void QTabBarPrivate::makeVisible(int index)
{
    ...
    if (available >= lastTabEnd) {
        // the entire tabbar fits, reset scroll
        scrollOffset = 0;
    } else if (tabStart < scrolledTabBarStart) {
        // Tab is outside on the left, so scroll left.
        scrollOffset = tabStart - scrollRect.left();
    } else if (tabEnd > scrolledTabBarEnd) {
        // Tab is outside on the right, so scroll right.
        scrollOffset = qMax(0, tabEnd - scrollRect.right());
    } else if (scrollOffset + entireScrollRect.width() > lastTabEnd + 1) {
        // fill any free space on the right without overshooting
        scrollOffset = qMax(0, lastTabEnd - entireScrollRect.width() + 1);
    }
    ...
}