From Xojo Documentation
| This property is only available on the macOS platform.
For cross-platform compatibility, use ＃If...＃Endif with the Target... specifiers to make sure you will not attempt to use this property on an incompatible platform.
Controls the appearance of the SegmentedButton on macOS.
Use MacButtonStyles to set/get this property.
Not all button styles are available on all versions of macOS.
This code is in the Initialized event of the control but will only take effect when building for macOS. It sets the style to Rounded.