Setting active tab

Silverstripe version: 4.13

I am extending LeftAndMain to add a custom subscriber management form. I’m creating instances of GridField and then adding them one Tab at a time to a TabSet, but when I load the form, the final (third) tab is always selected. Is there a way to make sure the first tab is selected instead?

$fields = FieldList::create(
    $root = TabSet::create(

More context, I’m basing this off SecurityAdmin and using the template SilverStripe\Forms\CMSTabSet to add these tabs to the top nav. If I go to /admin/subscribers with no trailing slash it works fine, but with the trailing slash, the first tab is no longer selected, so this may be an URL pattern issue.

I think I’ve just run into a bug of some kind. If I just have 2 tabs, the first is selected as expected, but if I have more than 2, the 3rd one is always selected.

OK, I figured it out. For anyone else who encounters this, I changed:
$form->addExtraClass('ss-tabset cms-tabset-nav ' . $this->BaseCSSClasses());

$form->addExtraClass('ss-tabset cms-tabset-nav-primary ' . $this->BaseCSSClasses());