bug#4710: 23.1.50; Bad display of underlines crossing line boundaries

[Stefan Monnier]

>> "If it hurts, don't do that" comes to mind.
> But it does look really ugly.  :-)  This single behavioural tic is what
> makes underlines undesirable.  If somebody had a good idea how to fix
> this in general, that would be a win.

Maybe someone could come up with a neater way to display "face
continuations" (face that applies to the text where a line is
wrapped).  For underline, we could put a short bit of dotted underline
as in:
              foo bar baz
                  ——————-⋯
              toto titi turlu
             ⋯————


        Stefan