/* ==========================================================
   Da Yan Academic Homepage — Core Theme & Layout (Critical)
   Design Tokens, Base, Nav, Hero, Contact, Footer
   ========================================================== */

/* ---------- Design Tokens ---------- */
:root {
  /* Fonts — Mac-inspired typography system */
  --font-sans: -apple-system, BlinkMacSystemFont, "SF Pro Text", "SF Pro", "PingFang SC", "Hiragino Sans GB", "Inter", system-ui, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  --font-display: -apple-system, BlinkMacSystemFont, "SF Pro Display", "SF Pro", "PingFang SC", "Hiragino Sans GB", "Inter", system-ui, -apple-system-headline, sans-serif;
  --font-serif: -apple-system-ui-serif, "New York", "Songti SC", "PingFang SC", "Charter", "Bitstream Charter", Georgia, "Times New Roman", serif;
  --font-mono: ui-monospace, "SF Mono", "Menlo", Monaco, "Cascadia Code", "Source Code Pro", Consolas, monospace;

  /* Typography mapping */
  --font-body: var(--font-sans);
  --font-ui: var(--font-sans);
  --font-label: var(--font-sans);
  --font-meta: var(--font-sans);

  /* Light Theme — Apple Mac Inspired (Clean, Vibrant, Modern) */
  --bg: #F5F5F7;
  --bg-elevated: #FFFFFF;
  --bg-subtle: #EBECEF;
  --bg-subtle-hover: #E1E2E7;
  --bg-nav: rgba(245, 245, 247, 0.82);
  --featured-bg: #FCFAF6;
  --featured-border: #FF9500;
  --featured-badge-bg: rgba(255, 149, 0, 0.12);
  --featured-badge-text: #B25E00;

  --text: #1D1D1F;
  --text-soft: #515154;
  --text-faint: #86868B;

  --accent: #0066CC;
  --accent-strong: #004FB8;
  --accent-soft: rgba(0, 102, 204, 0.08);
  --accent-border: rgba(0, 102, 204, 0.22);

  --border: #E5E5EA;
  --border-strong: #D2D2D7;
  --border-light: #F2F2F7;

  --code-bg: #EFEFF2;
  --code-text: #1D1D1F;
  --code-border: #E5E5EA;

  --badge-bg: #0066CC;
  --badge-text: #FFFFFF;

  --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.04);
  --shadow: 0 2px 8px -1px rgba(0, 0, 0, 0.06), 0 1px 3px rgba(0, 0, 0, 0.03);
  --shadow-hover: 0 10px 26px -4px rgba(0, 0, 0, 0.09), 0 3px 8px rgba(0, 0, 0, 0.04);
  --shadow-popover: 0 16px 36px -4px rgba(0, 0, 0, 0.12), 0 4px 12px rgba(0, 0, 0, 0.05);

  --radius-sm: 6px;
  --radius: 10px;
  --radius-lg: 16px;
  --radius-full: 9999px;
  --maxw: 1200px;
}

/* Dark Theme — macOS Dark & Apple Night Accent */
[data-theme="dark"] {
  --bg: #161617;
  --bg-elevated: #1C1C1E;
  --bg-subtle: #2C2C2E;
  --bg-subtle-hover: #3A3A3C;
  --bg-nav: rgba(22, 22, 23, 0.82);
  --featured-bg: #1F1B14;
  --featured-border: #FF9F0A;
  --featured-badge-bg: rgba(255, 159, 10, 0.2);
  --featured-badge-text: #FFB340;

  --text: #F5F5F7;
  --text-soft: #A1A1A6;
  --text-faint: #76767B;

  --accent: #2997FF;
  --accent-strong: #64B5F6;
  --accent-soft: rgba(41, 151, 255, 0.14);
  --accent-border: rgba(41, 151, 255, 0.32);

  --border: rgba(255, 255, 255, 0.11);
  --border-strong: rgba(255, 255, 255, 0.20);
  --border-light: rgba(255, 255, 255, 0.06);

  --code-bg: #1C1C1E;
  --code-text: #F5F5F7;
  --code-border: rgba(255, 255, 255, 0.12);

  --badge-bg: #2997FF;
  --badge-text: #FFFFFF;

  --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.3);
  --shadow: 0 3px 10px -2px rgba(0, 0, 0, 0.5), 0 1px 3px rgba(0, 0, 0, 0.3);
  --shadow-hover: 0 10px 28px -4px rgba(0, 0, 0, 0.65), 0 2px 8px rgba(0, 0, 0, 0.4);
  --shadow-popover: 0 18px 40px -8px rgba(0, 0, 0, 0.75);
}

