Re: Question about display engine

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 19 Aug 2019 18:13:05 +0200
> From: Ergus <spacibba@aol.com>
> Cc: Eli Zaretskii <eliz@gnu.org>, emacs-devel@gnu.org
> 
> Extending the face is a detail that most users won't even notice a
> change while it works somehow. Actually we have had different behaviors
> between gui and tui for years and nobody complained up to now.

I think you over-simplify the situation.  First, both GUI and TTY
frames behave the same when extension of face background color is
concerned, they only differ in how they handle extension of other
attributes (of which only the underline is relevant to TTY frames).
(And I did hear over the years a couple of complaints about how TTY
frames extend the underline attribute.)

And second, I refer you to the renewed discussion of bug#15934 a day
or two ago, from which my take is that users will notice and do care
about such changes in at least a couple of important use cases, as a
soon-to-be-pushed changes will prove.  So I don't think we can change
this behavior at will on the assumption that "no one will notice".

> I am wondering about over-specifications and over-engineering for such a
> detail, when most of the users only need to extend the background color.

This discussion established that even supporting just the background
color non-extension as a user option will require to have almost all
of the machinery in place: the extend bit, the generation of a special
face without the background color, etc.  And once we have that, adding
other face attributes to the soup is relatively easy and won't require
any design changes.