I would have thought that A had similar structure to B, as in action#1 was interrupted by action#2:
A: Te ________ hasta que me aburrí y me fui.
I was waiting for you until I got bored and left
B: Ella estaba lavándose el pelo cuando él llegó.
She was washing her hair when he arrived.
But the answer to A was “estuve esperando” not “estaba esperando.”
Does it mean that in B the woman didn’t stop washing her hair even the man arrived, but in A the waiting totally completed?