I took a look at this from the NS port point of view.

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.

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

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.

Steps to repeat:

   emacs -Q

   C-u 8 0 x RET

        ;; This inserts a line wide enough to reach the right side of the screen without wrapping)

   (set-frame-parameter (selected-frame) 'left-fringe 50)

        ;; This makes the left fringe wide, the text area still holds the 80 x:s nicely.

   (window-width)

        ;; 80

   (set-frame-parameter (selected-frame) 'left-fringe 1)

        ;; Narrow left fringe. Text area still holds the x:s

        ;; The line of x:s wrap

        

    -- Anders