Round-4 regression, root-caused:
- The rail overlay (position:absolute) painted above the in-flow input
no matter the DOM order — CSS paints positioned elements after
in-flow ones — so the bar sliced across the disc ('disc behind the
bar').
- The wrapper's height was unpinned: the rail's top-1/2 centered
against a stretched box while the disc rode the input at the top —
vertical drift, which also exaggerated the disc's overhang and made
the original-size 16px disc read oversized.
Fix: the input is absolute inset-0 too (DOM order now governs — the
disc rides above the rail), and the wrapper is pinned to h-4 so rail
center and disc track share one box. Geometry otherwise unchanged from
round 4: rail spans the thumb-center range, 0/100 flush at the very
ends, solid ticks, no rim.