Description
If you use rollup-plugin-off-main-thread while bundling a service worker script, and one of the files that is pulled in via importScripts() inside of a Promise ends up calling addEventListener('fetch', ...), it triggers a Fetch event handlers must be added during the worker script’s initial evaluation error message in Firefox 74.
You can reproduce this by clearing out previous service worker registrations and then visiting https://proxx.app/ or https://jeffy.info/ in Firefox.
This was previously discussed at GoogleChromeLabs/proxx#470 (in which @jakearchibald said it shouldn't be an issue), but recently came up again in Workbox's issue tracker at GoogleChrome/workbox#2448
As far as I can tell, this error message doesn't affect functionality (i.e. the fetch handlers added via the imported scripts still appear to work), so this might just be inaccurate error reporting in Firefox.
Even if this behavior is technically allowed by the SW spec, I wonder if it's possible for rollup-plugin-off-main-thread to ship an optional loader template that's tailored to the SW importScripts() use case, in which the entire import flow isn't wrapped in a Promise? I've tried making that change myself in a custom template, but never got very far, because I wasn't sure how it would fit in with the require + registry logic. I feel like it might be the sort of thing that would take me days to figure out, and take someone familiar with the codebase significantly less time.