Of course, and that's why I asked if GTK+3 is officially supported and tested only with Mutter. It's not a big surprise, and I'm offended or anything, but this resulted in silence.

The issues above and what you mentioned are why Qt is so popular. It may not be 100% native, and neither is GTK+3, but their macOS and Windows support is good enough that even big commercial projects use it. If you look at it from the issues you outlined, we should be happy that Qt has a commercial business model and therefore allocates development resources to those.

With all of that being said, let's please not forget that the GNOME developers have been refactoring GTK+ again and are improving the "million lines of C", so I get what you're saying and feel their pain. However, it's fair to report in a friendly way and express our frustration as to regressions introduced in 3.x and still not fixed after years of it being stable.