The SVOS retheme makes .rail load-bearing in two files owned by two different
agents: this template's grid-cursor start, and base.html's --rail-h measuring
script that publishes the rail's height for scroll-margin-top (the rail wraps,
so no CSS number can know it).
Neither breaks loudly if it is renamed. Ours starts the cursor one tile too
high; theirs falls back to a fixed guess. design-dev's sheet carries the mirror
of this note above the .rail rule, so the coupling is documented from both ends
rather than from whichever side happened to notice.