Display bug using bold/italic faces & header line

[David Ponce]

Hi all,

In GNU Emacs 21.3.50.1 (i386-mingw-nt4.0.1381)
of 2002-07-15 on EBAT311
configured using `configure --with-gcc (2.95)'

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: ENU
 locale-coding-system: iso-latin-1
 default-enable-multibyte-characters: t

There is a nasty bug in the display engine of the latest CVS version
of Emacs 21 that displays garbaged text in buffers when scrolling up.

After some investigation I found that this occurred when both the
header line is displayed and the buffer contains text in bold or
italic faces.

A simple way to reproduce the bug is to start:

emacs -q--no-site-file

and display the Emacs manual in info.  Just scrolling the info buffer
page down twice or more, then page up twice or more shows the garbaged
display.  When I set `Info-use-header-line' to nil the bug
disappeared.

I also have this problem in other buffers when ruler-mode (which uses
the header line) is enabled because I use italic and bold font-lock
faces.

It seems that the following changes in xdisp.c caused the bug:

2002-07-07  Richard M. Stallman  <address@hidden>

* xdisp.c (make_cursor_line_fully_visible): Don't try short scrolls.
Instead just return 0 when there is something to be done.
(try_scrolling): If make_cursor_line_fully_visible returns 0,
retry scrolling as if cursor were off the bottom.
(try_cursor_movement): If make_cursor_line_fully_visible returns 0,
return CURSOR_MOVEMENT_MUST_SCROLL.
(redisplay_window): If make_cursor_line_fully_visible returns 0,
go to try_to_scroll.

After reverting the above changes all worked well again.

I hope all that will help.

Sincerely,
David