The deep-link highlight tint faded out over 700ms on the comment body
layers but the sticky header's background switched instantly, and its
4px bottom `after` gradient band recolored by class-switching that
`transition-colors` cannot animate. Both desynced from the body during
the fade, showing a white header and a pale seam under it.
Add `transition-colors duration-700` to the sticky shell so the header
background fades with the body, and make the `after` band derive its
color from the header via `bg-[inherit]` + a `mask-image` fade instead
of a per-state gradient color, so all three layers are driven by the
single header background transition.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>