Re: Feature proposal/request: Indentation driven by display engine

[Stephen Berman <Stephen.Berman@gmx.net>]

On Sat, 24 May 2008 14:15:59 +0200 Carsten Dominik <carsten.dominik@gmail.com> wrote:

> Hi,
>
> I have a proposal to make:
>
> Recently, I have experimented with an idea, to create
> indentation on the Emacs display.
>
> For example, in an outline structure
>
> * level 1
> text under level 1
> ** Level 2
> text under level 1
>
>
> I would like to display this as:
>
> * level 1
>   text under level 1
> ** Level 2
>    text under level 1
>
> without actually inserting white space at the beginning of
> each line.  So effectively I would like to shift the location
> of the left margin on a per-line basis.
>
> I have been trying to implement this using display properties
> on each newline characters, displaying "\n   " instead of
> just "\n".  This basically works, but it interacts badly
> with outlining:  If I fold the text below a headline, the
> first indentation is still displayed.

This sounds reminiscent of a bug I reported a while ago; see
<http://permalink.gmane.org/gmane.emacs.devel/86019>.  

> The next thing I tried was using overlays over each "\n",
> with an after-string property carrying the indentations.
> This works correctly with outline, but when I add thousands
> of overlays to a buffer this becomes slow and redisplay and
> editing become sluggish, probably because to the huge
> amount of markers in the buffer.
>
> It then occurred to me that it might be useful to support
> such a feature directly in the display engine.
> For example, if the line contains text with an `indentation'
> property, the display engine would add this amount of white
> space to before the beginning of the line, maybe also a vertical
> line indicating the location of the margin.
>
> Would that be something useful to add?  Anyone who would
> volunteer to implement this, because I frankly have no
> idea how the display engine works internally.
>
> Or are there other comments, for example how this dynamic
> indentation could be implemented in a different way?

I've attempted to develop a version of longlines-mode that implements
the same sort of left margin shifting you mention, also by means of a
similar display property (I use " \n" concatented with a customizable
string).  It works fairly well (also with adaptive filling, which does
not work with current longlines-mode) but among other problems suffers
from the display bug I referred to in the above URL, which sounds like
what you are also seeing.  In an earlier version I tried using
before-string overlays and also ran into scalability problems.  I don't
know if a before-string property would solve all issues, but it would be
nice to have the possibility.

Steve Berman