Total rotation required to reach the new index.
Note that this is correct because the active indicator holds its rotation value when it gets to an index. So when the user clicks on Tab 2, the indicator rolls there and holds. Imagine that it rolls 180°, or pi radians, and then is followed up by a click on thie third tab, requiring yet another 180° or pi radians. This number indicates that to reach the third tab, a total rotation value of 2pi radians is required, but already stored is that the active indicator has rotated pi radians (or 180°).
double get radiansToRotate => newIndex * radiansPerSide * sidesPerListItem;