Tabs control: tabpanel needs to be in tab order if there's no focusable tabpanel content

[carmacleod]

Steps:

• Open the Tabs control example
• Type the Tab key - focus moves to "Tab Label 2"
• Type the Tab key again - focus should move into the tabpanel (i.e. the div containing "Content for second tab...") but instead, it goes to the browser's location bar.

The APG Guidance for Tabs component says that if focus is in the tablist and the user types the Tab key:

... move focus to the next element in the page tab sequence outside the tablist, which is the tabpanel unless the first element containing meaningful content inside the tabpanel is focusable.

The Carbon Tabs component doesn't need to know what focusable element inside the tabpanel contains meaningful content. :)
It only needs to know whether or not any of the tabpanel content is focusable. If none are, then the tabpanel itself needs to be focusable (i.e. have tabindex="0").

Here's how one Tabs component decided on a prop for this: point 2 in juanca/react-aria-components#6
based on this comment in issue w3c/aria-practices#323 (comment).

Happy to talk this through before anything is coded.

[dakahn]

Wow, that's a really good catch. The use case you describe in w3c/aria-practices#323 (comment) is totally reasonable. Thank you so much for bringing this to our attention!