bug#38049: C mode fontification broken with reposition-window

[Alan Mackenzie <acm@muc.de>]

Hello, Juri.

On Tue, Nov 05, 2019 at 21:07:37 +0000, Alan Mackenzie wrote:
> On Sun, Nov 03, 2019 at 22:28:05 +0200, Juri Linkov wrote:
> > Version: 27.0.50

> > This is a reproducible test case:

> > 0. emacs -Q
> > 1. C-x C-f emacs/lib-src/emacsclient.c
> > 2. M-: (progn (search-forward "create-frame" nil t) (reposition-window))

> > Then half screen displays unfontified lines.

> > Fontification doesn't fail in other modes, only in C mode.

> > This has something to do with interaction between c-font-lock
> > and buffer navigation in reposition-window.

> Indeed it does.

> Briefly,
> (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.

[ .... ]

I have fixed this bug by widening around c-font-lock-fontify-region.  I
am closing the bug with this post.

-- 
Alan Mackenzie (Nuremberg, Germany).