I'll mention two others I find very hard. The compositor. Dealing with ancient Win32 API, with his semi intelligent windows where seemingly every window featur

I'll mention two others I find very hard. The compositor. Dealing with ancient Win32 API, with his semi intelligent windows where seemingly every window feature is in weird undocumented ways coupled. Also, the race condition when rendering, just resize any window and see it jiggle. We seem to have collectively given up on fixing it. Text Input. While not terrible difficult, so many features. From drag and drop, copy / paste, page up / down, CTRL up down left right, Home / End, wrapping. Text is by far the most complex GUI.