It seems window messages are being blocked, is this still working for you? It did for me just a couple of weeks ago.

A few weeks ago I recommended to stop the propagation of consumed message events to increase Observable's security. Looks like they've now implemented this change. Thanks for letting me know! Sending the notebook off to the graveyard now. :)

Understandable! I am now using a Mutation.observer to see whether cells were added/removed to the sandbox, but was wondering if a native editor cue could be intercepted to trigger dependent cells to update.

If you don't mind using hacks, have a look at https://github.com/observablehq/runtime to access the runtime instance. Other than that MutationObserver is certainly a good choice, but be aware that moved elements will be recorded as remove + add.