This box appears in the center of my Emacs frame with certain actions.
The most reliable way to trigger it is attempting to move the cursor past the boundaries of the buffer.
It disappears after switching to a different buffer, or scrolling enough to replace all the text in view. Marking the area also solves it.
I don't think it's an accessibility feature because of the unusual way in which it disappears. Anyone else recognize this?
My version: GNU Emacs 24.5.1 (x86_64-apple-darwin15.0.0, NS apple-appkit-1404.13) of 2015-11-30
Installed with Macports on El Capitan