Brackets close around the label instead of underlining it.
Two pseudo-elements, four pixels wide, with one border side dropped so each reads as a bracket. They slide in from twenty pixels out and fade at the same time. The label is untouched — no weight change, no underline, no reflow — so a dense nav bar stays perfectly still while the pointer moves through it.
The first link carries aria-current="page", so its marks stay closed permanently — the same visual doing double duty as the active-page indicator.