bug#37840: Missing in the Emacs manuals:

[martin rudalics]

> I just noted that the ediff-control-panel does not react on any of
> the display-buffer-alist possibilities to customize appearance of
> frames.

Customize 'ediff-control-frame-position-function'.  You can stuff
anything you want into the return value of that function, not just the
location.  Consult 'ediff-control-frame-parameters' in ediff-wind.el
for how ediff sets things up if you do not interfere.

And be aware that Emacs has many other frame creating packages that
disregard 'display-buffer-alist'.

martin