= f.inputs t('title.line_items') do
  %table#line_items
    %thead
      = render 'line_items/list_header', :invoice => invoice
    %tbody
      = f.fields_for :line_items do |line_item|
        = render 'line_items/form', :line_item => line_item
    %tfoot
      = render 'line_items/list_footer', :total_amount => invoice.amount

= f.inputs t('title.line_items') do
  %table#line_items
    %thead
      = render 'line_items/list_header', :invoice => invoice
    %tbody.sortable
      = f.fields_for :line_items do |line_item|
        = render 'line_items/form', :line_item => line_item
    %tfoot
      = render 'line_items/list_footer', :total_amount => invoice.amount

Add .sortable to line_items list form.
Add .sortable to line_items list form.
