Don't update order when not enough stock

There's a user-facing change here. When you tried to update the quantity
of a line item beyond available stock, two things used to happen:

1. A warning was displayed.
2. The item's quantity was updated to the highest possible.

Unfortunately, the logic to update the line item was also reloading the
page and the warning message disappeared before it could be
acknowledged. The easiest fix was to skip the update request. And in my
opinion, it's even better to let the user decide if they still want to
update or cancel the update.

Eventually, we want to replace all this custom Javascript logic with
StimulusJS anyway. So let's not put too much effort into this. It was
important though to resolve the flaky spec which made many builds fail.

app/assets/javascripts/admin/spree/orders/variant_autocomplete.js.erb

@@ -51,10 +51,10 @@ $(document).ready(function() {
         quantity = maxQuantity;
         save.parents('tr').find('input.line_item_quantity').val(maxQuantity);
         ofnAlert(t("js.admin.orders.quantity_adjusted"));
+      } else {
+        adjustItems(shipment_number, variant_id, quantity, true);
       }
-      toggleItemEdit();
 
-      adjustItems(shipment_number, variant_id, quantity, true);
       return false;
     }
     $('a.save-item').click(handle_save_click);

spec/system/admin/order_spec.rb

@@ -339,34 +339,27 @@ describe '
     login_as_admin
     visit spree.edit_admin_order_path(order)
 
-    quantity = order.line_items.first.quantity
-    max_quantity = 0
+    item = order.line_items.first
+    quantity = item.quantity
+    max_quantity = quantity + item.variant.on_hand
     total = order.display_total
 
     within("tr.stock-item", text: order.products.first.name) do
       find("a.edit-item").click
       expect(page).to have_input(:quantity)
-      max_quantity = find("input[name='quantity']")["max"].to_i
       fill_in(:quantity, with: max_quantity + 1)
       find("a.save-item").click
     end
 
-    pending "Reload bug"
-    # The following modal is displayed but disappears straight away because the
-    # page is reloaded after the click on the save button. Somehow this happens
-    # only the first time the page is loaded and after that this logic seems to
-    # work fine.
-    sleep 1
     within(".modal") do
-      expect(page).to have_content "Quantity unchanged from previous amount"
+      expect(page).to have_content "Insufficient stock available"
       click_on "OK"
     end
 
-    expect(page).to_not have_content "Loading..."
     within("tr.stock-item", text: order.products.first.name) do
-      expect(page).to have_text(max_quantity.to_s)
+      expect(page).to have_field :quantity, with: max_quantity.to_s
     end
-    expect(order.reload.line_items.first.quantity).to eq(max_quantity)
+    expect { item.reload }.to_not change { item.quantity }
   end
 
   it "there are infinite items available (variant is on demand)" do