Fluentui: FocusTrapZone's trapping elements need to be hidden

[phkuo]

The focus trap zone uses focusable <div>s to trap focus (called "bumpers"). Because these are tab stops, the screen reader ascribes special behaviors to them. But they aren't actually meant to be noticed by the user. Therefore, they should have aria-hidden="true" added to them.

https://github.com/microsoft/fluentui/blob/e7b119a05ef8a4aa254df00320f3672298617ade/packages/office-ui-fabric-react/src/components/FocusTrapZone/FocusTrapZone.tsx#L113

[paulgildea]

@smhigley - Thoughts on the right a11y behavior here?

@phkuo You willing to submit a PR 😄 ?

[xugao]

Agreed. @phkuo - feel free to shoot a PR :)

[msft-github-bot]

:tada:This issue was addressed in #14376, which has now been successfully released as [email protected].:tada:

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