If I understand you, I think it's a bit much to expect ScalaJS to somehow change the platform it is targeting, which is the browsers as accessed via Javascript (ie. without requiring plugins which would have made it DOA for most use cases). If browsers don't want threads that share data via Javascript control, and it appears they don't, and by design, then what is the alternative for ScalaJS? Same situation as with Javascript.

Mozilla is working on a type of array that can be shared.

The DOM is your structurally shared data. You just need to find a way to represent your data as (hidden) DOM nodes.

That actually doesn't work in this case, because Web Workers can't access the DOM.