bug#11943: 24.1.50; Emacs unusably slow when looking at large files (bidi support at fault)

[Eli Zaretskii <eliz@gnu.org>]

> From: Lawrence Mitchell <wence@gmx.li>
> Date: Sun, 15 Jul 2012 12:58:17 +0100
> 
> This appears to be an issue with buffers that have no strongly
> R2L or L2R characters near point-min.

More accurately, it was an issue with _paragraphs_ that have no strong
directional characters.

> C-x b *tmp* RET
> M-x fundamental-mode RET
> M-: (dotimes (i 100000) (insert "1\n")) RET
> M-<
> 
> next-line now has a noticeable delay (especially when holding
> down next-line, display doesn't update the cursor position).
> 
> Now insert an 'a' somewhere near the beginning of the buffer,
> display updates appear instantaneous again.  If we instead go to
> the end of the buffer and insert an 'a', no such change in the
> response occurs.

However, if you insert "a\n\n" at the beginning, the slowdown comes
back again.  Which is to say that the problem is not with what we have
at point-min, but what we have in the paragraph around point.  The
fact that setting bidi-paragraph-direction to a non-nil value makes
the problem go away is another telltale sign of that.

Anyway, it's solved now.

Thanks.