Scarlett eventually interrupted and drove the neighbor cat away by standing up on her hind legs and pawing vigorously at the glass.
The second time, the neighbor cat stayed nose-to-nose with Scarlett through the window for several minutes. They both just stared at each other peacefully before the Siamese eventually wandered away. Scarlett stared after it wistfully for about fifteen minutes.
It would seem that my kitties have made a friend!