// Contain-fit of an image into a parent width with a height cap. Pure and
// display-only; all measurement math stays on source pixels.
// Null when the parent is not laid out yet (clientWidth 0) — the caller must
// not finalize CSS size until it is, to avoid a wrong-aspect flash.
export function fitDisplay(
  parentW: number, imgW: number, imgH: number, maxH: number,
): { fitW: number; fitH: number } | null {
  if (parentW <= 0 || imgW <= 0 || imgH <= 0) return null;
  const fitW = parentW;
  let fitH = (imgH / imgW) * fitW;
  if (fitH > maxH) {
    // Cap height and shrink width to preserve aspect; never stretch.
    fitH = maxH;
    return { fitW: (fitH * imgW) / imgH, fitH };
  }
  return { fitW, fitH };
}