Wayland can expose displays in any arbitrary order, and doesn't have the native concept of a primary display. However, there are games that presume that the first listed display is the primary, which can lead to problems if that output isn't necessarily the ideal one, as they may use that display to build a resolution list or as the default fullscreen output. This sorts displays by position, then attempts to find the primary display, first by querying the explicit ordering hint, then the GNOME DBus property, then tries to determine the 'best' display according to the criteria that is generally ideal for games and media playback.
The makes the list of displays deterministic, as long as the desktop configuration remains static, with a reasonably appropriate one prioritized as primary, even if there is no explicit way to retrieve the primary display. In the case where a user has a particularly esoteric physical display configuration, the new hint enables explicitly overriding the sorting and selection logic, allowing the entire display order to be customized, if necessary.
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