Tests for moving focus across details element. The existence of shadow tree on details element should not affect the focus sequential navigation ordering.
To manually test, press tab key five times and then shift+tab five times. It should traverse focusable elements in the increasing numerical order and then in the reverse order.

1. Focusable element with tabindex=1
2. Focusable element in details with tabindex=2
3. Focusable element in details with tabindex=3
4. Focusable element in summary with tabindex=4
5. Focusable element in details with tabindex=5
6. Focusable element in details with tabindex=6
5. Focusable element in details with tabindex=5
4. Focusable element in summary with tabindex=4
3. Focusable element in details with tabindex=3
2. Focusable element in details with tabindex=2
1. Focusable element with tabindex=1