bug#57207: 29.0.50; Fontification is slow after e7b5912b23 (Improvements to long lines handling)

[Gregory Heytings]

> > I can reproduce that, too.  The "Narrow" indication disappears with the 
> > next redisplay cycle, so it seems to be a not-too-grave issue (but a 
> > bug nonetheless).
>
> It doesn't disappear if you keep typing fast enough, or lean on a key.

Indeed, it disappear and reappears.

> > It could probably be solved by moving the calculation of "BUF_BEGV (b)
> > > BUF_BEG (b) || BUF_ZV (b) < BUF_Z (b)" from decode_mode_spec to
> > mark_window_display_accurate_1, which would set a one-bit field in 
> > struct buffer.
> >
> > WDYT?
>
> That's too late, I think?  How do we ensure after that the update of the 
> mode-line (in case the hook really wants to change the restriction)?

Hmmm...  I guess I need to think about this a bit more!