It seems that "force-" in this use flag is redundant and not in line
with "qt5" and "gtk3" which are already use flags in these packages.
It would be better to be consistent and have all of the gui toolkit use
flags named the same way. This also falls in line with the qa team's
recommendations for use flags in this situation.
Sorry for the delay on a promised comment here. Anyhow:
The GNOME teams stance on this technically is, that there shouldn't be any gtk2 USE flag in any form as per https://wiki.gentoo.org/wiki/Project:GNOME/Gnome_Team_Ebuild_Policies#gtk3
The application maintainer should select which gtk version is best at the time - and choose gtk3 if they are as good (no feature regressions or rendering bugs with gtk3 or so).
But with applications like firefox, things aren't so black and white, and as this is not a gnome maintained package, I'd say you can choose what you deem best, but we are happy to discuss things to align things across the tree better.
I believe the force-gtk2 flag was used due to this point in our policy:
* This means that having USE=gtk3 to enable gtk+-3 instead of gtk+-2 support is forbidden.
combined with USE=gtk2 being very similar to that, so implying the same for gtk2 with the versions reversed in that sentence.
So I'm not really sure what here is best.
As far as we are concerned, then in an ideal there would be no USE=gtk3 anywhere either, but the reality is different due to some libraries being too hard to split into two separate slots (manpower demand not worth it). Take for example avahi - in an ideal world, we would have a avahi:2 that ships the gtk2 libs and avahi:3 that ships the gtk3 libs, but it was too hard to split them up like this just for the ideal USE flags desire.
Most applications we said to pick one, moving to gtk3 when things are good enough there. This did mean a huge vocal minority (or majority, who knows) backlash on them being forced on gtk3 early in some cases, but it's 2016 now. But firefox is a rather big thing, and I would be willing to look another way here, especially as gtk3 still has problems, yet it would be very helpful to be able to test things out and help upstream improve things.
(In reply to Mart Raudsepp from comment #1)
> especially as gtk3 still has problems
Here I of course mean firefox gtk3 having problems. gtk3 is problem-free, I promise, especially gtk3.20 themeing changes won't cause issues at all :D
From qa's point of view, there are to be no non-versioned gtk use flags
in the tree. We recommend that either the flag be renamed to gtk2 instead
of force-gtk2, or the flag be dropped.
Also, please keep the gtk3 use flag as is.
I have pushed an update that converts force-gtk2 to just gtk2 thanks for reporting.
This is still in discussion and USE=gtk2 is meant for library cases as far as we are concerned.
But I guess I can't mind this flag changing back and forth in whatever way a maintainer happens to go for until this discussion is concluded.