code: func (b *Button) SetBlurFunc(handler func(key tcell.Key)) *Button {
  	b.blur = handler
  	return b
  }
docs: // SetBlurFunc sets a handler which is called when the user leaves the button.
  // The callback function is provided with the key that was pressed, which is one
  // of the following:
  //
  //   - KeyEscape: Leaving the button with no specific direction.
  //   - KeyTab: Move to the next field.
  //   - KeyBacktab: Move to the previous field.