bug#20100: 24.4.91; Can't scroll a window while in minibuffer

[martin rudalics <rudalics@gmx.at>]

 > I'm trying to write a completion package using `read-from-minibuffer'
 > combined with `post-command-hook'.
 >
 > Everything is working OK, except when I want to sync the current
 > completion candidate (one of buffer's lines) with the corresponding line
 > in the buffer.
 >
 > This code, specifically `recenter', doesn't work:
 >
 > (with-current-buffer buf
 >    (goto-char (point-min))
 >    (forward-line (1- num))
 >    (recenter))

I suppose that at the time you invoke `recenter', the selected window
doesn't show `buf'.  When with emacs -Q I do

(let ((buffer (get-buffer-create "*buffer*"))
       (line 0))
   (set-window-buffer nil buffer)
   (with-current-buffer buffer
     (while (< line 100)
       (insert (format "%02d\n" line))
       (setq line (1+ line))))

   (with-current-buffer buffer
     (goto-char (point-min))
     (forward-line 25)
     (recenter)))

then I see the line starting with "25" centered in the selected window.
Can you try modifying my example such that it "doesn't work for you"?

 > However, this code will scroll the window (`buf' and `wnd' correspond to
 > each other):
 >
 > (with-current-buffer buf
 >    (goto-char (point-min))
 >    (forward-line (1- num))
 >    (setf (window-point wnd)
 >          (point)))

The fact that you apparently have to

(setf (window-point wnd) (point))

seems to suport my claim above, namely that `wnd' is not selected at
that time.

 > Still, it's not a perfect solution, since even after setting
 > `window-point', (window-start wnd) and (window-end wnd t) will not
 > return the correct thing.

What is the wrong thing and what would the correct thing be?  The only
function that should work "correctly" here is `set-window-start'.

 > I'm pretty sure it's a bug and the code above should work as is, but I'd
 > also appreciate pointers for achieving the scrolling and window bounds
 > re-calculation for the current Emacs.

Beyond what you can read in the documentations, explaining `recenter' is
hardly possible without explaining redisplay as well.  You have to look
at its implementation.

martin