Cart item quantities not re-rendered when new values are passed down via the API

[manospsyx]

Describe the bug

One issue I attempted to tackle here, is this:

When custom code is used to keep the quantities of two (or more) items in sync, the quantities of "dependent" items fail to render correctly.

To reproduce

Here's some code you can use to play around with this state-related problem:

add_action( 'woocommerce_after_cart_item_quantity_update', function( $cart_item_key, $quantity = 0 ) {

	if ( isset( WC()->cart->is_syncing ) && WC()->cart->is_syncing ) {
		return;
	}

	if ( ! empty( WC()->cart->cart_contents[ $cart_item_key ] ) ) {

		$cart_item = WC()->cart->cart_contents[ $cart_item_key ];

		if ( ! $quantity || $quantity < 0 ) {
			$quantity = 0;
		} else {
			$quantity = $cart_item[ 'quantity' ];
		}

		if ( in_array( $cart_item[ 'data' ]->get_name(), array( 'Hoodie', 'Belt' ) ) ) {

			foreach ( WC()->cart->cart_contents as $search_item_key => $search_item ) {

				if ( 'Belt' === $search_item[ 'data' ]->get_name() ) {

					if ( 'Hoodie' !== $cart_item[ 'data' ]->get_name() ) {
						continue;
					}

				} elseif ( 'Hoodie' === $search_item[ 'data' ]->get_name() ) {

					if ( 'Belt' !== $cart_item[ 'data' ]->get_name() ) {
						continue;
					}

				} else {
					continue;
				}

				WC()->cart->is_syncing = true;
				WC()->cart->set_quantity( $search_item_key, $quantity, false );
				WC()->cart->is_syncing = false;
			}

		}
	}
}, 10, 2 );

What this code does, in a nutshell: Whenever you change the quantity of an item called "Hoodie", it searches for an item called "Belt", and tries to keep the quantity of the latter in sync with the former. Here's how this works in the legacy cart:

Screen.Recording.2021-11-23.at.05.15.18.p.m.mp4

And here's what's happening in the block cart:

Screen.Recording.2021-11-23.at.05.17.51.p.m.mp4

Note that I need to refresh to get the qty of the "synced" item to update, although it's coming back just fine through the API.
This is a roadblock that we'll need to overcome to make the new Cart/Checkout Blocks compatible with Force Sells, Product Bundles, Composite Products, Mix and Match Products, and likely other extensions that rely on the woocommerce_after_cart_item_quantity_update filter to keep cart item quantities in sync.

Expected behavior

Cart item quantities should always render the values passed down via the API.

Screenshots

See above.

Environment

WordPress (please complete the following information):

• Core version: [e.g. 5.8.2]
• WooCommerce version: [e.g. 5.9.0]
• WooCommerce Blocks version: add/change-quantity-with-steps