bug#22466: 25.0.50; disable-theme apparently forces a redisplay and causes a screen flash

[Artur Malabarba]

1. From “emacs -Q” evaluate `(load-theme 'tango-dark)'. Any other theme
with a dark background will also do.

2. Then, notice how every time you evaluate the following code, the
screen flashes white for a moment, and then resumes the exact same
appearance.

    (progn (disable-theme 'tango-dark)
           (load-theme 'tango-dark))

Of course, “resuming the same appearance” is expected, the problem is
that the white flash is distracting, unnecessary, and eye-hurting.

I understand _why_ it happens (after `disable-theme' the background is
actually white), but I don't see why it _has_ to happen. That is, why
does Emacs redisplay the frame in the middle of lisp evaluation, instead
of waiting until the end of the command loop?

More importantly, is this redisplay (or “partial redisplay”, or whatever
it is) be avoided, so Emacs doesn't flash white on my pretty little eyes?

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ps: In case it helps, investigation shows that this is caused by this
form inside `disable-theme':

    (set-frame-parameter frame 'background-color
                             (custom--frame-color-default
                              frame :background "background" "Background"
                              "unspecified-bg" "white"))

I'm not saying this form is wrong. I'm just pointing to where it happens.