This issue seems to have been fixed in Opera 57. There is no longer a 1px line along the top of the window when maximised. However, O57 introduces Netflix ads in the Start Page news feed, and the extension recommendations (eg. Amazon, Aliexpress, ebay, etc.) are more annoying than ever. O52 is probably still the "golden version" of the new Opera.
Pick your poison: update to O57 or downgrade; recommended O52 or thereabouts - this will eliminated the 1px line.
