#app {
  display: grid;
  grid-template-rows: var(--topbar-h) 1fr;
  grid-template-columns: minmax(320px, 42%) 1fr;
  grid-template-areas:
    "topbar topbar"
    "sidebar main";
  height: 100vh;
  width: 100vw;
  padding: var(--sp-3);
  gap: var(--sp-3);
  transition: grid-template-columns var(--dur-med) var(--ease-out);
}

@media (min-width: 1400px) {
  #app { grid-template-columns: minmax(320px, 520px) 1fr; }
}

/* ---------- Focus / fullscreen toggle ---------- */
.focus-toggle {
  position: fixed;
  top: 14px;
  right: 14px;
  z-index: 200;
  width: 34px;
  height: 34px;
  border-radius: 10px;
  display: grid;
  place-items: center;
  background: var(--glass-bg-strong);
  backdrop-filter: blur(14px) saturate(160%);
  -webkit-backdrop-filter: blur(14px) saturate(160%);
  border: 1px solid var(--glass-border-strong);
  color: var(--slate-400);
  box-shadow: var(--glass-shadow);
  transition: color var(--dur-fast) var(--ease-out), border-color var(--dur-fast) var(--ease-out),
    background var(--dur-fast) var(--ease-out), top var(--dur-fast) var(--ease-out), right var(--dur-fast) var(--ease-out);
}
.focus-toggle:hover { color: var(--brass-bright); border-color: var(--brass-soft-strong); }
.focus-toggle svg { width: 15px; height: 15px; }

/* While in focus mode the topbar and sidebar chrome are hidden, so this
   button becomes the only thing sitting at the very top edge — nudge it
   in slightly and keep it above everything, including the sidebar overlay. */
body.is-focus-mode .focus-toggle { top: 10px; right: 10px; }

/* ---------- Focus mode: hide chrome, let content take the full screen --- */
body.is-focus-mode .topbar { display: none; }
body.is-focus-mode .sidebar__header,
body.is-focus-mode .sidebar__tabs,
body.is-focus-mode .sidebar__footer { display: none; }
body.is-focus-mode #app {
  grid-template-rows: 1fr;
  grid-template-areas: "sidebar main";
}
@media (max-width: 900px) {
  body.is-focus-mode #app { grid-template-areas: "main"; }
  body.is-focus-mode .sidebar { top: 0; }
}

/* ---------- Top bar ---------- */
.topbar {
  grid-area: topbar;
  display: flex;
  align-items: center;
  gap: var(--sp-4);
  padding: 0 var(--sp-5);
  background: var(--glass-bg-strong);
  backdrop-filter: blur(var(--glass-blur)) saturate(180%);
  -webkit-backdrop-filter: blur(var(--glass-blur)) saturate(180%);
  border: 1px solid var(--glass-border);
  border-radius: var(--radius-lg);
  box-shadow: var(--glass-shadow);
  z-index: 30;
  transition: backdrop-filter var(--dur-med) var(--ease-out), background var(--dur-med) var(--ease-out),
    box-shadow var(--dur-med) var(--ease-out), border-color var(--dur-med) var(--ease-out);
}

/* Apple-"material" behavior: once content has scrolled underneath, the
   topbar's glass thickens — more blur, more saturation, more opacity. */
.topbar.is-scrolled {
  background: var(--glass-bg-strong);
  backdrop-filter: blur(calc(var(--glass-blur) + 18px)) saturate(200%);
  -webkit-backdrop-filter: blur(calc(var(--glass-blur) + 18px)) saturate(200%);
  border-color: var(--glass-border-strong);
  box-shadow: var(--glass-shadow-lg);
}

.topbar__brand {
  display: flex;
  align-items: center;
  gap: var(--sp-2);
  font-family: var(--font-display);
  font-weight: 700;
  font-size: var(--fs-md);
  white-space: nowrap;
}

.topbar__brand .mark {
  width: 26px; height: 26px;
  border-radius: 8px;
  background: linear-gradient(155deg, var(--teal-bright), var(--brass));
  display: grid;
  place-items: center;
  color: var(--on-accent);
  font-weight: 800;
  font-size: 13px;
  box-shadow: 0 3px 10px var(--brass-soft-strong);
}
.topbar__brand .wordmark { color: var(--parchment); font-weight: 700; }

