Re: support for tty frames from windowed emacs session

[Richard Stallman]

    I think there's more to it, though.  It might be that the C display
    code only uses either the X path or the tty path, but not both.  But
    I'm not sure, and don't have time to refresh my memory by looking at
    the code, sorry.

That would be easy to fix.

    > It seems that a major obstacle is that `window-system' (and similar)
    > is a variable rather than a function taking a frame as argument.

We would want to get rid of all uses of window-system.
We already have a way to check whether a display has a window system:
display-multi-frame-p.