I believe there's a bug in the interaction between the Storefront API's new Cart object and Shopify's Script Editor scripts. The bug is that Line-Item scripts do not execute on Cart related mutations.
I can copy & paste GraphQL queries & responses all day. But the gist of it is, using the latest `v.2021-10` GraphQL endpoint, the published Shopify Script Editor Line-Item scripts properly execute on the `checkoutLineItemsAdd` mutation but does not execute whatsoever on the `cartLinesAdd` mutation. This statement of behavior is true for all the other mutations of those objects.
My Script Editor App's published Line-Item script is fully functional. It is, take my word for it. It's easy to write a simple script to verify this, do it.
I assume Shopify created the new Cart object to lighten the processing and database load related to the heavier Checkout object. There's a lot of customer `line_item` activity that never makes it to checkout so why should we burden the checkout infrastructure unnecessarily, right? So, once a customer gets their Cart items locked-in as desired, they then get promoted to Checkout items (eventually). But what if customers change their mind last minute and still want to change some checkout line_items, right? Well that's why keeping both implementations makes sense. So I get that the CartLine object is not a replacement for the CheckoutLineItem object. As a result, there are two places to edit `line_items` that are or will eventually be Checkout related `line_items`. We all understand.
"A Cart represents the merchandise that a buyer intends to purchase"
And maybe "intends" there means it operates more like a semi-functional Cart; a casual wish-list?
But I still can't really understand why it would make sense to not enact my cart `line_item` modifications executed by the Script Editor on that Cart object. For the customer, this is a very disorienting & misleading experience. Imagine the customer not experiencing the Script Editor `line_item` modifications while browsing the website and using the Cart... and then all of a sudden a rush of `line_item` changes unexpectedly appear when their `line_items` are promoted from Cart to Checkout.
As a result of this current behavior, whether it be a bug or not, I'm forced to continue using the Checkout object site-wide and ignore the new Cart object completely. That means the benefits Shopify intended to realize will not be realized. I don't think this is the intended effect Shopify was moving towards when they conceived of the Cart object concept.
Does this mean one of: