bug#64391: more info

[Andrew Cohen]

Mattias had flagged this code at line 2948 in editfns.c as a likely culprit. 

/* Record the accessible range of the buffer when narrow-to-region
     is called, that is, before applying the narrowing.  That
     information is used only by internal--label-restriction.  */
  Fset (Qoutermost_restriction, list3 (Qoutermost_restriction,
                                       Fpoint_min_marker (),
                                       Fpoint_max_marker ()));

Just for fun I simply commented it out (since the test code doesn't use
labeled restrictions) and ran the test code, and the problem disappears:

Emacs 30.0.50
Modify after narrowing:  0.076 s elapsed, 0 GCs, 0.000 s GC, 0.076 s non-GC
Modify before narrowing: 0.070 s elapsed, 0 GCs, 0.000 s GC, 0.070 s non-GC
-- 
Andrew Cohen