bug#56682: locked narrowing

[Stefan Monnier]

> Search in xdisp.c for "->base_line_number = 0", and you will see that
> we invalidate the cache when the narrowing changes or when other
> similar calamities happen.

OK, after another search I found (in `redisplay_window` and
`try_scrolling`) that we do:

          if (!just_this_one_p
              || current_buffer->clip_changed
              || BEG_UNCHANGED < CHARPOS (startp))
            /* Forget any recorded base line for line number display.  */
            w->base_line_number = 0;

So IIUC `w->base_line_number` can be used if:

          w->base_line_number != 0
          && !current_buffer->clip_changed
          && BEG_UNCHANGED < w->base_line_pos

?

Why do we flush the cache when `just_this_one_p` is false?


        Stefan