Red windows on unused desktops

I like to use as many desktops as possible when I do my homework, I'll keep iTunes playing on one; a CAS running in another. I do this for hours, and when ever I return back to my least frequently used desktop, usually the one providing sound or one with an old google searched queried, the window is bright red and unresponsive for about 2-3 seconds.

does anyone know what/why/how this is; & if its benign or even fixable?

I run Sierra 10.12.6 on a 2017 MacBook Pro 15 inch