White blip onclick

All-

I have the following code on a click event

window.open("intro.html", "_self");

when the page loads there's a white blip that happens

I have both of my background edge files set to black but that doesn't solve the problem

Both edge files are set to 100% Min W 1024 Max W 1200 Once the browser goes beyond 1200, i can see white

i could set up an animatoin with a white background before window.open but i'd like to understand where the white blip originates/background originates

thx,

Z