bug#4911: mouse-face property should merge face attributes, not replace

[Lars Ingebrigtsen <larsi@gnus.org>]

Dave Aspinall <daveaspin@googlemail.com> writes:

> Perhaps this has been noted already: the low-level behaviour of the
> mouse-face property seems ugly: it simply overwrites the face property 
> for characters under the mouse.  For example in Info, blue underlined
> links turn black without the underline when the mouse is hovered over 
> them to give the green background from the highlight face.  This feels
> unnatural.
>
> In Proof General (http://proofgeneral.inf.ed.ac.uk) we use the
> mouse-face property on programming language text which is heavily 
> decorated with font-lock.  Users complain that when the mouse is over
> a region the normal fontification is obliterated.

To reproduce:

(insert (propertize "hello" 'face 'underline 'mouse-face 'highlight))

and put the mouse pointer over the "hello": The underline goes away.

I think it might make sense to merge the properties...  but, on the
other hand, this may make the text illegible.

For instance, this

(insert (propertize "hello" 'face '(:foreground "blue") 'mouse-face 'highlight))

would become a blank, blue box if the highlight face didn't define both a
foreground colour.

So I'm not sure this would work.  Any opinions?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no