Thanks, got it working now!
In the end I sidestepped the collab extension, and used a system similar to the split view example (except sending the ChangeSets over the network), using the map function on the ChangeSets as necessary and using the EditorView.dispatch() function to apply the change sets.
However, I ran into a problem with exceptions from the view when testing with simulated network delay and several clients:
Uncaught DOMException: Failed to execute ‘collapse’ on ‘Selection’: The offset 74 is larger than the node’s length (16).
at eval (webpack-internal:///./node_modules/@codemirror/next/view/dist/index.js:3085:28)
at DOMObserver.ignore (webpack-internal:///./node_modules/@codemirror/next/view/dist/index.js:4269:20)
at DocView.updateSelection (webpack-internal:///./node_modules/@codemirror/next/view/dist/index.js:3066:32)
at DocView.update (webpack-internal:///./node_modules/@codemirror/next/view/dist/index.js:2945:18)
at EditorView.measure (webpack-internal:///./node_modules/@codemirror/next/view/dist/index.js:4783:30)
at EditorView.readMeasured (webpack-internal:///./node_modules/@codemirror/next/view/dist/index.js:4849:18)
at EditorView.posAtCoords (webpack-internal:///./node_modules/@codemirror/next/view/dist/index.js:4979:14)
at queryPos (webpack-internal:///./node_modules/@codemirror/next/view/dist/index.js:3963:20)
at Object.get (webpack-internal:///./node_modules/@codemirror/next/view/dist/index.js:3995:30)
at MouseSelection.select (webpack-internal:///./node_modules/@codemirror/next/view/dist/index.js:3793:36)
It seems to happen when doing a deletion at the same time as the document receives a deletion from the server (I was selecting a long text with the mouse and replacing the selection with a few letters, in rapid sequence on each client that had their network updates throttled to once every 5 seconds).
Am I just using the library wrong (perhaps I should use EditorView.update() instead of EditorView.dispatch() to apply changes?), or should I file a bug report for this?