This mistake gets repeated over and over again, in every single GUI framework of any kind. I don't understand why does it have to be that way.

Just use multi-threading. Run each library in a separate thread. Use actors approach to pass data between libraries, application and finally to submit data to update GUI.

Using single thread for everything and expecting that developer will create separate threads for heavy work is obvious approach, but it never worked. Developer doesn't care. And user gets inferior experience.