This seems to be related to my making use of the “toolbarItems” property. That’s how I’m getting that “Submit” button to appear.
private func configureToolbar()
{
let leftSpacer = UIBarButtonItem(barButtonSystemItem: .flexibleSpace, target: nil, action: nil)
let submitButton = UIBarButtonItem(title: "Submit", style: .plain, target: self, action: #selector(submitButtonPressed))
let rightSpacer = UIBarButtonItem(barButtonSystemItem: .flexibleSpace, target: nil, action: nil)
self.toolbarItems = [leftSpacer, submitButton, rightSpacer]
}