`shouldCollapseRail` was keyed on `!railExpanded`, mirroring the expand
helper instead of negating it. Once a hover revealed the button it could
never collapse back into the rail, so the collapse looked broken after the
first restore.
Key the predicate on `railExpanded` being true, restore the mirror
relationship with `shouldExpandRail`, and add regression coverage for the
expand -> leave -> collapse round trip. Also lengthen the rail strip.