bug#26959: Feature request: bold underlines

[Eli Zaretskii]

> Cc: 26959@debbugs.gnu.org
> From: Clément Pit--Claudel <clement.pitclaudel@live.com>
> Date: Wed, 17 May 2017 14:59:56 -0400
> 
> > What all this means is that currently the exact visual effect of the
> > underline attribute is deliberately not well-defined: about the only
> > thing you can rely on is that you will get a horizontal line somewhere
> > in the lower portion of the characters.
> > 
> > Implementing your suggestion would require that we define the behavior
> > much better, which is not easy given the different font drivers and
> > fonts, on which the user has almost no control.  E.g., we will need to
> > decide whether thickness customization overrides the font-dependent
> > scaling, and if not, how these two play together.  And if we want to
> > allow customization of the underline position (why not?), we will have
> > to decide what to do with it when the font size changes.  And then we
> > will need to decide what to do if the font doesn't support scaling.
> 
> That makes sense, but I'm not sure all of this is needed. I agree that it 
> would be nice, but is it really necessary? 

Perhaps not.  But any subset of this we choose to implement should be
consistent and should make sense to users.

> In terms of code, my suggestion would translate into multiplying the 
> `thickness' variable in xftfont by 2 when :bold t is specified in the 
> underline's property list.

Even if the bold attribute starts in the middle of a consecutive run
of underlined characters?  IOW, should this override the current
behavior of computing the thickness only once per such run?