So I need a mask on this layer only for a few frames somewhere in the middle but not anything before and after that region.
What's a good way to do this? I see that the none/add/substract dropdown option is not keyframable.
You could create a keyframe 1 frame before and one frame after your mask, where it is the size of screen.
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.
You could also split the layer immediately before and after where you want the mask so your masked layer is its own very short layer. Command+Shift+D (or its PC equivalent) will split it at the spot of the playhead.