Updates the tip based on user cursor movement.

def _cursor_position_changed(self):
        """ Updates the tip based on user cursor movement.
        """
        cursor = self._text_edit.textCursor()
        position = cursor.position()
        document = self._text_edit.document()
        char = to_text_string(document.characterAt(position - 1))
        if position <= self._start_position:
            self.hide()
        elif char == ')':
            pos, _ = self._find_parenthesis(position - 1, forward=False)
            if pos == -1:
                self.hide()