GTK 3.99.1 Released As The Latest Development Step Towards GTK4
GTK 3.99.1 ties up "a number of loose ends" with their API ahead of the GTK 4.0 milestone, including a simplification of the button class hierarchy. There has also been a lot of work going into gtk4-demo as the demo collection for showing off the various toolkit widgets. Included are also a number of new demos.
Rounding out GTK 3.99.1 is a lot of bug fixing as well as some performance optimization work. One of the most exciting is fixing a long-standing issue with their OpenGL renderer that caused wrong clipping.
The GNOME developers working on GTK4 are comfortable in encouraging other developers now to begin porting their software to the GTK 4.0 interfaces as well as providing feedback on the API.
GTK 4.0 development isn't over yet with at least another development release still planned for introducing their at-spi back-end for new accessibility interfaces.
More details on the GTK 3.99.1 release via GTK.org.