Bug in incremental undrawing of mouseover highlighting

[Bob Rogers]

   It seems that mouseover background highlighting is not undrawn if
replaced by the same unhighlighted text.  This happens in the CVS
version as of late on 11-Nov.  (I have seen this bug for some time now,
but only just thought of a (relatively) easy way to reproduce it.)

   To reproduce:

   1.  "emacs -Q"

   2.  "C-x d RET".  This gets a dired buffer; the exact directory
doesn't matter as long as it has at least a few files in it.  (And, for
some reason, "." and ".." don't count.)

   3.  "C-x h M-w C-x b *scratch* RET C-y M-<".  This gets a copy of the
dired buffer contents; sometimes they come with dired fontification, and
sometimes not.  The same text should be displayed in the same character
positions in both dired and *scratch* buffers.

   4.  "C-x b RET" to get back to the dired buffer, and move the mouse
over a file name so that it is highlighted in green.

   5.  "C-x b RET" to return to *scratch*.  Notice that the same
characters in the same position are still highlighted in green.

   6.  Move the mouse to a different file name.  Usually the
highlighting goes away, but sometimes it doesn't (and it doesn't
correlate with fontification).  If it does persist, type "C-x b RET" to
return once more to the dired buffer, and there will be two green file
names -- this can be repeated until every file name is green.

   7.  "C-l" (or anything else that causes redisplay from scratch) will
restore the display to normal; this is why I think it's a redisplay bug.

   My apologies if this is a known problem; I find it hard to follow the
redisplay threads, so I may have missed the relevant description.

                                        -- Bob Rogers
                                           http://rgrjr.dyndns.org/