7a58f80cd3823.
[1] https://gitlab.gnome.org/GNOME/mutter/-/blob/9a1fa7e13b5feefb76902287ae7ca25... [2] https://gitlab.gnome.org/GNOME/mutter/-/blob/9a1fa7e13b5feefb76902287ae7ca25... [3] https://gitlab.gnome.org/GNOME/mutter/-/blob/9a1fa7e13b5feefb76902287ae7ca25...
[4] https://github.com/KDE/kwindowsystem/blob/1399ec9c2f05107c36bc8661593df35bad... [5] https://invent.kde.org/plasma/kwin/-/blob/b488f3b6c6200aca9fa1f61cbf0253eeda... [6] https://invent.kde.org/plasma/kwin/-/blob/b488f3b6c6200aca9fa1f61cbf0253eeda...