.topbar__course-switch {
  display: flex;
  align-items: center;
  gap: var(--sp-2);
  background: var(--glass-bg);
  border: 1px solid var(--glass-border);
  border-radius: 999px;
  padding: 6px var(--sp-3);
  font-size: var(--fs-sm);
  color: var(--slate-400);
  transition: border-color var(--dur-fast) var(--ease-out);
}
.topbar__course-switch:hover { border-color: var(--brass-soft-strong); color: var(--parchment); }
.topbar__course-switch .course-dot {
  width: 8px; height: 8px; border-radius: 50%;
  background: var(--current-course-color, var(--brass));
}

.topbar__search {
  flex: 1;
  max-width: 420px;
  position: relative;
}
.topbar__search input {
  width: 100%;
  background: var(--glass-bg);
  border: 1px solid var(--glass-border);
  border-radius: 999px;
  padding: 8px var(--sp-3) 8px 34px;
  font-size: var(--fs-sm);
  color: var(--parchment);
  transition: border-color var(--dur-fast) var(--ease-out), background var(--dur-fast);
}
.topbar__search input::placeholder { color: var(--slate-600); }
.topbar__search input:focus { border-color: var(--brass); background: var(--glass-bg-strong); outline: none; }
.topbar__search svg {
  position: absolute;
  left: 10px; top: 50%;
  transform: translateY(-50%);
  width: 15px; height: 15px;
  color: var(--slate-500);
  pointer-events: none;
}

.topbar__meta {
  margin-left: auto;
  display: flex;
  align-items: center;
  gap: var(--sp-4);
}

.lang-switch {
  display: flex;
  align-items: center;
  gap: 6px;
  padding: 6px 10px;
  border-radius: 999px;
  border: 1px solid var(--glass-border);
  background: var(--glass-bg);
  color: var(--slate-400);
  font-family: var(--font-mono);
  font-size: var(--fs-xs);
  font-weight: 600;
  transition: border-color var(--dur-fast), color var(--dur-fast);
}
.lang-switch:hover { border-color: var(--brass-soft-strong); color: var(--parchment); }

.theme-switch {
  display: flex;
  align-items: center;
  justify-content: center;
  width: 32px;
  height: 32px;
  border-radius: 50%;
  border: 1px solid var(--glass-border);
  background: var(--glass-bg);
  color: var(--slate-400);
  transition: border-color var(--dur-fast), color var(--dur-fast);
}
.theme-switch:hover { border-color: var(--brass-soft-strong); color: var(--brass-bright); }
.theme-switch svg { width: 15px; height: 15px; }

.progress-ring-wrap {
  display: flex;
  align-items: center;
  gap: var(--sp-2);
  font-size: var(--fs-xs);
  color: var(--slate-400);
}

.sidebar-toggle {
  display: none;
  color: var(--slate-400);
  padding: var(--sp-2);
}

/* ---------- Sidebar ---------- */
.sidebar {
  grid-area: sidebar;
  background: var(--glass-bg);
  backdrop-filter: blur(var(--glass-blur)) saturate(180%);
  -webkit-backdrop-filter: blur(var(--glass-blur)) saturate(180%);
  border: 1px solid var(--glass-border);
  border-radius: var(--radius-lg);
  box-shadow: var(--glass-shadow);
  display: flex;
  flex-direction: column;
  min-height: 0;
  overflow: hidden;
}

.sidebar__header {
  padding: var(--sp-4) var(--sp-4) var(--sp-2);
  display: flex;
  align-items: baseline;
  justify-content: space-between;
}

.sidebar__header h2 {
  font-size: var(--fs-xs);
  text-transform: uppercase;
  letter-spacing: 0.12em;
  color: var(--slate-500);
  font-family: var(--font-mono);
  font-weight: 500;
}

