It appears that life in the highchair is over. Elspeth is now completely refusing to eat while strapped into it; in fact she sits there yelling until I let her down. Now nothing will do but to sit in her very own chair at her very own (mismatched) table to eat her dinner.
She's been 18 months old for five days, and in those five days it seems like she's really made the transition from baby to toddler. It's been coming on for some time, but now I think it's complete. Wish me luck!