Hi, I am encountering this problem in Jasper studio version 6.17 from quite a while.
Whenever I am opening more than one reports in my project and try to access the design tabs for more than one report, I almost always encounter a black window. I can use the source and preview windows comfortably, but the design window appears to be only black. The only fix I have come up until now is to close the application without closing the reports and then again open the application. With this, I can view the design windows for all reports.