I don't understand why a sizable gap was left between the mission configuration options and start button.
Reducing the space would completely eliminate the need for the right side scroll bar.
Seeing 2/3 of the start button just looks messy.
I also noticed that if you access the arcade via the missions tab - it does not show anywhere the associated CR cost for not having completed the previous Tier. This differs then accessing the gate directly from the arcade.
This isn't covered in any of the tutorials either.
You will likely have players who use the mission interface exclusively - you may want to consider adding this info.
Another fix would be to just make the whole missions window a bit taller.
What are you talking about?
What scroll bar? seeing half the start button? make the missions window taller?
I see no problems on my game. I bet the problem is the resolution/monitor setup.
If they 'fix' it for your configuration, they may 'break' it for mine.