I think that it'a a real good opportunity for Flarum to have an audience in France. The association which manage this forum is really enjoyed by flarum but I'm not sure that the lack of this feature will not be too important to block the migration.
My fear about the third party extension is that firstly it will not be maintained when the feature will be in the core and secondly how migrating from extension to core in future ? Do you see what I mean ?
The private best messaging extension is this ? https://packagist.org/packages/fof/byobu
Here the first screenshots of the result 😉 Many many thanks for your help !
