bug#27103: Should show-trailing-whitespace highlight specified spaces?

[Eli Zaretskii]

> Cc: 27103@debbugs.gnu.org
> From: Clément Pit--Claudel <clement.pitclaudel@live.com>
> Date: Sun, 28 May 2017 13:53:09 -0400
> 
> > But this again asks that the display engine could produce 2 different
> > display elements out of the same buffer position, which means it would
> > need to scan that buffer position twice.  Right?
> 
> Possibly — I don't think I understand this part well.  What happens for 
> before-string properties?

There's a string there, so those properties are processed when the
display engine traverses the string characters.

> Could the same thing happen here?

No, because there's no string.  There's just buffer text and nothing
else.

Maybe we could extend the line-prefix property to support left-fringe
and right-fringe specs.

> > Are overlays really so slow that you are prepared to jump through such
> > hoops?
> 
> Definitely :) I need these markers for a literate-programming buffer where 
> roughly every other line has a marker.  On a 10k lines file, overlays are 
> entirely impractical.

Can you show a simple example where having 5K overlays in a buffer
cause significant slowdown?  I mean, just having 5K overlays, without
any additional bells and whistles.  What exactly is slowed down?