Arrows routed behind a node showed straight through the fill-none
boxes. The white (visited/current/monochrome) boxes now carry a 50%
opacity fill of the standard app background color (fill-background/50):
an edge passing behind composites UNDER the fill — dimmed to half, not
shown through — while the no-fill look survives on the background
surface. Upcoming (grayed) nodes keep their solid card fill.