/* ================================================================
   Jeffrey E. Sun — Academic Website
   Ground-up stylesheet
   ================================================================ */

/* ----------------------------------------------------------------
   CSS Variables
   ---------------------------------------------------------------- */
:root {
  --bg:           #ffffff;
  --text:         #2d2d2d;
  --text-muted:   #666666;
  --heading:      #111111;
  --accent:       #1a5276;
  --accent-hover: #154360;
  --nav-bg:       #1d2b3a;
  --nav-text:     #f0f0f0;
  --nav-hover:    #a8c4d8;
  --divider:      #e0e0e0;
  --nav-height:   60px;
}

/* ----------------------------------------------------------------
   Reset & Base
   ---------------------------------------------------------------- */
*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  font-size: 16px;
  scroll-behavior: smooth;
}

body {
  background-color: var(--bg);
  color: var(--text);
  font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, sans-serif;
  font-size: 1rem;
  font-weight: 400;
  line-height: 1.75;
  -webkit-font-smoothing: antialiased;
}

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

a {
  color: var(--accent);
  text-decoration: underline;
  text-decoration-color: transparent;
  transition: color 0.15s ease, text-decoration-color 0.15s ease;
}

a:hover, a:focus {
  color: var(--accent-hover);
  text-decoration-color: var(--accent-hover);
}

b, strong {
  font-weight: 600;
}

p {
  margin-bottom: 1rem;
}

hr {
  border: none;
  border-top: 1px solid var(--divider);
  margin: 2.5rem 0;
}

ul {
  list-style: none;
  padding: 0;
  margin: 0;
}

h1, h2, h3, h4 {
  font-family: 'Merriweather', Georgia, 'Times New Roman', serif;
  font-weight: 700;
  color: var(--heading);
  line-height: 1.3;
  letter-spacing: -0.01em;
}

/* ----------------------------------------------------------------
   Layout Container
   ---------------------------------------------------------------- */
.container {
  max-width: 860px;
  margin: 0 auto;
  padding: 0 28px;
}

/* ----------------------------------------------------------------
   Navigation
   ---------------------------------------------------------------- */
.site-header {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: var(--nav-height);
  background-color: var(--nav-bg);
  z-index: 100;
  display: flex;
  align-items: center;
}

.nav-inner {
  max-width: 860px;
  margin: 0 auto;
  padding: 0 28px;
  width: 100%;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.nav-brand a {
  font-family: 'Merriweather', Georgia, serif;
  font-size: 1.05rem;
  font-weight: 700;
  color: var(--nav-text);
  text-decoration: none;
  letter-spacing: 0.01em;
}

.nav-brand a:hover {
  color: var(--nav-hover);
  text-decoration: none;
}

/* Desktop nav links */
.nav-links {
  display: flex;
  align-items: center;
  gap: 2rem;
  list-style: none;
}

.nav-links a {
  font-size: 0.875rem;
  font-weight: 500;
  letter-spacing: 0.04em;
  text-transform: uppercase;
  color: var(--nav-text);
  text-decoration: none;
  transition: color 0.15s ease;
}

.nav-links a:hover {
  color: var(--nav-hover);
  text-decoration: none;
}

/* Mobile hamburger */
.menu-toggle-input {
  display: none;
}

.menu-toggle-label {
  display: none;
  flex-direction: column;
  justify-content: center;
  gap: 5px;
  cursor: pointer;
  padding: 6px;
}

.menu-toggle-label span,
.menu-toggle-label span::before,
.menu-toggle-label span::after {
  display: block;
  width: 22px;
  height: 2px;
  background: var(--nav-text);
  position: relative;
  transition: transform 0.3s ease;
}

.menu-toggle-label span::before,
.menu-toggle-label span::after {
  content: '';
  position: absolute;
}

.menu-toggle-label span::before { top: -7px; }
.menu-toggle-label span::after  { top:  7px; }

/* Mobile nav panel */
@media (max-width: 640px) {
  .menu-toggle-label {
    display: flex;
  }

  .nav-links {
    display: none;
    position: absolute;
    top: var(--nav-height);
    left: 0;
    right: 0;
    background-color: var(--nav-bg);
    flex-direction: column;
    align-items: flex-start;
    gap: 0;
    padding: 0.5rem 0 1rem;
    border-top: 1px solid rgba(255,255,255,0.1);
  }

  .nav-links li {
    width: 100%;
  }

  .nav-links a {
    display: block;
    padding: 0.75rem 28px;
    font-size: 0.9rem;
  }

