Re: [Emacs-diffs] master 29d1c72: Introduce new value t for compilation-context-lines to eliminate scrolling

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Mon, Aug 26, 2019 at 19:29:16 +0300, Eli Zaretskii wrote:
> > Date: Mon, 26 Aug 2019 16:26:51 +0000
> > Cc: monnier@iro.umontreal.ca, emacs-devel@gnu.org
> > From: Alan Mackenzie <acm@muc.de>

> > Should I have been aware of this?

> What is "this" in this context?  The display margins or how to use
> them with overlays?

I think, more the first of these.

> > Somehow, in the elisp manual, it seems buried under several levels
> > of chapters, sections, and sub-sections.

> Well, Emacs is a large program, and display features in particular are
> a legion...

I found implementing these margins (see patch in separate post) to be
particularly difficult.  There is no complete example of how to do this
on the "Display Margins" elisp manual page.  There, the word "property"
is used, rather than "text property or overlay property", leaving me
uncertain of whether I could have implemented this "=>" in the margin
solely with overlays.  I was uncertain, right up till near when I had
the thing ready, whether the variables implementing this need to be
buffer local or not.  (It also didn't help that I tried at first to put
the `display' text properties on overlay-arrow-string, which, of course,
can't work.)

Also, the setup seems wierd.  Why isn't there a direct `margin' overlay
property, much the same way there's a `before-string' overlay property?
Instead, what we have is a required "dummy" `before-string', which is
supplanted by a `display' text property on it.  It just seems a strange
way of implementing it.  This certainly doesn't promote easy
understanding.  IMHO, of course.

-- 
Alan Mackenzie (Nuremberg, Germany).