bug#56841: Emacs-28 C Mode: Fontification errors when arglist closing ) is on next line

[Alan Mackenzie <acm@muc.de>]

Hello, Emacs.

This is (a version of) a bug reported by Bill Sacks <sacks@ucar.edu> in
the correspondence for bug #56818.  I am separating it from that bug,
since it is a distinct bug, despite having similar symptoms.  In
Emacs-28:

1. Start emacs -Q.
2. Insert the following file in C Mode:

void myfunc(

  ) {

}

3. In line 2 (the first blank line) type "<TAB>int somevar".
4. Note that somevar is not fontified.  This is a bug, given that the
  arglist to myfunc is terminated with a ) on line 3.
5. Do something (e.g. typing M-x) to cause a redisplay.  somevar gets
  its correct face.
6. Note that any insertion or deletion in L2 causes somevar to lose its
  fontification.  This is a bug.
7. (After 6).  Move point onto somevar and do C-u C-x =.  This shows
  that the face text property is set on the character despite the face
  not appearing on the screen.

-- 
Alan Mackenzie (Nuremberg, Germany).