Focus trap in activity should stop trapping when activity no longer has tabbable elements

[compulim]

Screenshots

This screenshot shows document.activeElement pointing to a sentinel (a <FocusRedirector> created by the <FocusTrap>).

Version

4.15.1

Describe the bug

After the message content no longer contains any tabbable elements (say, card is disabled after submission), pressing TAB key should send the focus back to the chat history.

Steps to reproduce

1. Navigate to https://microsoft.github.io/BotFramework-WebChat/05.custom-components/l.disable-adaptive-cards/index.html
2. Use keyboard navigation keys to click on the "United States" button
3. After the result is submitted, press TAB key and observe the focus ring is gone

Expected behavior

As both "United States" and "Hong Kong" button is disabled, pressing TAB key should send the focus back to the chat history.

Additional context

When the button is disabled, we must not send the focus elsewhere or blur. This is for accessibility reasons. Say, when a button is disabled, the focus should stay on the button until the user moved it away.

When disabling a button, currently, we are leaving the button enabled until the focus is moved away. Then, we set the button as disabled. We could do it better by setting the tabindex on the button as -1. Then, we will not need to observe the blur event on the button. We just need to leave the tabindex="-1" on the element.

[Bug]