Divorce changes the child dependency requirements. A custodial parent doesn’t have to provide half the support but still gets the deduction. And can give that deduction away to the non-custodial parent even if the NCP doesn’t provide more than half the support. Gotta love the IRS. My brother and ex wife each take one kid even though they are twins and spend exactly the same amount of time with each parent, and my brother probably provides a little more in $$.
I don’t know the answer to the original question, but I’m running it both ways for my daughter this year as she did provide very close to half (or more) of her own support. I’m sure we can justify it either way (that I provided half+ or that she is self supporting) to the IRS if needed. I’ve never heard of the IRS disputing that a full time college student isn’t a dependent.