/* ============================================================
   ZHENG HAO-TIAN — shared site stylesheet
   Single source of truth for all four pages.
   Identity: dark navy #0c1421 + crimson accent + serif/sans pairing.
   ============================================================ */

/* ---------- Design tokens ---------- */
:root {
  /* Palette */
  --navy:        #0c1421;
  --navy-2:      #12203a;   /* card surface */
  --navy-3:      #16273f;   /* hover surface */
  --ink:         #eef2fb;   /* primary text */
  --ink-soft:    #c4cee4;   /* secondary text */
  --ink-faint:   #93a1c0;   /* tertiary / captions */
  --line:        rgba(255, 255, 255, 0.10);

  /* Institutional (Stony Brook) red palette */
  --accent:      #990000;   /* PRIMARY red RGB 153/0/0 — filled surfaces (buttons, pills, rules) */
  --accent-deep: #6b000d;   /* SECONDARY dark red RGB 107/0/13 — hover / pressed */
  /* SECONDARY bright red RGB 213/32/39 — the official bright brand red.
     Used for TEXT links/accents on dark navy: it is both on-brand and the
     only swatch with enough contrast (~4.7:1) to read as text. */
  --accent-soft: #d52027;
  --inst-yellow: #f1ea86;   /* official institutional yellow RGB 241/234/134 */
  --gold:        var(--inst-yellow); /* legacy alias → institutional yellow */

  /* Type */
  --serif: "Source Serif 4", "Charter", Georgia, "Times New Roman", serif;
  --sans:  "Inter", system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI",
           Roboto, "Helvetica Neue", Arial, sans-serif;

  /* Rhythm */
  --wrap: 1120px;
  --gutter: clamp(1rem, 4vw, 2.5rem);
  --radius: 14px;
  --radius-sm: 9px;

  /* Motion */
  --ease: cubic-bezier(0.22, 0.61, 0.36, 1);
  --dur: 0.45s;
}

/* ---------- Reset / base ---------- */
*, *::before, *::after { box-sizing: border-box; }

html {
  scroll-behavior: smooth;
  /* Solid base so long pages / overscroll never show the UA background */
  background-color: var(--navy);
}

body {
  margin: 0;
  padding: 0;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  font-family: var(--sans);
  color: var(--ink);
  /* Solid navy base only — the photographic backdrop (body::before) carries the
     visual identity, so no competing gradient here. */
  background-color: var(--navy);
  -webkit-font-smoothing: antialiased;
  text-rendering: optimizeLegibility;
  line-height: 1.6;
}

/* Photographic backdrop. A gentle vertical navy veil keeps text legible while
   the research illustration (added by body::after below) shows through. */
body::before {
  content: "";
  position: fixed;
  inset: 0;
  z-index: -2;
  background-color: var(--navy);
  background-image:
    linear-gradient(180deg,
      rgba(12, 20, 33, 0.44) 0%,
      rgba(12, 20, 33, 0.30) 40%,
      rgba(12, 20, 33, 0.52) 100%);
  background-repeat: no-repeat;
}

/* Research imagery (hand placing a savings coin into asset blocks) as a
   fixed, site-wide watermark in the lower-right of the viewport — visible on
   every page and every section. It sits at z-index 0 with header/main/footer
   raised above it, so it renders behind the copy without collapsing behind
   the body background. */
body::after {
  content: "";
  position: fixed;
  right: 0;
  bottom: 0;
  z-index: 0;
  width: clamp(340px, 44vw, 720px);
  aspect-ratio: 16 / 9;
  background: url("bg.png") no-repeat right bottom / contain;
  opacity: 0.7;
  pointer-events: none;
  /* Soft radial fade anchored to the corner so the whole illustration (hand +
     coin + asset blocks) stays visible, feathering only at the far edges. */
  -webkit-mask-image: radial-gradient(120% 120% at 100% 100%, #000 55%, transparent 96%);
  mask-image: radial-gradient(120% 120% at 100% 100%, #000 55%, transparent 96%);
}

/* Keep the readable content above the watermark. */
.site-header, main, .site-footer { position: relative; z-index: 1; }

img { max-width: 100%; height: auto; }

/* The `hidden` attribute must always win over component display rules
   (e.g. .publist { display:flex }) so expandable sections stay collapsed. */
[hidden] { display: none !important; }

main { flex: 1 0 auto; width: 100%; }

h1, h2, h3 {
  font-family: var(--serif);
  font-weight: 600;
  line-height: 1.15;
  letter-spacing: 0.01em;
  margin: 0 0 0.5em;
}

p { margin: 0 0 1em; }

a {
  color: var(--accent-soft);
  text-decoration: none;
  transition: color 0.2s var(--ease);
}
a:hover { color: var(--inst-yellow); text-decoration: underline; text-underline-offset: 3px; }

