[emacs-21.2.1] display bug after truncated long line

[kwzh-b]

In GNU Emacs 21.2.1 (i386-redhat-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2003-02-19 on porky.devel.redhat.com
configured using `configure  --host=i386-redhat-linux --build=i386-redhat-linux 
--target=i386-redhat-linux-gnu --program-prefix= --prefix=/usr 
--exec-prefix=/usr --bindir=/usr/bin --sbindir=/usr/sbin --sysconfdir=/etc 
--datadir=/usr/share --includedir=/usr/include --libdir=/usr/lib 
--libexecdir=/usr/libexec --localstatedir=/var --sharedstatedir=/usr/com 
--mandir=/usr/share/man --infodir=/usr/share/info --with-gcc --with-pop 
--with-sound'
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: nil
  locale-coding-system: nil
  default-enable-multibyte-characters: nil

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

(Tested in both an 80-column X frame and a termcap frame.)

Create a buffer (*scratch* will do) containing a line of
579 chars and two other lines, then let that first line
get truncated in the display.

$ emacs -q
C-x h C-w C-u 579 x RET yyy RET zzz RET
M-: (setq truncate-lines t) RET

Expect to see:  long row of xxxxxx, then yyy, then zzz.
Actually see:   long row of xxxxxx, then zzz.
The "yyy" line is missing.  (And bad things seem to happen when
I try to move point there, but I'm not going to investigate that
any further tonight.)