`emacs -nw', `framep' returning `t', `display-graphic-p' returning `nil'

[Drew Adams]

I guess I don't really understand "terminal mode" and the use
of Emacs with a character-only display.  (The last time I used
Emacs regularly without a graphic display was before Emacs had
any support for graphic display!)

See https://emacs.stackexchange.com/a/37371/105 and the
question it answers.

The problem exposed there is that, by default, library dired+.el
binds key `M-S-o' (aka `M-O').  Apparently, when Emacs is used
with some terminals, or in some terminal mode(s)?, that key
sequence is sent by the arrow keys.  (On MS Windows, I don't
see the problem using `emacs -nw', but I don't claim that's
the case for all users.)

(A similar problem arose for `M-S-b', `C-M-S-b', `C-M-S-r',
`C-M-S-g', `C-M-S-m', and `C-M-S-t': they might not be
handled well in some "terminal" contexts.)

To help with the problem, I did this:

1. Added a Boolean user option,
   ` diredp-bind-problematic-terminal-keys' (t by default).

2. Prevented binding the key when the display is character-only
   AND the option is nil:

(when (or (display-graphic-p)
          diredp-bind-problematic-terminal-keys)
  (define-key dired-mode-map [(meta shift ?o)]
              'diredp-chown-this-file))

That apparently solved the problem for at least some users:
setting the option to nil prevented the key binding.

I was thinking that when `display-graphic-p' is non-nil the
problem wouldn't exist.  But apparently at least one user has
the problem even though `display-graphic-p' returns non-nil.

I use `display-graphic-p' here and there in my code, and I
wonder now if it is really doing what I have thought.  (I
used `window-sytem' before `display-graphic-p' existed.)

Searching a bit more, I see that `framep' returns plain `t'
"for a termcap frame (a character-only terminal)".  And the
user with this problem (in spite of non-nil `display-graphic-p')
does indeed get `t' for `(framep (selected-frame))'.

So my next attempt will likely be to use that instead of
`display-graphic-p'.

Before I try that, can someone please point out what I'm missing?

The doc for both `framep' and `display-graphic-p' seems to say
that the former returns `t' for a " a character-only terminal",
and the latter returns `nil' for a "text-only terminal".  But in
this user's case they apparently don't agree about his "terminal".
Is there some nuance here - a difference between a "character-only"
terminal and a "text-only" terminal?

Just what are the relations between `emacs -nw', `framep'
returning `t', and `display-graphic-p' returning `nil'?

What should I be testing for here, to detect a "terminal" or
a "terminal" mode (I'm not sure just what the problematic use
case is), so that it then makes sense to check the option value?

(Yes, I could always check just the option.  Still, I'll like
to understand this a bit better.  Is there a good test to use
here, which would show that the user's session can handle such
a key, in which case there is no need to test the option?)