Tangential: I haven't done GUI work in a long long long time, but I discovered FLTK in 1998 (then pre-1.0), and came back to it again and again, last time in 2012, for the simple reason that it never flickered thanks to built-in double buffering - and every other toolkit, even in 2012, had cases where it did.
I don't know if it's still a problem these days, but if it s - GUI toolkit developers, please pay attention to it.