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.
This property would still need to be added to every line. So
basically
all you're asking is a "before-string" text-property, maybe?
Hmm, nice translation of my request. Yes, basically what I am
asking for is a working before-string text property. I know
that adding the property sounds like a lot of effort, but I could
use the font-lock/jit-lock setup to get this done efficiently,
only for the pieces that actually get displayed.
The same has been requested for display properties (e.g. to put
something in the margin without hiding any of the current text, you
need an overlay).
A `before-string' text-property might be doable, but I couldn't tell
you
how easy it'd be to add. If someone wants to implement that, I might
accept it (e.g., as a form of `display' property), but if I were you
I wouldn't hold my breath (we have enough bugs to fix and are getting
close to feature freeze).