Run a localhost server on port 8000, you can run one using
python3 -m http.server 8000
Put a file named logo.png in the same directory as where you started the server
Once you get the permission prompt, wait for 5 seconds. The iframe will navigate away.
The test is to make sure that on giving permission, the page reloads correctly and you see the subresource now.
Expected result
This page will try to load an image named logo.png in an iframe hosted on your localhost server running on port 8000.
You should get a localhost permission prompt, and blocking should cause it to not load, and allowing should make it appear.