/* ---------- Base ---------- */
* {
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
  scroll-padding-top: 80px;
}

body {
  margin: 0;
  font-family: var(--font-body);
  font-size: 16px;
  line-height: 1.62;
  letter-spacing: -0.006em;
  color: var(--text);
  background: var(--bg);
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-rendering: optimizeLegibility;
  transition: background-color 0.25s ease, color 0.25s ease;
}

.container {
  max-width: var(--maxw);
  margin: 0 auto;
  padding: 0 24px;
}

a {
  color: var(--accent);
  text-decoration: none;
  transition: color 0.15s ease, text-decoration-color 0.15s ease;
}

a:hover {
  color: var(--accent-strong);
  text-decoration: underline;
  text-underline-offset: 3px;
}

img {
  max-width: 100%;
  display: block;
}

section[id] {
  scroll-margin-top: 80px;
}

::selection {
  background: var(--accent-soft);
  color: var(--accent-strong);
}

:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
  border-radius: var(--radius-sm);
}

/* Custom macOS-style scrollbar */
::-webkit-scrollbar {
  width: 8px;
  height: 8px;
}

::-webkit-scrollbar-track {
  background: transparent;
}

::-webkit-scrollbar-thumb {
  background: var(--border-strong);
  border-radius: 9999px;
  border: 2px solid transparent;
  background-clip: padding-box;
}

::-webkit-scrollbar-thumb:hover {
  background: var(--text-faint);
  background-clip: padding-box;
}

html {
  scrollbar-color: var(--border-strong) transparent;
  scrollbar-width: thin;
}

/* ---------- Skip link ---------- */
.skip-link {
  position: absolute;
  left: 16px;
  top: -60px;
  z-index: 200;
  padding: 8px 16px;
  border-radius: var(--radius);
  background: var(--accent);
  color: #fff;
  font-family: var(--font-ui);
  font-size: 0.88rem;
  font-weight: 500;
  text-decoration: none;
  transition: top 0.2s ease;
}

.skip-link:focus {
  top: 14px;
}

/* ---------- Navigation ---------- */
.site-nav {
  position: sticky;
  top: 0;
  z-index: 50;
  background: var(--bg-nav);
  -webkit-backdrop-filter: blur(20px) saturate(180%);
  backdrop-filter: blur(20px) saturate(180%);
  border-bottom: 1px solid var(--border);
  transition: background-color 0.25s ease, border-color 0.25s ease, box-shadow 0.25s ease;
}

.site-nav.scrolled {
  box-shadow: var(--shadow-sm);
  border-bottom-color: var(--border-strong);
}

.nav-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  height: 60px;
  gap: 16px;
}

.nav-brand {
  display: inline-flex;
  align-items: center;
  flex-shrink: 0;
  text-decoration: none !important;
}

.nav-brand-img {
  display: block;
  height: 36px;
  width: auto;
  transition: filter 0.25s ease, opacity 0.2s ease;
}

.nav-brand:hover .nav-brand-img {
  opacity: 0.85;
}

/* Invert signature ink in dark mode */
[data-theme="dark"] .nav-brand-img {
  filter: invert(0.92) brightness(1.2);
}

.nav-right {
  display: flex;
  align-items: center;
  gap: 8px;
}

.nav-links {
  display: flex;
  align-items: center;
  gap: 4px;
  margin-left: auto;
  margin-right: 8px;
}

.nav-links a {
  font-family: var(--font-ui);
  font-size: 0.92rem;
  font-weight: 500;
  color: var(--text-soft);
  padding: 6px 12px;
  border-radius: var(--radius);
  white-space: nowrap;
  text-decoration: none !important;
  transition: color 0.15s ease, background-color 0.15s ease;
}

.nav-links a:hover {
  color: var(--accent);
  background: var(--bg-subtle);
}

.nav-links a.active,
.nav-links a[aria-current="true"] {
  color: var(--accent);
  background: var(--accent-soft);
  font-weight: 600;
}

/* ---------- Theme Switcher Button ---------- */
.theme-toggle {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 36px;
  height: 36px;
  padding: 0;
  border: 1px solid var(--border);
  border-radius: var(--radius-full);
  background: var(--bg-elevated);
  color: var(--text-soft);
  cursor: pointer;
  flex-shrink: 0;
  transition: color 0.15s ease, border-color 0.15s ease, background-color 0.15s ease, transform 0.2s ease;
}

.theme-toggle:hover {
  color: var(--accent);
  border-color: var(--accent-border);
  background: var(--bg-subtle);
  transform: scale(1.05);
}

