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bug#38049: C mode fontification broken with reposition-window
From: |
Alan Mackenzie |
Subject: |
bug#38049: C mode fontification broken with reposition-window |
Date: |
Wed, 6 Nov 2019 20:56:05 +0000 |
User-agent: |
Mutt/1.10.1 (2018-07-13) |
Hello, Eli.
On Wed, Nov 06, 2019 at 19:13:44 +0200, Eli Zaretskii wrote:
> > Date: Tue, 5 Nov 2019 21:07:37 +0000
> > From: Alan Mackenzie <acm@muc.de>
> > Cc: 38049@debbugs.gnu.org
> > (i) reposition-window narrows to (2758 3940) in
> > repos-count-screen-lines.
> > (ii) This latter function uses vertical-motion to count the lines.
> > (iii) vertical-motion triggers jit-lock fontification.
> > (iv) This calls (eventually) c-font-lock-fontify-region.
> > (v) c-font-lock-fontify-region attempts to examine buffer text before
> > the start of the jit-lock chunk to find syntactic context.
> > (vi) This is outside the visible region, so Emacs raises an exception.
> > (vii) The exception is caught and discarded by an unwind-protect in
> > c-font-lock-fontify-region.
> > (viii) The jit-lock chunk remains unfontified.
> > As Stefan M has sometimes remarked, narrowing is often not a good idea.
> Narrowing is part of life.
Indeed.
> Why can't c-font-lock-fontify-region be fixed so as not to signal an
> error in these cases? If we do, will that fix the original problem?
Yes. Thinking about it, font locking might present
c-font-lock-fontify-region a chunk in any state of narrowing. It just
doesn't happen that way very often, though. So CC Mode needs to widen
the buffer to make sure it has adequate access.
> > It would seem undesirable for the vertical-motion in (iii) to trigger
> > font-locking, since it is merely trying to count lines.
> vertical-motion calls functions from the display engine, which
> fontifies the text it traverses. It doesn't only count lines, it also
> counts columns in canonical character width units (i.e. actually in
> pixels). So it cannot possibly avoid fontifications because
> fontification might affect how many pixels characters take on
> display. Fontifications can also affect the line count, if some
> fontification code actually puts display properties on the fontified
> text -- something that you rarely if ever see, but Lisp programs can
> rightfully expect that. So this idea:
> > Perhaps there should be a macro `without-fontifying' which could be
> > wrapped around this call to vertical-motion, if there isn't such a
> > thing anyway.
> cannot fly, sorry.
Yes, accepted and understood.
> Let's try to fix the problem in c-font-lock-fontify-region instead,
> okay?
I'll do that. It just needs a simple widen in a save-restriction.
--
Alan Mackenzie (Nuremberg, Germany).