Fix transition for Replace ModalBar with 2 lines

This is for #7396.

For Replace, the modal bar can only ever be 1 or 2 lines, so I went with a simpler approach. I created a second `offscreen2` class for closing a 2 line modal bar, and then conditionally apply the correct class.

I don't see any problem with the transition that opens a 2 line modal bar, so This new class is only used for close.

cc @larz0