see test code.
current scroller focus movement is different to previous movement without focus manager.
here is legacy focus behavior.
|state in viewport\behavior|press tab key|press arrow key|on focus|
|no focusable content |focus next object outside of scroller|move scroll bar|focus scroller itself.|
|focusable content with next focus object| if the next object is not last leaf of focus tree, focus next object of focusable content. otherwise, focus next object outside of scroller.|if the next object is at arrow direction, focus next object of focusable content. otherwise, move scroll bar.|focus the top content of focus tree.|