A bowler or a bonnet

Mrs Hyde had never taken drugs in her life, not even an aspirin. She had met Jekyll at the neighbour's party the day before and discussed the merits of hat wearing or not! A bowler, a boater or a bonnet? She said. No further proof was needed I could only stare at the evidence in front of me, noting the arched comma of an eyebrow and THAT hat. A bowler or a bonnet. Or what?