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Porting the entire desktop to GTK+ 3 is not going to happen in Xfce 4.12, it will be a more subtle approach. Xfce 4.12 has the following targets:
Eventually Xfce has to work with to GTK+ 3, so we can optionally support this in a couple of applications to make the migration quicker and it becomes easier to track the status of GTK+ 3. This involves the following tasks:
Depending on the time we'll see how this goes, but it give everybody the opportunity to work on this during the 4.12 release cycle. Keep in mind that GTK+ 2 is still the target for Xfce 4.12, so this should work at all times.