Subject: Re: bug#10114: 24.0.91; (not (eq (current-buffer) (window-buffer (selected-window))))

From: martin rudalics <[email-masked]>
Date: Wed, 23 Nov 2011 11:06:44 +0100

> When you remove the current buffer from the selected window,
 > the buffer that gets displayed in the selected window isn't always
 > the same as the buffer that gets made the new current buffer.
 > The command M-x test (see below) manifests the problem
 > for me from runemacs -Q.
 >
 > Output on Emacs 23.3:
 > Current: *Test*<2>; Selected: *Test*<2>; [...]
 > Current: *Test*<1>; Selected: *Test*<1>; [...]
 >
 > Output on Emacs bzr trunk:
 > Current: *Test*<2>; Selected: *Test*<2>; [...]
 > Current: *Test*<1>; Selected: *scratch*; [...]

The output on Emacs bzr trunk should be

Current: *Test*<2>; Selected: *Test*<2>; [...]
Current: *scratch*; Selected: *scratch*; [...]

because *scratch* was the buffer shown before *Test*<2>*.

 > (defun test ()
 >   (interactive)
 >   (progn
 >     ;; This seems to sow enough confusion.
 >     (switch-to-buffer-other-window "*Test*<1>")
 >     (switch-to-buffer-other-window "*Test*<2>")
 >     (delete-other-windows))
 >   (while
 >       (progn
 >         (message
 >          "Current: %s; Selected: %s; \
 > type k to kill current buffer, \
 > any other key to quit"
 >          (current-buffer)
 >          (window-buffer (selected-window)))
 >         (and (eq (read-event) ?k)
 > 	     (kill-buffer))))
 >   (clear-this-command-keys t))
 >
 > In GNU Emacs 24.0.91.1 (i386-mingw-nt6.1.7601)
 >  of 2011-11-20 on MACHINE
 > Windowing system distributor `Microsoft Corp.', version 6.1.7601
 > configured using `configure --with-gcc (4.6)'

The problems happens because `kill-buffer' makes another buffer current
when killing the current buffer and calls `replace-buffer-in-windows'
which ends up calling `switch-to-prev-buffer' for the selected window.
The latter refuses to change the current buffer because the selected
window's buffer is _not_ the current buffer (a behavior needed to make
`switch-to-prev-buffer' work within buffer excursions).

So the solution is to have `kill-buffer' call `replace-buffer-in-windows'
_before_ making another buffer current.

Thanks for the excellent report, martin