This cannot be reproduced under normal working conditions, on any of our working and testing machines, on any supported platform.
This is a machine-specific issue, probably caused by faulty graphics drivers. Unfortunately, many device drivers cannot be updated to valid versions anymore on Windows 7 because of lack of vendor support. Also note that Windows 7 is not one of our mainstream supported platforms for current versions of PixInsight (actually, this happened also with the previous 1.8.5 version).