Never needed an extender, but never flew United before until a couple of months ago, no issues in the polaris seats coming to the US, but on the first class flights intercontinental I needed an extender on one flight and just managed close it on the second flight. I was quite surprised as I had lost 18 pounds since the start of covid and had no problems on Delta and American that I frequented before covid for intercontinental flights in the US. Big relief that it was the seat belts and not me despite my weight loss.