While I have no question it was onside, I personally get a bit irked when fans insist on calling stuff like that a disallowed goal... it wasn't.
If someone scores THEN a flag goes up and a decision made, then it's disallowed. When the flag goes up, the whistle goes, and one player continues to play for several seconds then scores against no opposition under no pressure, it's not a disallowed goal, it's a guy who wants to put the ball in the net to make a point. Doesn't mean he would have anyway, Hart came out but made no real effort, may have fouled him, remember Giggs against Arsenal, similar position, beat the keeper, then put it over the bar(i still love that moment both as an Arsenal and football fan, it was just hilarious).
Much like the Arsenal fans saying but for several offside calls and disallowed goals we'd have scored more against City and saying Giroud had a disallowed goal. No, the whistle went, the keeper AND two defenders stopped playing, then Giroud turned and shot past 3 players who didn't try and stop the ball. It was not a disallowed goal, it wasn't a goal full stop.