reproduce

• define tabbox with 2 available tabs
• 2nd tab can be disabled or hidden via databinding, e.g. disabled="@load(vm.disabled[tab])"
• select 2nd tab (still available at that moment)
• send command, which disables 2nd tab via databinding
• tab gets disabled but tabbox is still visible

same with "visible" state. This is a bug, because disabled/invisible tabs should not be selectable!

A client-side workaround could look as following - but server-side handling would be probably better:
{{zk.afterLoad('zul.tab', function () {
new zul.tab.Tab(); // force init

var orgFunc = zul.tab.Tab.prototype.setDisabled;
zul.tab.Tab.prototype.setDisabled = function() {
orgFunc.apply(this, arguments);
if (this.isDisabled() && this.isSelected())

{ this.getTabbox().setSelectedIndex(0); }

}
});}}