.theme-icon-sun,
.theme-icon-moon {
  width: 17px;
  height: 17px;
  stroke: currentColor;
  transition: transform 0.3s ease;
}

/* Light mode: show moon to switch to dark */
:root:not([data-theme="dark"]) .theme-icon-sun {
  display: none;
}
:root:not([data-theme="dark"]) .theme-icon-moon {
  display: block;
}

/* Dark mode: show sun to switch to light */
[data-theme="dark"] .theme-icon-moon {
  display: none;
}
[data-theme="dark"] .theme-icon-sun {
  display: block;
}

/* ---------- Hero Section ---------- */
.hero {
  padding: 48px 0 24px;
}

.hero-grid {
  display: grid;
  grid-template-columns: 1fr 180px;
  gap: 40px;
  align-items: start;
}

.hero-text {
  min-width: 0;
}

.hero-name {
  margin: 0 0 6px;
  font-family: var(--font-display);
  font-weight: 700;
  font-size: clamp(2.2rem, 5.5vw, 3.1rem);
  line-height: 1.12;
  letter-spacing: -0.028em;
  color: var(--text);
}

.hero-suffix {
  font-family: var(--font-sans);
  font-style: normal;
  font-weight: 400;
  color: var(--text-faint);
  font-size: 0.5em;
  letter-spacing: -0.01em;
  margin-left: 4px;
}

.hero-tagline {
  margin: 0 0 16px;
  font-family: var(--font-ui);
  font-size: 1.05rem;
  line-height: 1.45;
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 6px;
}

.tagline-role {
  color: var(--accent);
  font-weight: 600;
  letter-spacing: 0.01em;
}

.tagline-sep {
  color: var(--text-faint);
  font-weight: 400;
  user-select: none;
}

.tagline-affiliation {
  color: var(--text-soft);
  font-weight: 500;
  text-decoration: none !important;
  transition: color 0.15s ease;
}

.tagline-affiliation:hover {
  color: var(--accent);
  text-decoration: underline !important;
  text-underline-offset: 3px;
}

.hero-bio {
  font-size: 1.02rem;
  color: var(--text-soft);
}

.hero-bio p {
  margin: 0 0 12px;
}

.hero-bio a {
  color: var(--accent);
  font-weight: 500;
  text-decoration: underline;
  text-underline-offset: 3px;
}

.hero-bio ruby rt {
  font-size: 0.6em;
  color: var(--text-faint);
}

.hero-photo {
  display: flex;
  justify-content: flex-end;
  align-items: center;
}

.hero-photo-frame {
  position: relative;
  display: inline-block;
  width: 180px;
  aspect-ratio: 412 / 536;
  border-radius: 50%;
  overflow: hidden;
  border: 3px solid var(--bg-elevated);
  box-shadow: 0 0 0 1px var(--border), var(--shadow);
  background: var(--bg-subtle);
  transition: transform 0.3s ease, box-shadow 0.3s ease;
}

.hero-photo-frame::after {
  content: "";
  position: absolute;
  inset: 0;
  border-radius: 50%;
  box-shadow: inset 0 0 0 1px var(--border), inset 0 0 16px rgba(0, 0, 0, 0.1);
  pointer-events: none;
  transition: box-shadow 0.3s ease;
}

.hero-photo-frame:hover {
  transform: scale(1.025);
  box-shadow: 0 0 0 1px var(--accent-border), var(--shadow-hover);
}

.hero-photo-frame:hover::after {
  box-shadow: inset 0 0 0 1.5px var(--accent-border), inset 0 0 12px rgba(0, 0, 0, 0.05);
}

.hero-photo-frame img,
.hero-photo img {
  display: block;
  width: 100%;
  height: 100%;
  aspect-ratio: 412 / 536;
  object-fit: cover;
  object-position: top center;
  border-radius: 50%;
  transition: transform 0.35s ease;
}

.hero-photo-frame:hover img {
  transform: scale(1.04);
}

/* ---------- Hero Action Pills ---------- */
.hero-links {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 9px;
  margin: 22px 0 0;
}

.link-pill {
  display: inline-flex;
  align-items: center;
  gap: 7px;
  padding: 7px 15px;
  border: 1px solid var(--border);
  border-radius: var(--radius-full);
  background: var(--bg-elevated);
  color: var(--text);
  font-family: var(--font-ui);
  font-size: 0.88rem;
  font-weight: 500;
  line-height: 1.2;
  box-shadow: var(--shadow-sm);
  text-decoration: none !important;
  cursor: pointer;
  transition: border-color 0.15s ease, color 0.15s ease, background-color 0.15s ease, transform 0.15s ease, box-shadow 0.15s ease;
}

a.link-pill:hover,
summary.link-pill:hover,
button.link-pill:hover {
  border-color: var(--accent-border);
  color: var(--accent);
  background: var(--bg-subtle);
  transform: translateY(-1px);
  box-shadow: var(--shadow);
}

