As someone who's been programming for over 15 years, I've found that I agree with the Zen of Python here that usually explicit is better than implicit. Also, in my experience, when faced with an issue like this, the best overall results are gained by trying to help the programmer to understand exactly what the code is doing rather than hide those details away. I know it's common that beginner programmers want to avoid getting caught up in the gory details, but when they eventually do hit problems that knowledge really helps them find the cause and fix it.
Is running in the background important for most actions, or just a few special (but common) scenarios, like triggering a dialog? I've coded in a few different GUI toolkits, and usually event handlers are run on the main thread. Modal dialogs blocking the app is, in other toolkits, expected behavior, as often in those cases you need a response from the user before the app can continue. If you don't want that behavior there is usually a separate option for showing dialogs non-blocking, where you send in a dialog finished callback function or a delegate.