GTK 4 release plan. It is noted that it will take about a year to bring GTK 4 to its proper form (GTK 4 since summer 2016). Until the end of 2019, it is planned to prepare another experimental release of the GTK 3.9x series, then in the spring of 2020 the final test release of GTK 3.99 will be offered, including all the intended functionality. GTK 4 is expected to be released in early fall 2020, at the same time as GNOME 3.38.
There are five planned feature changes to be completed before the final release, including work on replacing fixed widgets with scalable views, a new API for animation and translation of effects and progress indicators to it, completion of the redesign of the pop-up menu system (development of ideas related to nested submenus and drop-down menus), replacing the old system of hot keys with event handlers, finalizing the new API for Drag&Drop operations.
Among the optional features that I would like to have time to add before the release of GTK 4 are the UI designer widget, improved top panel layouts, and a widget repository through which experimental widgets can be shipped without being integrated into the main GTK package. The development of tools for porting applications to GTK4 is also mentioned, for example, the preparation of appropriate variants of the GtkSourceView, vte and webkitgtk libraries, as well as the provision of platform support. For example, the OpenGL-based rendering system works well on Linux, while the Vulkan-based rendering system still needs some work. On Windows, the Cairo library is used for rendering, but an alternative implementation based on (layer for translating OpenGL ES calls to OpenGL, Direct3D 9/11, Desktop GL and Vulkan). For macOS, a fully functional rendering backend is not yet available.
Source: opennet.ru