bug#9186: patch for buggy drag behavior affecting emacs >=23

[Andrew Schein]

Below is my fix to bug#9186.  I located the area of the C code that
deals with recording click events and noticed that there is logic that
looks for extremely precise pixel placement rather than buffer
position.  In my opinion this leads to unexpected behavior; humans can not
move their hands so precisely as they can reliably move their mouse to
the area defined by fuzz.  On the other hand, there are plenty of cues
to indicate when the mouse is placed back into the original position.

In proposing this fix I am a a little concerned about how the overall
system will be effected.  Why was this click_fuzz logic introduced to
begin with?


*** ./src/keyboard.c    2011-08-15 16:25:02.516887000 -0700
--- ../copy2/trunk/src/keyboard.c    2011-08-17 08:58:01.792927000 -0700
***************
*** 5630,5637 ****
              event->modifiers |= click_modifier;
              ignore_mouse_drag_p = 0;
            }
!         else if (EQ (Fcar (Fcdr (start_pos)), Fcar (Fcdr (position))))
!           /* Mouse has returned to its original position.  */
            event->modifiers |= click_modifier;
          else
            {
--- 5630,5649 ----
              event->modifiers |= click_modifier;
              ignore_mouse_drag_p = 0;
            }
!         else if (xdiff < double_click_fuzz && xdiff > - double_click_fuzz
!              && ydiff < double_click_fuzz && ydiff > - double_click_fuzz
!           /* Maybe the mouse has moved a lot, caused scrolling, and
!              eventually ended up at the same screen position (but
!              not buffer position) in which case it is a drag, not
!              a click.  */
!             /* FIXME: OTOH if the buffer position has changed
!                because of a timer or process filter rather than
!                because of mouse movement, it should be considered as
!                a click.  But mouse-drag-region completely ignores
!                this case and it hasn't caused any real problem, so
!                it's probably OK to ignore it as well.  */
!             && EQ (Fcar (Fcdr (start_pos)), Fcar (Fcdr (position))))
!           /* Mouse hasn't moved (much).  */
            event->modifiers |= click_modifier;
          else
            {

--
Andrew I. Schein
www.andrewschein.com