Consider the existing input sources MRU only valid if it contained
more than one element to pick from. Fixes the following situation
with initial-setup sessions:
- Initial setup Session starts, with several input sources already
configured ("us" between them)
- InputSourceManager initializes, only the default "us" keymap is
available
- MRU list is constructed, "us" is picked
- InputSourceManager catches up with session configuration, the
other extra sources are added
- MRU list is reconstructed, "us" is already the most recent
- Session ends up with "us" picked, regardless of its position in
the list, and no MRU existing prior to startup
If we consider the intermediate single-element MRU list invalid,
it is still possible to pick the best default source between all
the configured ones (the one that was put first in the list,
basically), after initialization is complete.
But also, it is unnecessary to have if there is a single source to
pick from. After the sources list has two elements or more, the
MRU list will become effective and preserved during changes to
the available sources.
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5873
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2495>
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