Fix transition override logic.

When someone calls `.show()` on a region, it starts a close animation that
will trigger the actual show transition when it's complete. If someone else
tries to show while that fade out is going on, the newest view will be
snown. The override was reset only after a fade in was complete, which is
wrong. As soon as a region starts fading out, any new region can come and
close that new region. This commit fixes that.