From Xojo Documentation
| This item was deprecated in version 2019r2.
Please use PagePanel.SelectedPanelIndex as a replacement.
The number of the currently selected panel (page/tab). The first panel is number zero.
To change the panel (page/tab) that is displayed, set this value. This is also how you change the displayed panel for the TabPanel control, which is subclassed from PagePanel. Altering this value calls the PagePanel.Change event.