The top bar's bottom border and the sidebar logo band's bottom border draw
one horizontal line across the top of the app, and they sat a pixel apart.
Both declared h-16, so the number was never the problem. Every box in the
app is border-box: the top bar carried `h-16 border-b` on one element, which
puts the line inside the 4rem at 63 to 64 px, while the sidebar put `border-b`
on the wrapper and `h-16` on the link inside it, which puts the line after the
4rem at 64 to 65 px. Same number, different line.
The height and the border now travel together as SHELL_HEADER_BAND in
shell-metrics.ts, next to the sub-shell scroll floor that exists for the same
reason. The element carrying that class is the band; neither call site
restates a height, and the sidebar link stretches with h-full. Border colour
stays per-surface, because the two chrome surfaces are tinted apart by design
and only the geometry has to match.
Two guards. chrome-header-seam.test.tsx asserts the structural property, that
each band renders every token of the shared constant and declares no height of
its own; a second height class on either element replaces the shared one
through twMerge, so both halves fire on the same edit. The e2e spec measures
the painted result at 768, 900, 1024 and 1280 px in both the expanded sidebar
and the collapsed rail, and checks each band actually paints a border at the
same weight first, so two missing lines cannot pass as two aligned ones.
