Cursor location in a form

I have a form with several fields (a,b,c) and a button.  The button should only be enabled when
the cursor is in field b.  How can I tell when the cursor is in that field?

builder newButtonFor: self
        action: #openLessonListEditor
        getEnabled: #hasSelectedCurrentLesson
        label: 'Edit current lesson'
        help: 'Edit current lesson'.    

I want #hasSelectedCurrentLesson to return true only when the
cursor is in field b.

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