Hey, junbear. Moved this over to the Mac support forums. This sounds a bit odd occurrence with the invisible window/mouse. If you try pressing CMD+M while Heroes of the Storm is open, does this help bring the invisible window?
There’s a few known issues with Catalina, if you are on the new OS build, we have a list of Mac issues here.
Hopefully it’s just another weird issue with the fullscreen mode and windowed mode will work. If it still happens, do let us know 