There’s an absolutely brilliant hack in here.
Part of the goal is to allow existing poorly written third-party scripts - analytics stuff for example - to run in a sandbox in a web worker rather than jamming up the main thread.
Problem is, those scripts need to access the page DOM API.
Partytown uses a devious trick where a proxy object provides getters and setters which then make blocking API calls to a separate service worker, using the mostly-forgotten xhr.open(…, false) parameter.
It’s so clever!