Hello Lilian! I just tested with this code (on Page Show event):
var navbar = $("[data-role='navbar']:visible");
if($('.tg-state-persist', navbar).length == 0) {
$(".ui-btn-active", navbar).addClass('tg-state-persist');
} else {
$(".ui-btn-active", navbar).removeClass('ui-btn-active');
$(".tg-state-persist", navbar).addClass('ui-btn-active');
}
it works. I think you didn't set selected tab (as Max wrote). Please select MobileNavBarItem which you want to be highlighted and in Properties check "Active" checkbox.