サイトヘッダー（レスポンシブナビ）

/* このスニペット配下だけ box-sizing を揃える（既存のリセットとは競合しません） */
.site-header,
.site-header *,
.site-header *::before,
.site-header *::after {
  box-sizing: border-box;
}

.site-header {
  --header-height: 60px;
  --header-bg: #ffffff;
  --header-border: #e5e7eb;
  --header-fg: #111827;
  --header-muted: #6b7280;
  --header-accent: #2563eb;
  --header-breakpoint: 768px;

  position: sticky;
  top: 0;
  z-index: 50;
  background: var(--header-bg);
  border-bottom: 1px solid var(--header-border);
  color: var(--header-fg);
  font-size: 0.875rem;
}

.site-header__inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  max-width: 1152px;
  height: var(--header-height);
  margin: 0 auto;
  padding: 0 1rem;
}

.site-header__logo {
  flex: none;
  color: inherit;
  font-size: 1.125rem;
  font-weight: 700;
  text-decoration: none;
}

.site-header__list {
  display: flex;
  align-items: center;
  gap: 1.5rem;
  margin: 0;
  padding: 0;
  list-style: none;
}

.site-header__link {
  color: var(--header-muted);
  text-decoration: none;
  transition: color 0.15s;
}

.site-header__link:hover,
.site-header__link[aria-current="page"] {
  color: var(--header-fg);
}

.site-header__link[aria-current="page"] {
  font-weight: 600;
}

.site-header__cta {
  display: inline-block;
  padding: 0.5rem 1rem;
  border-radius: 6px;
  background: var(--header-accent);
  color: #ffffff;
  text-decoration: none;
  white-space: nowrap;
}

.site-header__cta:hover {
  opacity: 0.9;
}

.site-header a:focus-visible,
.site-header button:focus-visible {
  outline: 2px solid var(--header-accent);
  outline-offset: 3px;
  border-radius: 4px;
}

/* ハンバーガーボタン（広い画面では非表示） */
.site-header__toggle {
  display: none;
  align-items: center;
  gap: 0.5rem;
  padding: 0.5rem;
  border: 0;
  background: none;
  color: inherit;
  font: inherit;
  cursor: pointer;
}

.site-header__toggle-text {
  position: absolute;
  width: 1px;
  height: 1px;
  overflow: hidden;
  clip-path: inset(50%);
  white-space: nowrap;
}

/* 3本線。開いているときは×に変形する */
.site-header__bars,
.site-header__bars::before,
.site-header__bars::after {
  display: block;
  width: 20px;
  height: 2px;
  background: currentColor;
  transition: transform 0.2s, opacity 0.2s;
}

.site-header__bars::before,
.site-header__bars::after {
  content: "";
  position: absolute;
}

.site-header__bars::before {
  transform: translateY(-6px);
}

.site-header__bars::after {
  transform: translateY(6px);
}

.site-header--open .site-header__bars {
  background: transparent;
}

.site-header--open .site-header__bars::before {
  transform: rotate(45deg);
}

.site-header--open .site-header__bars::after {
  transform: rotate(-45deg);
}

@media (max-width: 767px) {
  .site-header__toggle {
    display: inline-flex;
    position: relative;
  }

  .site-header__nav {
    display: none;
    position: absolute;
    inset: var(--header-height) 0 auto;
    background: var(--header-bg);
    border-bottom: 1px solid var(--header-border);
    box-shadow: 0 8px 24px rgb(0 0 0 / 8%);
  }

  .site-header--open .site-header__nav {
    display: block;
  }

  .site-header__list {
    flex-direction: column;
    align-items: stretch;
    gap: 0;
    padding: 0.5rem 1rem 1rem;
  }

  .site-header__link {
    display: block;
    padding: 0.75rem 0;
    border-bottom: 1px solid var(--header-border);
  }

  .site-header__cta {
    margin-top: 0.75rem;
    text-align: center;
  }
}

@media (prefers-color-scheme: dark) {
  .site-header {
    --header-bg: #111827;
    --header-border: #374151;
    --header-fg: #f9fafb;
    --header-muted: #9ca3af;
    --header-accent: #3b82f6;
  }
}

/* ここから下はプレビュー表示用です。コピーして使うときは不要です */
body {
  margin: 0;
  background: #ffffff;
  font-family: system-ui, -apple-system, "Segoe UI", "Hiragino Sans",
    "Noto Sans JP", sans-serif;
  color: #111827;
}

@media (prefers-color-scheme: dark) {
  body {
    background: #111827;
    color: #f9fafb;
  }
}

const header = document.querySelector(".site-header");
const toggle = header.querySelector(".site-header__toggle");
const nav = header.querySelector(".site-header__nav");

function setOpen(open) {
  toggle.setAttribute("aria-expanded", String(open));
  header.classList.toggle("site-header--open", open);
}

toggle.addEventListener("click", function () {
  setOpen(toggle.getAttribute("aria-expanded") !== "true");
});

// メニュー内のリンクを押したら閉じる
nav.addEventListener("click", function (event) {
  if (event.target.closest("a")) setOpen(false);
});

// Escape で閉じてボタンにフォーカスを戻す
document.addEventListener("keydown", function (event) {
  if (event.key === "Escape" && toggle.getAttribute("aria-expanded") === "true") {
    setOpen(false);
    toggle.focus();
  }
});

// 広い画面に戻したときに開いた状態を持ち越さない
window.matchMedia("(min-width: 768px)").addEventListener("change", function (event) {
  if (event.matches) setOpen(false);
});