I'm looking at a 19h30m long Direct trip with Flixbus.
I wonder if there are any longer stops during the trip or if you're stuck in your seat for the entire duration?
There will usually be stops, either planned stops to drop and pick up passengers or mandatory stops for the driver's breaks. Unfortunately, it's sometimes difficult to know in advance exactly when and how long the bus will stop but 14 hours is a very long time to be driving (and illegal in most places, even outside the EU). I have even seen drivers stop more frequently than required (e.g. if the toilets are broken), you can always ask.
If there is only one driver, any trip longer than 4 hours 30 in the EU will have at least one break (could be two or be covered by time at the station but a driver is not allowed to drive more than 4/4.5 hours straight. The only time I had a long (6-7 hours) Flixbus journey without a proper break was a night trip with two drivers. Since neither of them drove longer than 4 hours, they were not required to rest and just switched in the middle of the night, with no time to get out of the bus.
I just completed 3 long Flixbus journeys and this is what I found out:
There are stops, but how many probably depends on the route and the time of day. The stops are announced a few minutes before and usually the driver will tell you how many minutes you got - but the drivers dialect might be hard to understand so if you're unsure then ask the driver or a fellow passenger.
I'd say at least 1 longer stop (30 minutes) at a gas station with restaurants.
The longest journey I did was over 19 hours long and there were 2 stops, one because we had to take a ferry and then after that just one more.
Some stops at bus stations along the way can be longer ones, I suggest checking the departure time in the Flixbus app and see how much time you will have there - it could be anything from 30 minutes to 2 minutes depending on traffic but only 5 minutes is usually scheduled. The bus will not leave the station before the scheduled time but may arrive earlier/later.
As mentioned, the stops where the driver is changed are just up to 5 minutes and most likely not at a gas station. They are not announced and you're not intended to get off the bus.
Something to keep in mind is that the driver will leave at the announced time so you must be back by then. The driver will not check if everyone are back.
We travelled on the Flixbus service from Munich to Zagreb departing at 3.15am recently. There were two drivers onboard and we stopped at a service station in Austria after a couple of hours on the road where we stopped for about 15 minutes and were able to get off and stretch our legs and the drivers swapped over. We then stopped in Ljubljana for 20 minutes where once again we were able to get off, before arriving in Zagreb at 11am so I would guess on a longer trip there should be a few stops at least.