Hi Ruediger,
Admittedly, we still have to do a lot of work to support multiple monitor setups correctly on all platforms, especially on Windows and macOS. Right now we are concentrating all of our efforts to evolve the PixInsight platform in several critical aspects, including image processing tasks and astronomical applications where we must achieve the best possible implementations, as well as other fields where we must prevent the risk of becoming less competitive. Unfortunately, our resources are very limited, and we cannot cover everything. Necessarily, some corner cases must remain uncovered for now, since we simply cannot afford to invest the required time and resources to work on them as necessary. I'll try to improve on the situation you are describing in a future version. Sorry for the inconvenience, and thank you for pushing us.