%div.flex
  %div.columns.three.text-center.checkout-tab.selected
    %span
      = t("split_checkout.your_details")
  %div.columns.three.text-center.checkout-tab
    %span
      = t("split_checkout.payment_method")
  %div.columns.three.text-center.checkout-tab
    %span
      = t("split_checkout.order_summary")      

%div.flex
  %div.columns.three.text-center.checkout-tab{"class": ("selected" if current_page?(checkout_your_details_path))}
    %span
      = t("split_checkout.your_details")
  %div.columns.three.text-center.checkout-tab{"class": ("selected" if current_page?(checkout_payment_method_path))}
    %span
      = t("split_checkout.payment_method")
  %div.columns.three.text-center.checkout-tab{"class": ("selected" if current_page?(checkout_summary_path))}
    %span
      = t("split_checkout.order_summary")      

Add selected class on form tab depending on the current path
Add selected class on form tab depending on the current path

