Before you can make changes on a sub-tab, you need to select it and bring its content in the foreground.

To select a different sub-tab on the Extra tab, one or more clicks are needed, depending of your current focus.

On the following screenshot, the focus is currently on the content of the Opening sub-tab.

Assuming you want to open the content of the Split sub-tab, click over the Split label. This brings in the focus the group of tabs, but not the Split sub-tab itself.

Now, click the Split tab label again. This brings the content of the Split sub-tab in the foreground.

To reselect the all sub-tabs, click on the empty space on the main tabs ribbon (right of the Extra tabs), and then click any of tabs.