When exploring the new Mix snapshots, I found that when you add a new snapshot and pop out the mixer to reveal the Snapshot tab, then type a description into the box below, your cursor-arrow disappears! Eventually I realized that pressing Esc brings it back. However, typing into the description of the box a second time, or any of the other snapshots, does not cause the arrow to disappear.
This is an known issue of the Notepad. The same behaviour is if you use it in the Project window (tracks) or MixConsole (Channels).