Re: transient-mark-mode in 22.0

[Stefan Monnier]

>> I noticed that setting the transient-mark-mode variable no longer does
>> anything
>> 
>> As far as I can see, it still does what it always did.
>> Did you actually observe that it fails to work?

> Only in so far as I expected transient-mark-mode to allow mouse-drag-
> region to return upon mouse-up, which it used to do under 21.x.  So in
> that sense, it fails to work.  It may well be that all other features of
> transient-mark-mode do work when the variable is set.

> In Emacs 21.X, when bound to a mouse-down event, this:

> (defun do-drag (event)
>   (interactive "e")
>   (let ((transient-mark-mode t)) ; transient mark
>     (message "DRAG STARTED")
>     (mouse-drag-region event)
>     (message "DRAG FINISHED")))

> and this:

> (defun do-drag (event)
>   (interactive "e")
>   (let ((transient-mark-mode nil)) ; no transient-mark
>     (message "DRAG STARTED")
>     (mouse-drag-region event)
>     (message "DRAG FINISHED")))

> produce different behavior w.r.t. mouse-up.  In 22.0, they produce the
> same behavior.  Unfortunately, it's not the behavior I need.

I believe this is due to the change

   revision 1.252
   date: 2004/10/27 17:44:59;  author: rms;  state: Exp;  lines: +49 -48
   (mouse-show-mark): Do most processing the same
   regardless of transient-mark-mode.
   ----------------------------

This code in mouse-show-mark is also responsible for the problem where
flyspell's mouse-2 also does a yank, reported a while back, and probably
a few other similar bug reports.  Before 1.252 the problem was only
affecting users who don't use transient-mark-mode.

To fix this problem, I maybe mouse-show-mark should be rewritten to not use
(read-event), but instead to use something like pre-command-hook.

Looking at the code of mouse-show-mark, it's pretty messed up:

- in the code below,

        (and (consp event)
             (or (eq (car event) 'switch-frame)
                 (eq (posn-point (event-end event))
                     'vertical-scroll-bar))
             (let ((keys (vector 'vertical-scroll-bar event)))
               (and (key-binding keys)
                    (progn
                      (call-interactively (key-binding keys)
                                          nil keys)
                      (setq events nil)))))))

  if the event is `switch-frame', we'll do a key lookup for
  [vertical-scroll-bar switch-frame] which of course will fail.

- since the while loop is expected to execute code such as
  handle-switch-frame (which it luckily doesn't), the selected buffer might
  get changed before we reach the end where we do (delete-region (mark t)
  (point)), so if we fix the above error, we bump into another.

- the overlay management has apparently been rendered completely useless by
  the use of the temporary transient-mark-mode.  So all that this code
  really does is implement the mouse-region-delete-keys.  This variable was
  introduced in 1996, but is not documented anywhere.  Does anybody use
  this?  (I expect that people who want such a functionality probably use
  delete-selection-mode instead anyway).  I.e. if could drop
  mouse-region-delete-keys, then we could completely remove mouse-show-mark
  and the associated bugs.


        Stefan