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

[Alan Mackenzie]

In article <mailman.552.1572813126.13325.bug-gnu-emacs@gnu.org> you 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.

Yes.  This is an interesting bug.

The unfontified area seems to be one jit-lock chunk immediately below
the line containing "create-frame".

> 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.

reposition-window, as far as I've been able to make out so far, seems to
assume that point starts in the current visible window.  That is not the
case here, where window-start is still at BOB, but point is at line 116.

I'll look at this in more detail when it's not so late.

-- 
Alan Mackenzie (Nuremberg, Germany).