When creating a slide animation where masks are involved, the only thing that is allowed right now is to move a mask and/or zoom a mask from the Start position to the Finish position of the animation.
I would like to be able to change the shape (i.e., length and width INDEPENDENTLY) of the mask while it is moving. Just spread the change equally over the duration of the animation, regardless of how tall or how wide it is (so, larger changes would have to be faster than shorter changes in order for them to end at the same time).