UiRail takes `collapsible`: while no panel on its side is open, each item
shows its icon and label as a pill, and once one opens the labels fold away
to the icon rail, making room for the panel. The tip is not shown while the
label is on screen; the button keeps its accessible name and the visible
label is hidden from assistive technology. The side panel rails turn it on.
Each rail is now placed by its screen-side edge, at the rail clearance, so a
rail that widens grows toward the middle of the screen; collapsed, it sits
6px outside its stack as before.