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Re: Layered display API

From: Eli Zaretskii
Subject: Re: Layered display API
Date: Fri, 15 Aug 2014 09:39:55 +0300

> Date: Fri, 15 Aug 2014 05:20:12 +0400
> From: Dmitry Gutov <address@hidden>
> CC: address@hidden, address@hidden
> > This property we are discussing (that doesn't yet exist) will be acted
> > upon by the display engine.  The display engine always knows what
> > "things" are there at point, and the supporting logic for this
> > property will cause the display engine to ignore all the other
> > "things" and display only the overlay that has this property.  At
> > least that's the theory.
> Well, since we don't only need to replace the displayed characters, but 
> also modify them, this doesn't do a lot of work for us.

Not sure why you would need to modify them.  My understanding is that
you include in the overlay some of the displayed text, and also
include your own text.

What I'm suggesting will allow you not to duplicate the displayed
text, but just add an overlay with your own.

>  From what I understand, you're suggesting a property that would act 
> like a combination of overlay + display does currently. Why not just 
> document that current behavior and then stick to it, rather than 
> introduce the new property?

See above: because what exists doesn't do the job for you well enough.
Copying into your overlay existing text from buffer, other overlays,
and display strings is fragile design with known deficiencies (which
were revealed in this thread).  Having a special property that would
avoid all that is a win, IMO.  For example, you will be able to stop
worrying about line-prefix and images.

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