PrintWindow and BitBlt of hidden windows

Question

My program is taking screenshots of other application windows to automate some tasks on them. Those windows can be hidden offscreen or obscured by other windows from time to

Answer 1

After more than two months I finally realized that this only occurs with the Windows update 1809 from late 2018. Apparently, Windows changed the way it handles clipping with that update so that the Device Context contents are no longer updated for parts of windows that are located offscreen.

To answer the individual questions:

1) GdiFlush doesn't seem to make a difference, at least from what I can tell so far.

2) Still not 100% sure about it, but I also think it doesn't make a difference.

3) Still no idea.

4) See answer above.