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The GTK4 project was cancelled for multiple reasons. We originally began working on Relm4 to use GTK4 for COSMIC applets. While others on the team were also experimenting with alternative Rust GUI libraries.

It required a lot of effort to patch GTK4 to support the Wayland layer shell protocol. Getting those patches merged into GTK4 was also taking a much longer time. There were long delays between code reviews; and they also wanted a series of much larger refactoring changes to be made to GTK4 before exposing the layer shell feature. It was much easier to get layer-shell working with iced, as it is a much leaner and concise code base.

GTK does not support fractional scaling, which is something we want our applets to support on day one. This was one of our major concerns. A concern that didn't apply to iced.

It was also exceedingly difficult to create custom widgets with GTK in Rust. Even those of us with years of experience considered it to be unreasonably difficult. So it was not feasible to expect new hires on the team to be able to comfortably develop COSMIC components with it. In comparison, our team was able to develop custom widgets with iced with much less effort and with greater flexibility, so the demand for iced grew stronger.

At the end of the day, GTK is not a Rust toolkit, and its API is cumbersome to adapt to Rust. Use of GTK would always be a compromise that lessens the developer experience for COSMIC app and applet development. A compromise that would eventually require us to rewrite everything in a native Rust GUI library the moment it would become possible to do so.

Since we are developing a desktop environment from the ground up anyway, we decided that there would be much more value for our time if we contribute to the Rust ecosystem and utilize iced to make a fully featured GUI library for application development.