bug#16647: Imprecisions with window-resizing cursors

[martin rudalics]

> On current trunk version, from emacs -Q
>
> 1)
>
> (progn
>   (scroll-bar-mode -1)
>   (split-window-right)
>   )
>
> Slowly move the mouse from left to right across the vertical line splitting the 
windows. Once the mouse goes past the black line, it's still shown as <=>, 
although clicking-and-dragging on that area won't have any effect.

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.

> 2)
>
> C-x 2
>
> Move the cursor from top to bottom, across the mode-line. Once the cursor enters the 
mode-line, it won't change to a "resize" cursor, although it's already possible to 
drag.

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.

> 3)
>
> C-x 1
> (set-window-margins nil (car (window-margins)) 10)
> C-x 3
>
> Move the cursor from right to left, across the divider. Once the mouse enters the 
right margin, it will still be displayed as <=>, although it's not possible to 
resize by clicking there.

This is a arguably a bug.  I checked in a fix.  Please try again.

Thanks, martin