From Xojo Documentation
Determines whether the control is transparent on Windows. The default is False. Has no effect on macOS or Linux.
Transparent controls draw more slowly and use more memory in order to cache the background. Unless you absolutely require transparency, leave this set to False.
For projects that were created prior to 2018r1, this property defaults to True to emulate previous behavior. Consider changing it to False to improve performance if you do not require transparency.