bug#16647: Imprecisions with window-resizing cursors

[martin rudalics]

>> I can't reproduce that neither with GTK+ 3.4.2 nor on Windows XP.  The
>> cursor changes shape to <=> within one frame column distance on the left
>> of the vertical border (the "grabbable_width" in coordinates_in_window
>> IIUC) till right after that border.  When the cursor shows as <=> I can
>> also drag the border.  Clicking on the border doesn't have any effect.
>
> Sometimes it works fine. Perhaps you could try moving the border to different 
locations.

Still seeing what I said above: "When the cursor shows as <=> I can also
drag the border."  You could try one of the following:

(1) Check whether setting the right-divider frame parameter to 1 suffers
    from the same problem.

(2) Try putting `coordinates-in-window-p' in a loop where you simulate
    (by incrementing the car of the first argument) the course of the
    mouse and display the return value via a message.

(3) In window.c DEFVAR_LISP a new variable, in coordinates_in_window set
    that variable to the value coordinates_in_window is about to return,
    display the value of that variable in the modeline, and then move
    the mouse to find out what it reports.

>> Emacs doesn't show a vertical resize cursor on the mode- or header-line
>> (for historical reasons, I presume).  You can now use bottom dividers to
>> show such a cursor.
>
> Normally I'd say you know better, except I recall seeing a
>
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>
> cursor on mode-lines, for as long as I remember.

Hmmm... on 2013-11-30 I have

* xterm.h (struct x_output): Add vertical_drag_cursor.

martin