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UI libraries in general have always been single-threaded. This means that you can access the UI controls only from the thread that created it (thread-affinity). When running long-running operations you would typically use a background thread to do that job. However progress needs to be reported on the UI and the user needs to have a way to cancel the operation. Doing that from a background thread is not possible and hence you have to jump threads, ie. from the background-thread to the ui-thread.