full width page needs resize

I use SiteOrigin for page content. When I make a row FullWidth and render the page, the row has a white padding on the left. When I resize the page to make it repaint, it paints full but all content areas start out with a left padding.

I’ve tried to rule out SiteOrigin by switching to the TwentyFifteen theme. Under that theme all contents areas are full width from the first paint.