Re: redisplay - very long lines

[Eli Zaretskii <eliz@gnu.org>]

> From: David Reitter <david.reitter@gmail.com>
> Date: Mon, 16 Feb 2009 08:44:23 -0500
> 
> The redisplay code seems to be unable to deal efficiently with very  
> long lines, that is, lines that go beyond the currently displayed  
> window.

This is a known limitation of the Emacs display engine, but IIRC it
has nothing to do with display margins.  The same slowdown happens if
your display is as wide as the longest line.

> Would it be possible to add a termination condition in move_it_to to  
> stop after leaving the window vertically or horizontally (with  
> truncate-lines)?

If we can tolerate an incorrect display, maybe.  The invisible part of
the line could in principle include characters and faces whose
presence may have global effects on the rest of the displayed portion,
such as faces that use larger fonts, to say nothing of inline images.

But it's been a while since I last took a good look at display code,
so perhaps I'm misremembering something, or maybe things changed.