Root cause (found via a zoomed pixel-level screenshot check, not
guessed): the indicator span always exists in the DOM starting from
left:0/width:0/transparent, and the very first real position landed
via a CSS-transitioned change from that fallback rather than a snap —
so the underline visibly grew in from nothing over 140ms instead of
being there immediately, reading as entirely absent on a fast/slow
first paint alike depending on timing. Every subsequent navigation was
unaffected (always transitioning between two already-visible states).
Fixed with a one-render-only transition suppression
(indicatorSettledOnce, gated one tick behind the indicator's first
non-null commit) so the first placement snaps instead of animating in,
while every later hop still animates normally. Verified against the
exact repro: a fresh page load at the same short virtual-time-budget
that previously showed no underline now shows it immediately.
6d9a2fa9f6