As series has a sync option for unwatched items, so one you see that episode, it’s deleted from the synced content, that same option should be available when syncing movie collections. Also for single tv episodes or movies, an option to delete the synced content when watched would be nice too.
So far, movie collections are synced as 1 item, there’s no way to delete single movies in it.
Hmmm no, I didn’t know it…
Once the item is watched, is it deleted from the synced device, or it just sync them, but you have to manually delete them from the synced device one you watch them?
It should get deleted as soon as you refresh the sync jobs.
(on the mobile app, Downloads & Sync - Manage - ↻)
I’ve been trying to find the menu as the one you displayed, and I can’t find a dialog like that, nor in android, nor in windows, how do you access that conditional search to sync?
Custom Filters appear at the very bottom of the drop-down where the quick filters are.
In the library view, in the web app.
thank you! I think I never scrolled that down 
2021 clean-up: implemented (trigger sync job from custom filter)
