a cancellation message cannot interrupt a synchronous JavaScript loop already running in that worker. The worker must divide expensive work into chunks and yield between them, giving its event loop a chance to receive the cancellation message. setTimeout() is one way to schedule the next chunk; repeatedly awaiting Promise.resolve() is not an equivalent yield to incoming message events because microtasks run before the next task.

Thanks, the distinction between yielding to a new task and only flushing microtasks is especially helpful. For an image-processing example, measuring cancellation delay alongside total processing time would make the chunk-size tradeoff easier to see.