I just boarded a CRJ-700 where my seat was 2A. Stepping onto the aircraft, I saw one space left in the overhead. It was above 1CD. I stowed my bag in that spot and sat down.

The passenger in seat 1A began to stare me down. I noticed she had a bag in her hand, “waiting” for the spot which I had just taken. She must have boarded well before me. I would guess she needed to grab something from her bag before stowing it. While I sympathize with her plight as I’ve been in her exact situation, I had understood overhead space to be first-come, first-serve within the same class of service.

What should I have said or done?

What I ended up saying or doing was nothing. After intensely staring at me, 1A asked a flight attendant what to do about her bag. The flight attendant squeezed her bag right beside mine in the 1CD overhead and closed it.