bug#26909: 25.1; A face for margins

[Eli Zaretskii <eliz@gnu.org>]

> From: Yuri Khan <yuri.v.khan@gmail.com>
> Date: Sat, 13 May 2017 21:07:03 +0700
> 
> The last two actions demonstrate that customizing the faces of
> whatever is displayed in the margin is not sufficient. A mechanism is
> needed that will allow customization of the margin where nothing is
> displayed. A new face would serve nicely.

A face can only affect places where something is displayed using that
face.  Display margins only display text if the buffer specifies text
properties or overlays which display in the margins.  But what you
would like to do calls for having a face that would affect screen
space where _nothing_ is displayed, and such screen space in Emacs is
always displayed using the frame's background color, not by using some
face.

You could specify a light gray background color for the frame, and
another color for the default face, but I think this would cause
unpleasant effects elsewhere on display, e.g. in the text area beyond
EOB.

IOW, I don't think introducing a new face would help here.  Some
additional mechanism would be necessary.