bug#19661: wrapping before window-width (new wrap-column text property?)

[martin rudalics]

>> Layouts should be handled at the Elisp level.
>
> This is impossible with the current Emacs design, and you know it.
> The design is that Lisp programs _specify_ the layout, by setting up
> text properties, overlays, and local variables.  The actual _handling_
> of the layout is done by the display engine, which is not exposed to
> Lisp.
>
> So if a particular kind of layout is not supported by the display
> engine, you cannot specify it in Lisp.

The windows code does provide the display engine with a clipping
rectangle and two buffer positions where to start displaying text in
that rectangle and where to display the cursor (the latter may be
overridden by the display engine).  Together, these determine the basic
layout of buffer portions on screen and can be used by Lisp programs.

> I agree, but I don't think this can or should be done in Lisp.  Over
> the years, I've seen many features that attempted to produce fancy
> display traits not supported by the engine, and they all look kludgey
> to me.  They also break very easily.

With multiple columns we have to provide an API.  For example, to decide
whether the first character of a buffer's line is also the the first
character of a line in the rectangle displaying that line.  Otherwise,
we cannot provide the navigation facilities Ivan asked for.  If each
column is displayed in a separate rectangle, the first character of a
line is always the first character of the rectangle displaying that line
and you can handle this distinction, and thus provide the API, on the
Lisp level.

> The wheel should be round, then it's a wheel.

And it should spin freely.

martin