From: aaron peterson <metaxis@gmail.com>
Date: Mon, 28 Nov 2016 23:46:22 -0800

Invoking "ediff-buffers" starts ediff with no visible control panel, in
either multiframe or plain.  There is a frame, but it is not visible.
It can be minimized with (iconify-frame), and it is then visible in the
dock, even showing the ediff help menu in the icon.

I can't reproduce this, so it might be a Darwin specific problem.