.link-pill .icon {
  color: var(--accent);
  font-size: 1.05em;
  flex-shrink: 0;
}

.icon {
  width: 1em;
  height: 1em;
  flex: none;
  vertical-align: -0.125em;
}

/* ---------- About Expandable Box ---------- */
.about-details {
  display: contents;
}

.about-details summary {
  list-style: none;
}

.about-details summary::-webkit-details-marker {
  display: none;
}

.about-body {
  width: 100%;
  flex-basis: 100%;
  margin-top: 14px;
  padding: 16px 20px;
  background: var(--bg-subtle);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  font-size: 0.96rem;
  line-height: 1.7;
  color: var(--text-soft);
  animation: fadeIn 0.2s ease-out;
}

.about-body a {
  color: var(--accent);
  text-decoration: underline;
  text-underline-offset: 3px;
}

.about-label {
  font-family: var(--font-ui);
  font-weight: 700;
  color: var(--text);
  margin-right: 4px;
}

@keyframes fadeIn {
  from {
    opacity: 0;
    transform: translateY(-4px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

/* ---------- CV Language Dropdown ---------- */
.cv-menu {
  position: relative;
  display: inline-block;
}

.cv-menu-panel {
  display: none;
  position: absolute;
  top: calc(100% + 6px);
  left: 0;
  z-index: 100;
  min-width: 140px;
  padding: 6px;
  background: var(--bg-elevated);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  box-shadow: var(--shadow-hover);
  flex-direction: column;
  gap: 2px;
  animation: fadeIn 0.15s ease-out;
}

.cv-menu-panel::before {
  content: "";
  position: absolute;
  top: -6px;
  left: 0;
  right: 0;
  height: 6px;
}

.cv-menu.open .cv-menu-panel {
  display: flex;
}

@media (hover: hover) and (pointer: fine) {
  .cv-menu:hover .cv-menu-panel {
    display: flex;
  }
}

.cv-chev {
  font-size: 0.7em;
  color: var(--text-faint);
  transition: transform 0.2s ease;
}

.cv-menu.open .cv-chev {
  transform: rotate(180deg);
}

@media (hover: hover) and (pointer: fine) {
  .cv-menu:hover .cv-chev {
    transform: rotate(180deg);
  }
}

.cv-menu-item {
  padding: 8px 12px;
  border-radius: var(--radius-sm);
  color: var(--text);
  font-family: var(--font-ui);
  font-size: 0.88rem;
  font-weight: 500;
  white-space: nowrap;
  text-decoration: none !important;
  transition: background-color 0.15s ease, color 0.15s ease;
}

.cv-menu-item:hover {
  background: var(--bg-subtle);
  color: var(--accent);
}

/* ---------- Sections ---------- */
.section {
  padding: 34px 0 10px;
}

/* Performance optimization: lazy-render off-screen sections */
#publications,
#talks,
#contact {
  content-visibility: auto;
  contain-intrinsic-size: 0 500px;
}

.section-heading {
  position: relative;
  margin: 0 0 20px;
  padding-bottom: 12px;
  font-family: var(--font-display);
  font-weight: 650;
  font-size: 1.75rem;
  letter-spacing: -0.022em;
  color: var(--text);
  border-bottom: 1px solid var(--border);
  text-wrap: balance;
}

.section-heading::after {
  content: "";
  position: absolute;
  left: 0;
  bottom: -1px;
  width: 48px;
  height: 2.5px;
  background: var(--accent);
  border-radius: 2px;
}

/* ---------- Contact Section (Compact Badges) ---------- */
.contact-links {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 8px;
  margin: 12px 0 0;
}

.contact-pill {
  display: inline-flex;
  align-items: center;
  gap: 7px;
  padding: 7px 14px;
  border: 1px solid var(--border);
  border-radius: var(--radius-full);
  background: var(--bg-elevated);
  color: var(--text-soft);
  font-family: var(--font-ui);
  font-size: 0.88rem;
  font-weight: 500;
  text-decoration: none !important;
  box-shadow: var(--shadow-sm);
  cursor: pointer;
  transition: all 0.15s ease;
}

.contact-pill:hover {
  color: var(--accent);
  border-color: var(--accent-border);
  background: var(--bg-subtle);
  transform: translateY(-1px);
  box-shadow: var(--shadow);
}

