Using a set of preferred countries, the others being relegated to an optgroup, as
✨
Allow to set a list of preferred countries
RTBC
implements, is great for the widget.
But while it is a slight improvement for the views filter, it is not really what we want here:
What we need instead is a select that only lists countries that are really existing in the current dataset, Views Selective Filters → style.
Though some preliminary D8 port 📌 [meta] Views Selective Filters D8 Port Fixed of that project exists, this is going to be quite some work, as neither the D7 version nor the D8 port has a proper API. However, we want this to work out of the box, without introducing any additional configuration or error sources. In the end, this could become some kind of Selective Filters API (and might then be split off or be incorporated into D8 Views Selective Filters.
1.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
The current patch modifies the behavior of the filter instead of introducing a setting which would control $only_used. Which means that it cannot be committed in this form.
Views Selective Filters → has an alpha version since April 24. Just installed and activated it and it works perfectly with Address country.