I’ve encountered this, but only on my laptop screen. It uses Windows DPI scaling of 175%. Wix (at least sometimes) set the size of #BACKGROUND_IMAGE inline to match the size of the viewport in pixels (overriding the 100% percentage width already set), but the scaling seems to introduce a rounding error whereby this value is off by one pixel. If this is your issue you can override their CSS and return to 100% width using the Tracking & Analytics feature and an !important selector.