.contact-icon {
  width: 16px;
  height: 16px;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  color: var(--accent);
  flex-shrink: 0;
}

.contact-icon.mono {
  font-family: var(--font-ui);
  font-size: 0.76rem;
  font-weight: 700;
}

.contact-arrow {
  font-size: 0.8em;
  color: var(--text-faint);
  transition: transform 0.15s ease, color 0.15s ease;
}

.contact-pill:hover .contact-arrow {
  color: var(--accent);
  transform: translate(1px, -1px);
}

/* ---------- Site Footer ---------- */
.site-footer {
  margin: 48px 0 32px;
  padding-top: 20px;
  border-top: 1px solid var(--border);
  color: var(--text-faint);
  font-family: var(--font-ui);
  line-height: 1.6;
  font-size: 0.86rem;
}

.site-footer a {
  color: var(--text-soft);
}

.site-footer a:hover {
  color: var(--accent);
}

/* ---------- Back to Top Button ---------- */
.back-to-top {
  position: fixed;
  bottom: 24px;
  right: 24px;
  z-index: 90;
  width: 40px;
  height: 40px;
  border-radius: var(--radius-full);
  background: var(--bg-elevated);
  border: 1px solid var(--border);
  color: var(--text-soft);
  box-shadow: var(--shadow);
  display: flex;
  align-items: center;
  justify-content: center;
  opacity: 0;
  visibility: hidden;
  transform: translateY(8px);
  cursor: pointer;
  transition: opacity 0.2s ease, transform 0.2s ease, visibility 0.2s ease, color 0.15s ease, border-color 0.15s ease;
}

.back-to-top.visible {
  opacity: 1;
  visibility: visible;
  transform: translateY(0);
}

.back-to-top:hover {
  color: var(--accent);
  border-color: var(--accent-border);
  transform: translateY(-2px);
  box-shadow: var(--shadow-hover);
}

/* ---------- Scroll Reveal Animations ---------- */
.reveal {
  opacity: 0;
  transition: opacity 0.4s ease, transform 0.4s ease;
}

.reveal.in {
  opacity: 1;
}

.section-heading.reveal,
.year-label.reveal {
  transform: translateY(10px);
}

.section-heading.reveal.in,
.year-label.reveal.in {
  transform: none;
}

.pub-card.reveal,
.contact-card.reveal {
  transform: translateY(12px);
}

.pub-card.reveal.in,
.contact-card.reveal.in {
  transform: none;
}

/* ==========================================================
   Responsive Mobile Optimization (Core)
   ========================================================== */
@media (max-width: 680px) {
  .container {
    padding: 0 16px;
  }

  .nav-inner {
    display: flex;
    flex-wrap: wrap;
    align-items: center;
    justify-content: space-between;
    height: auto;
    padding: 10px 0 8px;
    gap: 8px 0;
  }

  .nav-brand {
    order: 1;
  }

  .nav-brand-img {
    height: 30px;
  }

  .nav-right {
    order: 2;
  }

  .nav-links {
    order: 3;
    width: 100%;
    margin: 4px 0 0;
    overflow-x: auto;
    gap: 4px;
    padding-bottom: 2px;
    scrollbar-width: none;
    -webkit-overflow-scrolling: touch;
  }

  .nav-links::-webkit-scrollbar {
    display: none;
  }

  .nav-links a {
    font-size: 0.85rem;
    padding: 5px 10px;
  }

  .hero {
    padding: 24px 0 16px;
  }

  .hero-grid {
    grid-template-columns: 1fr;
    gap: 20px;
  }

  .hero-photo {
    order: -1;
    justify-content: center;
  }

  .hero-photo-frame,
  .hero-photo img {
    width: 140px;
    height: auto;
    aspect-ratio: 412 / 536;
    object-fit: cover;
    object-position: top center;
    border-radius: 50%;
  }

  .hero-name {
    font-size: 2.1rem;
    text-align: center;
  }

  .hero-tagline {
    justify-content: center;
    text-align: center;
    font-size: 0.96rem;
    gap: 5px;
    margin-bottom: 14px;
  }

  .hero-links {
    justify-content: center;
    gap: 7px;
  }

  .link-pill {
    padding: 6px 12px;
    font-size: 0.84rem;
  }

  .contact-links {
    gap: 7px;
  }

  .contact-pill {
    padding: 6px 12px;
    font-size: 0.84rem;
  }
}

/* Reduced motion */
@media (prefers-reduced-motion: reduce) {
  html {
    scroll-behavior: auto;
  }

  *,
  *::before,
  *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
    scroll-behavior: auto !important;
  }

  .reveal {
    opacity: 1 !important;
    transform: none !important;
  }
}