Xfce - Wikipedia
https://en.wikipedia.org/wiki/XfceApplications developed by the Xfce team are based on GTK and self-developed Xfce libraries. Other than Xfce itself, there are third-party programs which use the Xfce libraries. Xfce provides a development framework which contains the following components:
releng:4.12:roadmap:gtk3 [Xfce Wiki]
https://wiki.xfce.org/releng/4.12/roadmap/gtk3Eventually Xfce has to work with 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: Allow building parallel GTK+ 2 and 3 libraries for libxfce4ui and exo. Core components can optionally compile against GTK+ 3 ...