From Xojo Documentation
Design Property (as Boolean)
| This property is only available on the macOS platform.
For cross-platform development, use ＃If...＃Endif with the Target... specifiers to make sure you will not attempt to use this property on an incompatible platform.
If True, decreases the vertical dimension of the tab panel on North/South tabs (macOS only). This property can be set only in the IDE.