I’ve just decided to ditch BtSync (v2 turns out not to have been worth waiting for) and as I was migrating my folders over to SyncThing I found I’d accidentally left a hidden .sync folder in. When I deleted it, SyncThing seemed to choke on syncing the deletion. I get “Folder “Name” isn’t making progress” in the UI, and "Puller (folder “Name”, dir “.sync”): delete: Remove D:\Name\.sync: The directory is not empty.
I’m not sure whether it’s because it’s a hidden folder, or whether the .sync folders had different amount of archive stuff in them at the point that I cut over, or because I deleted the folder whilst it was in the middle of syncing, but it’s clearly not happy. Don’t know whether it’s really a bug, but I would certainly suggest adding a default ignore pattern to skip over .sync folders!