Synchronous XHR deprecation warning in shop_events_listener.js (confirmed not theme/app related)

[NB1]

Short description of issue

Synchronous XHR deprecation warning in core Shopify script shop_events_listener.js

Link to Shopify Store

northbyone.com.au

Reproduction steps

1. Run any page of the store through Google PageSpeed Insights or Lighthouse (mobile or desktop)
2. Check the Best Practices section, “Uses deprecated APIs” row
3. Warning appears: “Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user’s experience.”
4. Source is shown as assets/shop_events_listener-[hash].js:1:6843, flagged as 1st party on the store domain
5. Confirmed via Chrome DevTools Network tab that this file loads from cdn.shopify.com/shopifycloud/shopify/assets/shop_events_listener-[hash].js, indicating it’s served from Shopify’s own platform CDN rather than theme or app code
6. Warning is present on every page tested (homepage, collection pages, product pages) and persists regardless of theme or app configuration

Additional info

Theme in use: Blum (SalesHunterThemes). Confirmed with theme vendor that this file is not part of their theme bundle.
Ruled out all installed apps as the source: systematically disabled every app embed (Klaviyo Onsite Javascript, Globo Pre-order, Google & YouTube Store widget, Online store chat/Inbox, Judge.me, Exchange credit banner/AfterShip Returns) one at a time and retested after each change. The warning persisted identically at the same file and line regardless of which apps were active or inactive, ruling out app-injected scripts as the cause.
This appears to be a recurring issue with this specific file. Found a related report from another merchant describing a different bug in the same shop_events_listener.js file (parseFormData deduplication issue): Shop_events_listener.js — add-to-cart analytics broken when same variant appears twice with different properties
Also seeing broader mobile performance issues on this store (PageSpeed Performance

What type of topic is this

Bug report

Upload screenshot(s) of issue

[Mateusz-Shopify]

Hi!

The synchronous request is triggered by https://sellup.herokuapp.com/upseller.js?shop=northbyone.myshopify.com:

I’m assuming this script comes from one of the apps. It may be the case that it’s embeeded directly in one of the theme files, which is why disabling different apps didn’t help.

Our listener just proxies those calls, that’s why it’s being blamed incorrectly.