Clean GUI C++ libraries like this always make me think that Microsoft deliberately chose to not implement such a nice C++ GUI API for Windows and instead gave us the horrible MFC in order to lock us in into Visual Basic and other proprietary projects.
If Microsoft had given us a clean C++ API for Windows, the world of programming would be very different today.
I mean, Win32 was a relatively clean C library. Not beautiful by any means, but it worked and was well documented. At the time it was huge, but it's pretty irrelevant today. If they did it in C++ instead of C, how could it have been different.
It worked and it was well documented, but that does not mean its design is very nice. To me, that is.
I guess the latter is the case for SerenityOS and other OSes with integrated GUIs.