The sticky .sidebar-header had two layered issues that let nav items
show through it while the user scrolled the sidebar:
- its background was a single linear-gradient of rgba() stops,
starting at ~14% accent on transparent — the upper portion of the
header was effectively translucent
- .sidebar > * sets z-index 1 on every sidebar child, so the header
and the nav buttons share a stacking level. Sticky alone doesn't
lift the header; with equal z-index the nav wins on DOM order
Layer the existing accent gradient over a solid rgb(18, 18, 18) base
(visual unchanged, fully opaque), and bump the header to z-index 2 so
it paints above the nav buttons as they scroll under it.
1b3751598d