Manifest V3 Extensions

An MV3 background script is not a long-lived page — it is a service worker the browser tears down whenever it looks idle. Almost every bug below is the same bug in different clothes: work that was started but never awaited, or state that was written from a stale snapshot. Both fail silently. There is no crash, no console error in front of the user — just a setting that reverts, a button that does nothing, or a suite that stays green over a broken handler.

Await every side effect before the handler resolves

A fire-and-forget chrome.storage.local.set can be killed with the worker. If a message handler's caller depends on that write, the write must complete before the response is sent.

MV3's onMessage closes the reply channel as soon as the listener returns unless it returns literal true: