Until the Web Locks API is universally available, a ServiceWorker is probably the best cross-platform mechanism for locking among contexts with the same origin. This is a proof-of-concept implementation of locking using a ServiceWorker.
See the ServiceWorker code and how it is invoked from a client (this would likely be abstracted into a library).
Locking requests are made from a client to the ServiceWorker using fetch with a special HTTP header providing the JSON request. The ServiceWorker holds the fetch request until it can be satisfied. A browser can stop the ServiceWorker at any time, including during an ongoing fetch, so clients must retry a request if rejected.
The global state of a ServiceWorker is reset whenever it is restarted, so lock state is persisted to IndexedDB.
Orphaned locks are detected by comparing the clientId of a lock holder to the active clients obtained by Clients.matchAll.
Each box on the demo page is an iframe context. New iframes can be added and existing ones removed.
Within each context you can request a shared or exclusive lock, or release an acquired lock. When a lock is requested, its type is highlighted in yellow while pending and in green once acquired.
If a context is removed while holding a lock, it will eventually be released for the next requester. This may take a few seconds.