If you resize a Window the new Edge generally can not keep up and has to draw black bars (as seen in the screenshot). On the current edge this area is rendered gray which is less jarring on most sites ( I assume here without proof that most pages are light). Even better than using a set color would be to try to asses the background color of the page and use that. I hope the Team could at least consider changing the default fill color to a light gray as it is in the current edge
