code: def tab_should_insert_whitespace():
      
      b = get_app().current_buffer
      before_cursor = b.document.current_line_before_cursor
  
      return bool(b.text and (not before_cursor or before_cursor.isspace()))
docs: When the 'tab' key is pressed with only whitespace character before the
      cursor, do autocompletion. Otherwise, insert indentation.
  
      Except for the first character at the first line. Then always do a
      completion. It doesn't make sense to start the first line with
      indentation.