On Tue, Oct 06, 2020 at 10:40:33PM +0300, Andrii Kolomoiets wrote:
> In 'emacs -Q':
> 
> 1. M-: (setq test-frame (make-frame '((left . 50) (top . 50))))
> 2. C-x 5 o
> 3. M-: (set-face-background 'internal-border "blue" test-frame)
> 4. M-: (modify-frame-parameters test-frame '((background-color . "black")))
> 
> Now (face-background 'internal-border test-frame) returns nil.
> If we skip step 2, then the internal border color remains.

This isn't an NS specific problem. Eli actually spotted the bug last
year and put in a FIXME.

I can't think of a good reason for the code to use the selected frame,
but perhaps I'm missing something.

Patch attached.
-- 
Alan Third