GTK+ 4.0 Likely Being Released In Spring Of 2019
Happening the past week was the GUADEC 2018 GNOME developer conference and now the GTK+ team has put out their notes from the planning and discussions that happened pertaining to the next major version of the tool-kit.
In case you missed the recent GTK news, a GTK+ 3.24 minor feature update release is being planned for this fall alongside GNOME 3.30. GTK+ 3.24 will serve as an interim release until GTK+ 4.0 is available and adopted.
When it comes to GTK4, among the items they are still working on are finishing up the drag-n-drop (DND) support, introducing GtkTopLevel and properly handling pop-overs, adding transformations, creating a shortcuts event controller, porting the GtkTextView to render nodes, ensuring the Cairo back-end is performing at least on-par of GTK3, porting various libraries to the new interfaces, and other miscellaneous work.
Items that still might arrive for GTK+ 4.0 but more than likely will be postponed until GTK+ 5.0 are constraint-based layout support, shader compiler and application-provided shaders support, and UI designer support. It was also brought up that while the GTK4 Windows support is turning out okay, the GTK4 macOS back-end with Quartz is in a "much worse state", but that might be improved with a proper continuous integration setup.
At this point they are looking to release GTK+ 4.0 officially ahead of GNOME 3.32 in the spring of 2019, but they will be firming up their plans still soon. More details over on the GTK.org blog.