That's nothing to do with the SD screen. It's because there's a viewport cut into the graphics plane for the grid itself. At the edge of that viewport, you'll sometimes get a line like you're seeing (I think it depends on the color you choose) where the grid element, which is on the video plane, transitions to the graphics plane. It falls outside of the safe zones, though, so it's likely to be cut off by the overscan on most televisions. You'll notice, as you scroll left and right that line is also where the posters get clipped.
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