is GTK 3.x a real cross-platform solution?
I have tasted both Qt 4.x/5.x and GTKmm 3.x and I really like GTKmm over Qt.
Now I have just discovered that GTKmm 3.x doesn't offer a Windows porting, they also do not offer a Mac porting, basically I can't find nothing but libraries/sources/binaries for Linux. Apparently the Windows support ( I'm not sure about the situation for the Mac OS stuff ) was dropped in the 2.x era and it's still not-existing today.
I have planned to use GTK 3.x for specific reasons, especially for some new features introduced by the 3.x branch, so using GTK 2.x is an option off the table.
Now I ask: there is something new for the Windows and/or Mac support for GTK 3.x ? There is something ? even experimental projects ?
Answer
2018-12-18 Update:
GTK+ 3 on Windows has been supported through MSYS2 for years.
Old answer:
GTK for Windows is currently provided "as-is". What it means, basically, is that there are no GTK maintainer that use Windows, and those using Linux have enough work maintaining the Linux builds.
This means that any contribution for GTK 3 on Windows is welcome, some people are using it, but that's not ready for the masses because nobody steps up to do the work, which won't be done by magic overnight. So it's usable, but don't expect reactive maintenance.
As for the build, there's a french dude providing an (unofficial) GTK 3 installer for Windows.