It probably came to rest on both the top and against some support....which would make it OOB.
If not, and it just came to rest on just the top, by rule, there is no violation and there is no AP (coming to rest on the flange is an AP since it would be basket interference to touch the ball, but not the top of the board). Time will expire in the quarter unless a player had enough hops to go knock it down or some gust of wind comes along to knock it down.
But, I think I'd apply rule 2-3 on this one to declare it a held ball. Or, you could blow your whistle, declare it an inadvertent whistle, then end up in a held ball just the same.
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