bug#8020: 23.2.93; assertion failure on fast frame resize

[YAMAMOTO Mitsuharu]

This is forked from Bug#7348, and some analysis can be found there.
(http://debbugs.gnu.org/cgi/bugreport.cgi?bug=7348)

Steps to reproduce:

  1. Build emacs with --enable-checking
  2. emacs -Q
  3. C-x 2
  4. Move your mouse to the corner of the frame, grab it, drag up,
     rescaling the vertically to minimum size

Result:

 ../src/xdisp.c:11515: Emacs fatal error: assertion failed: BUFFERP(w->buffer)

The value of w->buffer was nil when the assertion failure occurred.

                                     YAMAMOTO Mitsuharu
                                mituharu@math.s.chiba-u.ac.jp

In GNU Emacs 23.2.93.1 (x86_64-apple-darwin10.6.0, GTK+ Version 2.22.1)
 of 2011-02-09 on yamamoto-mitsuharu-no-iMac.local
Windowing system distributor `The X.Org Foundation', version 11.0.10402000
configured using `configure  '--enable-checking' 'LDFLAGS=-L/opt/local/lib' 
'CPPFLAGS=-I/opt/local/include' 'CFLAGS=-g''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: ja_JP.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t