He hadn't told either Phoebe or Deidre about his daughter. The child wasn't precisely a secret. She simply didn't visit often. The public had seen fit to ignore her existence in the long ago scandal, and Calder supposed he’d simply become used to being about Meggie.
(Celeste Bradley, The Duke Next Door)
The word fit seems like a noun functioning as the complement of seen. Dictionaries say that fit takes an article when used as a noun, so I'm not sure if the word is really used as a noun in the above example.
Is it a noun?