Re: Bug in incremental undrawing of mouseover highlighting

[Bob Rogers <rogers-emacs@rgrjr.dyndns.org>]

   From: David Kastrup <dak@gnu.org>
   Date: Sun, 19 Nov 2006 23:50:32 +0100

   Bob Rogers <rogers-emacs@rgrjr.dyndns.org> writes:

   >    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.

   For people wanting to debug this: I have the slight suspicion that the
   problem is masked usually because some well-meaning workaround
   suppresses the highlighting of partial lines that _should_ be
   highlighted when a buffer pops up under the mouse cursor.  Often the
   highlighting only starts once one moves the mouse cursor.  And I think
   that the problem might be triggered when only parts of the line are
   to be highlighted.

   I have no recipe for reproduction now, this is rather irregular and
   tends to occur in the newsreader for me, which means that there is no
   repeatable recipe.

   -- 
   David Kastrup, Kriemhildstr. 15, 44793 Bochum

Indeed; I first noticed this in vm-summary-mode, where each line
represents a message.  But there the whole line is highlighted.  When I
switch from the VM summary buffer to a buffer that matches the same line
in its entirety, all mouseover highlighting is preserved; when I switch
to a buffer that matches it partially, highlighting is correctly cleared
only for those positions that must be changed to display different
characters.

					-- Bob