  .menu-toggle-input:checked ~ .nav-links {
    display: flex;
  }
}

/* ----------------------------------------------------------------
   Main Content
   ---------------------------------------------------------------- */
main {
  padding-top: calc(var(--nav-height) + 2.5rem);
  padding-bottom: 4rem;
}

/* ----------------------------------------------------------------
   Hero / About Section
   ---------------------------------------------------------------- */
.about-section {
  display: grid;
  grid-template-columns: 220px 1fr;
  gap: 2.5rem;
  align-items: start;
  margin-bottom: 2.5rem;
}

.about-photo {
  width: 100%;
  border-radius: 3px;
}

.about-name {
  font-size: 1.6rem;
  margin-bottom: 0.75rem;
  margin-top: 0;
}

.about-body {
  font-size: 0.98rem;
  color: var(--text);
  line-height: 1.8;
}

.about-body p {
  margin-bottom: 0.85rem;
}

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

@media (max-width: 640px) {
  .about-section {
    grid-template-columns: 1fr;
  }

  .about-photo {
    display: none;
  }
}

/* ----------------------------------------------------------------
   Section Headings
   ---------------------------------------------------------------- */
.section-heading {
  font-family: 'Merriweather', Georgia, serif;
  font-size: 1.2rem;
  font-weight: 700;
  color: var(--heading);
  margin-top: 0;
  margin-bottom: 1.25rem;
  padding-bottom: 0.4rem;
  border-bottom: 2px solid var(--accent);
  display: inline-block;
}

.subsection-heading {
  font-family: 'Merriweather', Georgia, serif;
  font-size: 1rem;
  font-weight: 700;
  color: var(--text-muted);
  margin-top: 1.5rem;
  margin-bottom: 0.75rem;
}

/* ----------------------------------------------------------------
   Paper / Item Lists
   ---------------------------------------------------------------- */
.paper-list {
  list-style: none;
  padding: 0;
  margin: 0 0 0.5rem;
}

.paper-list li {
  padding: 0.55rem 0 0.55rem 1.1rem;
  border-left: 2px solid var(--divider);
  margin-bottom: 0.75rem;
  font-size: 0.965rem;
  line-height: 1.6;
  color: var(--text);
}

.paper-list li a {
  text-decoration-color: transparent;
}

.paper-list li a:hover {
  text-decoration-color: var(--accent-hover);
}

.paper-meta {
  font-size: 0.88rem;
  color: var(--text-muted);
  margin-top: 0.2rem;
}

.paper-links {
  font-size: 0.875rem;
  margin-top: 0.2rem;
}

.paper-links a {
  margin-right: 0.5rem;
  color: var(--accent);
  font-weight: 500;
}

/* ----------------------------------------------------------------
   Collapsible
   ---------------------------------------------------------------- */
.collapsible-toggle {
  display: inline-flex;
  align-items: center;
  gap: 0.4rem;
  background: none;
  border: none;
  border-left: 3px solid var(--accent);
  padding: 0.45rem 0.85rem;
  color: var(--accent);
  font-family: inherit;
  font-size: 0.9rem;
  font-weight: 600;
  cursor: pointer;
  margin-bottom: 0;
  transition: color 0.15s ease;
}

.collapsible-toggle:hover {
  color: var(--accent-hover);
}

.collapsible-toggle::after {
  content: '+';
  font-size: 1rem;
  font-weight: 700;
  line-height: 1;
}

.collapsible-toggle.active::after {
  content: '−';
}

.collapsible-body {
  max-height: 0;
  overflow: hidden;
  transition: max-height 0.25s ease-out;
  background: #f4f7fa;
  border-left: 3px solid var(--accent);
}

.collapsible-body-inner {
  padding: 1.25rem 1.5rem;
  font-size: 0.92rem;
  line-height: 1.75;
}

.collapsible-body-inner p {
  margin-bottom: 0.85rem;
}

.collapsible-body-inner ul {
  list-style: disc;
  padding-left: 1.5rem;
  margin-bottom: 0.85rem;
}

.collapsible-body-inner ul li {
  margin-bottom: 0.3rem;
}

/* ----------------------------------------------------------------
   Footer
   ---------------------------------------------------------------- */
.site-footer {
  border-top: 1px solid var(--divider);
  margin-top: 3rem;
  padding-top: 1.5rem;
  text-align: center;
  font-size: 0.82rem;
  color: var(--text-muted);
}