The focus management system has been refactored. Tab/Shift+Tab now properly cycles focus between interactive elements, and the focus manager is attached to the root node rather than being a standalone entity. The keyboard dispatch system now handles Tab key for focus cycling when not consumed by other handlers.
Evidence
Focus management changes (search for "focusManager" and "focusNext")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.