No, that's incorrect. It's not more efficient at all, and even if it was it would still be less efficient to do it in the toolkit instead of in a separate shim. I explained this in detail in my other comment.
Also the testing and validation always needs to happen irrespective of where the shim layer is shipped, because this is still a new major release of a relatively large library. Having a shim for one widget out of many isn't going to meaningfully reduce the need for testing.
If you could mention what these programs are that are somehow completing the rest of their GTK4 ports in a small amount of time, refactoring everything to use the new improved widgets and rendering system at incredible speed, but are taking hundreds of thousands of man hours to replace the word HBox with Box, maybe someone can look at them and advise them on how to do that faster.
Also the testing and validation always needs to happen irrespective of where the shim layer is shipped, because this is still a new major release of a relatively large library. Having a shim for one widget out of many isn't going to meaningfully reduce the need for testing.
If you could mention what these programs are that are somehow completing the rest of their GTK4 ports in a small amount of time, refactoring everything to use the new improved widgets and rendering system at incredible speed, but are taking hundreds of thousands of man hours to replace the word HBox with Box, maybe someone can look at them and advise them on how to do that faster.