bug#22891: 25.0.92; set-fringe-mode with left fringe 0 breaks window width calculations on Mac OS (again)

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 14 Apr 2016 07:33:15 +0200
> From: Anders Lindgren <andlind@gmail.com>
> 
> Setting the `left-frame' frame parameter to any from 1 and up change the width of the fringe, while retaining
> the width of the text area (80 characters). However, setting it to 0 makes the text area wrap on 79 characters
> (despite there being space for the 80:th character) while `window-width' still returns 80.

This is all as expected: setting any of the fringes to zero requires
displaying a continuation glyph in the text area, and since Emacs
supports bidirectional display, we need to usurp one character cell
from the other edge of the window as well, to make the geometry of L2R
and R2L screen lines symmetrical.

> Presumably, this is the root cause of the ansi-term problem.

If this causes trouble in ansi-term, then ansi-term should be fixed to
take this into consideration.  (Presumably, ansi-term uses wrong
interfaces to get at the width of the text area.)

> However, to check if this was a NS specific problem, I tested this on a freshly built GTK+ emacs on
> LinuxMint. It turned out that it, too, suffers from the same problem, so I'm handing over this to you, Martin.

This is not Martin's problem, this is a problem in ansi-term (AFAIU).