Eli Zaretskii <eliz@gnu.org> writes:

> When this happens, does it help to type "M-x redraw-display RET"?  If
> it does, then, in the absence of a reproducible test case, all I can
> suggest is gather more info to narrow the range of possible
> offenders.  Start by rebuilding Emacs with -DGLYPH_DEBUG=1, then start
> Emacs with stderr redirected to a file, toggle redisplay tracing with
> "M-x trace-redisplay RET", and wait till the problem happens.  When it
> does, make the window with incorrect display the currently selected
> window, and type "M-x dump-glyph-matrix RET".  Finally, send the
> contents of the redirected stderr here.

Unfortunately, I hadn't compiled Emac with -DGLYPH_DEBUG=1 (but am going
to do it now), but at least I managed to take some screen shots now,
which may give somebody an idea about what the problem is, perhaps.

(Sorry for the big email.)

The first image is me reading a blog post normally.  The second is me
typing `C-h f redisplay'.  Only the top half of the display is actually
refreshed, leaving the image apparently being displayed in the help
buffer.

And the last image is me lowering and raising the window, forcing a
redisplay, which makes Emacs look like it should again.