Consider focusables with tabindex="-1" to be unreachable

[NickGoward]

Consider focusables with tabindex="-1" to be unreachable

Description

Updates LRUD to ignore focusables with tabindex="-1", but still focus any focusable elements within any parent with tabindex="-1".

Motivation and Context

Elements with a tabindex of -1 should only be focusable programatically and not be part of the navigational flow of the document. This is in-line with behaviour of tabbing order in a standard HTML document. See https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/tabindex

The primary driver for this change is the existence of devices which only announce aria-live regions if they have a tabindex, but these regions should not be focusable.

How Has This Been Tested?

New unreachable.html layout created in tests and tests written.
Manual testing with this layout.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.