bug#45620: 28.0.50; Child frames should have their own border width and colour

[martin rudalics]

> This is an idea set off by this discussion:
> https://github.com/Alexander-Miller/treemacs/issues/242#issuecomment-753296634
>
> Basically the problem is that child frames need a distinct border colour
> to be properly visible (and just using a different background colour does
> not look nearly as good).
>
> That can be achieved by customising the `internal-border' face, however
> that face controls the appearance of the border of all frames, and as I
> have learned there is a non-trivial amount of users who by default use a
> large internal border as a margin for their frames. And for these users
> the `internal-border' should have the same colour as the default
> background.

Isn't the situation even worse than how you describe it here?  When I
customize 'internal-border' face, that affects all frames, including
those for which I have set it already via 'set-face-background'.  Which
means that whatever a package does to set that face for a specific
(child) frame, that setting is undone by a later customization.  IIUC
the discussion you refer to above arrived at the same conclusion.

> So there is a conflict between wanting a distinct border colour for
> child frames and using the default background for normal frames.

If I'm not mistaken we use that face for our tooltip frames too which
means one more conflict.

> Packages can work around that individually, for example posframe accepts
> an `internal-border-color' argument that overrides the `internal-border'
> face frame-locally. But that still means that every package using child
> frames needs its own user option for the child frames' border colour
> when such matters clearly belong under the domain of the user's theme.

"clearly" is clearly too strong here.  Ultimately, the package must have
the choice and its choice should prevail (it currently doesn't).

> I think that since child frames serve sufficiently distinct use cases
> than normal frames it makes sense for them to have their own border
> appearance controls.

So what should we do?  Provide a separate 'child-frame-internal-border'
face and then probably also a 'tooltip-internal-border-face'?

Customizing such a face would still override individual frame settings.
What we need is probably a strategy to avoid setting the background for
those frames that have their internal border already set.  But then we
should do that for all faces running through 'set-face-attribute'.

martin