.sidebar__actions {
  display: flex;
  gap: var(--sp-2);
}
.sidebar__actions button {
  font-size: var(--fs-2xs);
  color: var(--slate-500);
  font-family: var(--font-mono);
  padding: 2px 6px;
  border-radius: var(--radius-sm);
}
.sidebar__actions button:hover { color: var(--brass-bright); background: var(--ink-750); }
.sidebar__actions button.is-active { color: var(--brass-bright); background: var(--brass-soft); }

.tree-scroll {
  flex: 1;
  min-height: 0;
  display: flex;
  flex-direction: column;
  overflow: hidden;
  position: relative;
}
.tree-scroll .sidebar-list {
  flex: 1;
  overflow-y: auto;
  padding: var(--sp-2) var(--sp-2) var(--sp-6);
}

.sidebar__footer {
  padding: var(--sp-3) var(--sp-4);
  border-top: 1px solid var(--ink-700);
  font-size: var(--fs-2xs);
  color: var(--slate-600);
  font-family: var(--font-mono);
  display: flex;
  justify-content: space-between;
}

/* ---------- Main ---------- */
.main {
  grid-area: main;
  min-width: 0;
  min-height: 0;
  overflow-y: auto;
  background: var(--glass-bg-soft);
  backdrop-filter: blur(var(--glass-blur)) saturate(180%);
  -webkit-backdrop-filter: blur(var(--glass-blur)) saturate(180%);
  border: 1px solid var(--glass-border);
  border-radius: var(--radius-lg);
  box-shadow: var(--glass-shadow);
}

.content-wrap {
  max-width: 780px;
  margin: 0 auto;
  padding: var(--sp-6) var(--sp-6) var(--sp-8);
}

@media (max-width: 900px) {
  .mobile-back { display: inline-flex; }
  #app {
    grid-template-columns: 1fr;
    grid-template-areas:
      "topbar"
      "main";
  }
  .sidebar {
    position: fixed;
    top: var(--topbar-h);
    left: 0;
    right: 0;
    bottom: 0;
    width: 100vw;
    z-index: 40;
    transform: translateX(-100%);
    transition: transform var(--dur-med) var(--ease-out);
  }
  .sidebar.is-open { transform: translateX(0); }
  .sidebar-toggle { display: inline-flex; }
  .topbar__course-switch span.label { display: none; }
  .content-wrap { padding: var(--sp-5) var(--sp-4) var(--sp-7); }
}

.sidebar__tabs {
  display: flex;
  gap: 2px;
  padding: 0 var(--sp-3) var(--sp-2);
}
.sidebar__tab {
  flex: 1;
  font-size: var(--fs-2xs);
  font-family: var(--font-mono);
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--slate-500);
  padding: 6px 4px;
  border-radius: var(--radius-sm);
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 4px;
  transition: background var(--dur-fast), color var(--dur-fast);
}
.sidebar__tab svg { width: 12px; height: 12px; }
.sidebar__tab:hover { color: var(--parchment); background: var(--glass-bg); }
.sidebar__tab.is-active { color: var(--brass-bright); background: var(--brass-soft); }

.sidebar-list {
  display: flex;
  flex-direction: column;
  gap: 2px;
  padding: 0 var(--sp-2);
}
.sidebar-list__item {
  display: flex;
  align-items: center;
  gap: var(--sp-2);
  padding: 8px var(--sp-2);
  border-radius: var(--radius-sm);
  color: var(--slate-400);
  font-size: var(--fs-sm);
  text-align: left;
}
.sidebar-list__item:hover { background: var(--brass-soft); color: var(--parchment); }
.sidebar-list__item svg { width: 14px; height: 14px; flex-shrink: 0; }
.sidebar-list__crumb {
  display: block;
  font-family: var(--font-mono);
  font-size: 10px;
  color: var(--slate-600);
  margin-top: 1px;
}

.mobile-back {
  display: none;
  align-items: center;
  gap: 6px;
  color: var(--brass-bright);
  font-size: var(--fs-sm);
  font-family: var(--font-mono);
  margin-bottom: var(--sp-4);
}
.mobile-back svg { width: 14px; height: 14px; }

.scrim {
  display: none;
  position: fixed;
  inset: var(--topbar-h) 0 0 0;
  background: rgba(6,10,14,0.55);
  z-index: 35;
}
.scrim.is-visible { display: block; }