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There are a few solutions for preventing the UI (User Interface) from waiting for a thread to finish in Python Flask, depending on the specific use case and requirements.

1. Using a message queue: Instead of directly calling the time-consuming task in the Flask view function, you can use a message queue system, such as Celery, to handle the task asynchronously. You can then send a message to the task queue and immediately return a response to the user, without waiting for the task to finish. The task will be executed in the background, and the user can check the status later if needed.

2. Using a background thread: If you want to perform a task in the background without using a message queue, you can use a background thread to execute the task asynchronously. You can create a separate thread to run the task and return a response immediately to the user. However, you need to be careful about thread safety issues, such as avoiding access to shared resources or variables across threads.

3. Using a web worker: In some cases, you may want to perform a long-running task on the client-side, such as processing a large file upload. You can use a web worker to run the task asynchronously in the background without blocking the UI. The web worker can communicate with the Flask server using AJAX requests to retrieve or update the task status.

Overall, the best solution depends on the specific requirements and constraints of your application. However, using a message queue or a background thread is generally a safer and more scalable approach than using a web worker, which may require more complex coding and debugging.