Part of the Microsoft April 2019 patch update appears to perform a "Desktop Update". The process never completes and the desktop is never accessible again until Fences is uninstalled (within another administrator account). I've seen some other weirdness in the past few months where icons and fence titles get highlighted when a window passes over them. I suspect that MS really isn't paying enough attention to update quality on W7, but this has never been their strong point with any product.
Windows 7 64 bit
NVidea Quadro 600 using 18.104.22.16883 (GeForce 377.83) driver