::selection { background: var(--accent); color: #fff; }

/* ---------- Header / nav ---------- */
.site-header {
  position: sticky;
  top: 0;
  z-index: 100;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  flex-wrap: wrap;
  padding: 0.9rem var(--gutter);
  background: rgba(12, 20, 33, 0.82);
  backdrop-filter: saturate(140%) blur(10px);
  border-bottom: 1px solid var(--line);
}

.brand {
  font-family: var(--serif);
  font-weight: 600;
  font-size: clamp(1.05rem, 2vw, 1.35rem);
  letter-spacing: 0.02em;
  color: var(--ink);
  white-space: nowrap;
}
.brand a { color: inherit; }
.brand a:hover { color: var(--ink); text-decoration: none; }
.brand .zh { color: var(--accent-soft); font-weight: 500; margin-left: 0.4em; }

.site-nav ul {
  display: flex;
  flex-wrap: wrap;
  gap: 0.25rem;
  margin: 0;
  padding: 0;
  list-style: none;
}
.site-nav a {
  display: inline-block;
  padding: 0.45rem 0.9rem;
  border-radius: var(--radius-sm);
  font-size: clamp(0.8rem, 1.1vw, 0.95rem);
  font-weight: 600;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--ink-soft);
  transition: background 0.2s var(--ease), color 0.2s var(--ease);
}
.site-nav a:hover {
  background: var(--accent);
  color: #fff;
  text-decoration: none;
}
.site-nav a[aria-current="page"] {
  color: #fff;
  background: rgba(153, 0, 0, 0.32);
  box-shadow: inset 0 0 0 1px rgba(213, 32, 39, 0.55);
}

/* ---------- Shared layout helpers ---------- */
.wrap {
  width: min(var(--wrap), 100% - 2 * var(--gutter));
  margin-inline: auto;
}

.section-title {
  font-size: clamp(1.9rem, 4vw, 3rem);
  color: var(--ink);
  text-align: center;
  margin-bottom: 0.75em;
}
.section-title::after {
  content: "";
  display: block;
  width: 64px;
  height: 3px;
  margin: 0.5em auto 0;
  background: var(--accent);
  border-radius: 2px;
}

/* Card used across research / teaching / contact / about panels */
.card {
  background: rgba(16, 28, 50, 0.88);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: clamp(1.1rem, 2.4vw, 1.9rem);
  backdrop-filter: saturate(120%) blur(4px);
  box-shadow: 0 10px 30px rgba(0, 0, 0, 0.35);
}

/* ---------- Buttons ---------- */
.btn {
  display: inline-flex;
  align-items: center;
  gap: 0.5em;
  padding: 0.6rem 1.1rem;
  border: none;
  border-radius: var(--radius-sm);
  background: var(--accent);
  color: #fff;
  font-family: var(--sans);
  font-size: 0.95rem;
  font-weight: 600;
  letter-spacing: 0.02em;
  cursor: pointer;
  transition: transform 0.18s var(--ease), background 0.2s var(--ease),
              box-shadow 0.2s var(--ease);
  box-shadow: 0 4px 14px rgba(153, 0, 0, 0.4);
}
.btn:hover { background: var(--accent-deep); transform: translateY(-2px); }
.btn:active { transform: translateY(0); }
.btn--ghost {
  background: rgba(255, 255, 255, 0.08);
  box-shadow: none;
  color: var(--ink);
}
.btn--ghost:hover { background: rgba(255, 255, 255, 0.16); }

/* ---------- Footer ---------- */
.site-footer {
  margin-top: auto;
  padding: 1.4rem var(--gutter);
  text-align: center;
  color: var(--ink-faint);
  font-size: 0.85rem;
  letter-spacing: 0.03em;
  border-top: 1px solid var(--line);
  background: rgba(12, 20, 33, 0.5);
}

/* ---------- Reveal on scroll (subtle) ---------- */
.reveal {
  opacity: 0;
  transform: translateY(22px);
  transition: opacity var(--dur) var(--ease), transform var(--dur) var(--ease);
  will-change: opacity, transform;
}
.reveal.is-visible { opacity: 1; transform: none; }

/* ---------- Responsive ---------- */
@media (max-width: 760px) {
  .site-header { justify-content: center; }
  .brand { width: 100%; text-align: center; }
  .site-nav ul { justify-content: center; }
}

/* ---------- Reduced motion ---------- */
@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  *, *::before, *::after {
    animation-duration: 0.001ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.001ms !important;
  }
  .reveal { opacity: 1; transform: none; }
}