When this bug happens, if you kill (or quit) all thunar instances (e.g. with Thunar --quit or ps + kill $pid), and restart it (Thunar --daemon), does automounting work with the new instance?
When this bug happens, if you kill (or quit) all thunar instances (e.g. with Thunar --quit or ps + kill $pid), and restart it (Thunar --daemon), does automounting work with the new instance?