[A11Y] Navigation in background occurs while “Learn more”/“What is a web compatibility issue” popups are still displayed

[softvision-oana-arbuzov]

Browser / Version: Firefox Nightly 82.0a1 (2020-09-16), Chrome 85.0.4183.102 Operating System: Windows 10 Pro

Steps to Reproduce:

1. Navigate to https://webcompat.com/issues/new
2. Using "Tab" key, navigate to the URL field and fill it in.
3. Using "Tab" key, navigate to "Confirm" button.
4. Using "Tab" key, navigate to "Learn more about web compatibility" link.
5. Using "Tab" key, navigate to "Learn more" link.
6. Using "Enter" key, activate the popup.
7. Use "Tab" key a few times (4-5 times) and press "Enter".
8. Observe behavior.

Expected Behavior: Elements from the popup are navigated on (e.g "Privacy policy" button, "X" button) and if "Enter" is pressed the corresponding action is performed (redirect/close popup).

Actual Behavior: Nothing occurs on the popup, but in the background navigation to next elements is performed.

Notes:

1. Screenshot attached.
2. The issue is also reproducible for "What is a web compatibility issue?" popup.

Watchers: @softvision-oana-arbuzov @cipriansv