IDK if it is also doing it when he merely shifts to another window??? He mentioned something about a dual-display, which pretty much says 'higher end video card'. I;ve no idea what GPU he has, but I believe he is using Win10.
I'm thinking that offering a doubleblit video mode, available as a menu option, whether in full screen, or windowed mode, might be our best solution for this. 'Doubleblit: Offers greater compatibility on newer OSes, but may be slower.'