Tech repaired Samsung Smart TV this week for powering off issue. Regarding the shadow on the top 1/4 of the screen, he agreed with the source of the problem being the Xfinity HD box. This because the shadow does NOT appear when streaming (e.g. Netflix; Amazon, etc.) OR a DVD from the Blu-ray. He suggested requesting a new box. (But I won't since I"m ditching Xfinity and going to YouTube TV)