tabCenter method
Center of tab index: the end tabs' pills sit at the padding, and the
others are evenly spaced between them.
Implementation
double tabCenter(int index) {
final double first = _padding.left + pillWidth / 2;
final double last = _size.width - _padding.right - pillWidth / 2;
if (_count == 1) {
return (first + last) / 2;
}
return first + index * (last - first) / (_count - 1);
}