bug#57669: 29.0.50; C-n, C-p off under long lines

[Gregory Heytings]

> C-n'ing through a real-world logfile languageclient.out, which I attach 
> again here, chops up at around line 44.

That line has lots of brackets.  Try M-: (setq bidi-inhibit-bpa t) RET.

> This led me to write contrived tests, which of course are frowned upon 
> in your school of measurement, whose modus operandi is interminable 
> catechism between a complainant who laboriously and imprecisely 
> describes slowness, and an inquisitor auto-replying "doesn't happen 
> here."

What is on master does not claim to make Emacs equally fast in all 
circumstances, which is simply impossible.  Emacs remains responive, but 
yes, it is still (a bit) slow in some cases, and that seems unavoidable 
(without e.g. turning the BPA algorithm off unconditionally, which would 
be unwise).