I have been quiet for a while trying to fix the deceptively named mvderwin() function in libcurses. Going by the name you would imagine that if derwin() creates a sub-window with coordinates relative to the origin of the parent window then mvderwin() should move a sub-window with coordinates relative to the origin of the parent window. Except it doesn't. What mvderwin() really does is create a mapping of the specified portion of the parent window into the sub-window, the sub-window does not change location at all.
Fixing the mvderwin code mainly involved tweaking around the screen refresh code so that when it found a sub-window that had been changed using mvderwin() the refresh code copied the characters from the correct place in the parent window into the sub-window region. Working on the refresh code in libcurses is probably the most difficult thing to do as it is quite complex and easy to break in strange ways. Fortunately, I have the automated tests for curses to help pick up any problems which is exactly why I wrote the curses automated testing in the first place. Being able to run a test suite enables me to check I have not broken previously working code and also check the behaviour of my fixes to ensure they are outputing exactly what I think they should be. The latter is probably the hardest thing to do in curses testing. It is quite easy to make some changes that displays correctly but is doing things badly in an invisible way. Something like outputting blanks when it does not need to, just by looking it would be difficult to tell whereas the automated testing suite flags the output as unexpected right away.