When launched, the BIAB window prevent any other application to overdisplay a windows on the top. This could lead to blocking situation as follows.
If you launch the leadsheet window as a fullscreen and then try to access the Opt function of it, you block your PC since BIAB waits you click on the OK button of the OPT window but in the same time prevent the related Opt window to overlap over the leadsheet windwo. As a a result, your PC is blocked.
A minor consequence is that you can't access the OK button and have to reboot the all thing. Worse you may loose all the work you did before if by any chance, you hav'nt taken the precaution to save your work before.

Apart from that, I'm very happy with BIAB 2021that I use in conjunction with Cubase. This a very helpfull application.