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bug#12170: save-excursion fails boundary case with recenter

From: martin rudalics
Subject: bug#12170: save-excursion fails boundary case with recenter
Date: Sat, 11 Aug 2012 16:22:39 +0200

> Your recipe also calls set-window-point, which moves point on its own,
> and also does this:
>   /* We have to make sure that redisplay updates the window to show
>      the new value of point.  */
>   if (!EQ (window, selected_window))
>     ++windows_or_buffers_changed;
> The windows_or_buffers_changed flag will force a thorough redisplay.
> Given this, do we still have something unexplained?

Yes, because `set-window-point' doesn't set windows_or_buffers_changed
in the case at hand since the window is the selected window.  Or am I
missing something?

>> If you agree, then we'd have to explain why a subsequent invocation of
>> `set-window-start' with NOFORCE t can override the setting of the window
>> start position implied by the last invocation of one of the functions
>> mentioned above.
> You didn't just call set-window-start, you also called
> set-window-point.  If I remove that second call, the result of your
> code is very different, and the window start position as the macro set
> it is still in effect when control is returned.

But `set-window-point' should be equivalent to `goto-char' here because
the window is the selected window.

>>  >> w->force_start 1 will cause redisplay to honor the start position set up
>>  >> by `recenter'
>>  >
>>  > Only if point will be visible when window is displayed starting at
>>  > startp.
>> I completely miss you here.
> I meant this code:
>        w->optional_new_start = 0;
>        start_display (&it, w, startp);
>        move_it_to (&it, PT, 0, it.last_visible_y, -1,
>              MOVE_TO_POS | MOVE_TO_X | MOVE_TO_Y);
>        if (IT_CHARPOS (it) == PT)
>    w->force_start = 1;
>        /* IT may overshoot PT if text at PT is invisible.  */
>        else if (IT_CHARPOS (it) > PT && CHARPOS (startp) <= PT)
>    w->force_start = 1;
> It only sets the force_start flag if displaying the window starting at
> startp will show point visible inside the window.  The call to
> move_it_to moves in a simulated display lines, and stops either at
> point or at the last pixel position visible in the window, whichever
> happens first.  The subsequent test that IT_CHARPOS (it) == PT
> verifies that it stopped at point and not because it reached the end
> of the text displayed in the window.
> Ergo, sometimes setting the window start position will not be
> honored.  That's what the comment above all this means when it says:
>   /* If someone specified a new starting point but did not insist,
>      check whether it can be used.  */
> "Did not insist" means that whoever set w->optional_new_start did not
> also set w->force_start.  The "check whether it can be used" part
> describes what I just explained.

I believe you.  But it remains a mystery to me why `set-window-point'
should make a difference here.  As a matter of fact, if I do

  (defmacro save-this-window-excursion (&rest body)
    (let ((start (make-symbol "start"))
      (point (make-symbol "point")))
      `(let ((,start (copy-marker (window-start)))
         (,point (copy-marker (window-point))))
       (progn ,@body))
     (set-window-start (selected-window) ,start t)
     (with-current-buffer (window-buffer (selected-window))
       (goto-char ,point)))))

  (defun f (n)
     (save-this-window-excursion (forward-line (- n)) (recenter 0)))

   (let ((buffer (switch-to-buffer "foo"))
         (height (1- (window-height (get-buffer-window "foo")))))
     (insert-char 10 (* height 2))
     (let ((pt (point)))
       (f height)
       (message "height %s old %s new %s" height pt (point)))))

I get the same results as with `set-window-point'.  IMHO the
`set-window-start' call makes the difference but I don't understand why.


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