Function traverseHiddenHeaders

traverseHiddenHeaders(
    hiddenAt: HiddenHeadersAt | undefined,
    index: number,
    direction: Direction,
    limit?: number,
): number
Scans from index in direction until it lands on a visible (non-hidden) header.
Parameters
- hiddenAt: HiddenHeadersAt | undefined
  Hidden-header port. When omitted, index is returned unchanged.
- index: number
  The index to start from.
- direction: Direction
  The direction to scan.
- Optionallimit: number
  The furthest visible, in-bounds header the scan may reach (inclusive). When every header from index to the grid edge is hidden, the scan would otherwise march off the grid and return an out-of-range index; instead it stops and returns limit. Omit to scan unbounded.
Returns number

Remarks
Only safe when the caller guarantees a visible header exists ahead.