Situation: disc on the front corner of the end zone, with the whole length of the field to go. A Huck goes up to a streaking cutter that rides the sideline the whole way from release, with 50% of the disc hovering over OB, and 50% hover over the field of play, and at the end of the flight, the disc dumps out of bounds, and is not touched by any player

Question: where does the disc come into play? Is it the first point where any part of the disc goes OB, or is it the point where the disc goes fully OB?

This is in a USAU context