/* ============================================================
   site.css  —  layout first; colour/typography polish deferred
   ============================================================ */

:root {
    --topbar-height: 64px;
    --sidebar-width: 220px;
    --content-max: 740px;
    --gap: 1.5rem;
    --color-bg: #fafafa;
    --color-surface: #fff;
    --color-border: #ddd;
    --color-text: #222;
    --color-muted: #666;
    --color-link: #003366;
    --color-active-bg: #f0f4f8;
    --color-active-bar: #003366;
}

/* ── Reset ───────────────────────────────────────────────── */

*,
*::before,
*::after {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

body {
    font-family: Georgia, serif;
    font-size: 1rem;
    line-height: 1.65;
    color: var(--color-text);
    background: var(--color-bg);
}

a {
    color: var(--color-link);
}
a:hover {
    text-decoration: underline;
}

/* ── Top bar ─────────────────────────────────────────────── */

.topbar {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    height: var(--topbar-height);
    background: var(--color-surface);
    border-bottom: 1px solid var(--color-border);
    display: flex;
    align-items: center;
    padding: 0 var(--gap);
    z-index: 100;
    gap: 1.5rem;
}

.topbar .site-title {
    font-weight: normal;
    font-size: 1rem;
    flex: 1;
    text-decoration: none;
    color: var(--color-text);
}

.topbar nav {
    display: flex;
    gap: 1.25rem;
}

.topbar nav a {
    text-decoration: none;
    color: var(--color-text);
    font-size: 0.95rem;
    font-family: system-ui, sans-serif;
}

.topbar nav a:hover {
    text-decoration: underline;
}

.topbar .logo {
    height: 48px;
    width: auto;
    flex-shrink: 0;
    display: block;
}

.menu-toggle {
    display: none;
    background: none;
    border: none;
    font-size: 1.4rem;
    cursor: pointer;
    padding: 0 0.25rem;
    color: var(--color-text);
    margin-right: auto;
}

/* ── Page wrapper ────────────────────────────────────────── */

.page-wrapper {
    display: flex;
    padding-top: var(--topbar-height);
    min-height: 100vh;
}

/* ── Sidebar ─────────────────────────────────────────────── */

.sidebar {
    width: var(--sidebar-width);
    flex-shrink: 0;
    position: sticky;
    top: var(--topbar-height);
    height: calc(100vh - var(--topbar-height));
    overflow-y: auto;
    border-right: 1px solid var(--color-border);
    padding: var(--gap) 1rem;
    background: var(--color-surface);
    font-family: system-ui, sans-serif;
}

.sidebar .sidebar-heading {
    font-size: 0.7rem;
    text-transform: uppercase;
    letter-spacing: 0.1em;
    color: var(--color-muted);
    margin-bottom: 1rem;
}

.sidebar .chapter {
    margin-bottom: 1.25rem;
}

.sidebar .chapter-title {
    font-size: 0.875rem;
    font-weight: bold;
    color: var(--color-text);
    display: block;
    padding: 0.15rem 0;
    text-decoration: none;
}

.sidebar .chapter-title:hover {
    text-decoration: underline;
}

.sidebar ul {
    list-style: none;
    padding-left: 0.6rem;
    margin-top: 0.2rem;
}
.sidebar ul li {
    margin: 0.1rem 0;
}

.sidebar ul a {
    font-size: 0.85rem;
    text-decoration: none;
    color: var(--color-text);
    display: block;
    padding: 0.2rem 0.5rem;
    border-left: 2px solid transparent;
    border-radius: 0 3px 3px 0;
}

.sidebar ul a:hover {
    background: var(--color-active-bg);
}

.sidebar ul a.active {
    border-left-color: var(--color-active-bar);
    background: var(--color-active-bg);
    font-weight: bold;
    color: var(--color-active-bar);
}

/* ── Main content ────────────────────────────────────────── */

.main-content {
    flex: 1;
    min-width: 0;
    padding: calc(var(--gap) * 1.5) calc(var(--gap) * 2);
}

.content-body {
    max-width: var(--content-max);
}

.content-body h1 {
    font-size: 1.75rem;
    font-weight: normal;
    margin-bottom: 1.25rem;
    border-bottom: 1px solid var(--color-border);
    padding-bottom: 0.4rem;
}

.content-body h2 {
    font-size: 1.2rem;
    font-weight: normal;
    margin: 1.75rem 0 0.5rem;
    border-bottom: 1px solid var(--color-border);
    padding-bottom: 0.2rem;
}

.content-body p {
    margin-bottom: 0.9rem;
}

.content-body ul,
.content-body ol {
    padding-left: 1.5rem;
    margin-bottom: 0.9rem;
}

.content-body li {
    margin-bottom: 0.3rem;
}

.content-body code {
    background: #eee;
    padding: 0.1em 0.35em;
    border-radius: 3px;
    font-size: 0.88em;
}

/* ── Previous / Next footer ──────────────────────────────── */

.page-nav {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-top: 3rem;
    padding-top: 1rem;
    border-top: 1px solid var(--color-border);
    font-size: 0.9rem;
    font-family: system-ui, sans-serif;
    max-width: var(--content-max);
}

.page-nav a {
    text-decoration: none;
    color: var(--color-link);
}
.page-nav a:hover {
    text-decoration: underline;
}
.page-nav .prev::before {
    content: "← ";
}
.page-nav .next::after {
    content: " →";
}

/* ── Catalogue ───────────────────────────────────────────── */

.controls {
    margin: 1.25rem 0;
    padding: 0.875rem 1rem;
    background: var(--color-surface);
    border: 1px solid var(--color-border);
    border-radius: 6px;
    font-family: system-ui, sans-serif;
}

.controls strong {
    margin-right: 0.5rem;
    font-size: 0.9rem;
}

.controls button {
    margin-right: 0.4rem;
    padding: 0.3rem 0.7rem;
    border: 1px solid #999;
    border-radius: 4px;
    background: var(--color-surface);
    cursor: pointer;
    font-size: 0.875rem;
}

.controls button:hover {
    background: #eee;
}

.controls button.active {
    background: var(--color-text);
    color: #fff;
    border-color: var(--color-text);
}

#status {
    margin: 0.75rem 0;
    color: var(--color-muted);
    font-size: 0.875rem;
    font-family: system-ui, sans-serif;
}

.catalogue {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
    gap: 1rem;
    margin-top: 1rem;
}

.card {
    background: var(--color-surface);
    border: 1px solid var(--color-border);
    border-radius: 8px;
    padding: 1rem;
    font-family: system-ui, sans-serif;
}

.card h2 {
    font-size: 0.95rem;
    font-weight: bold;
    margin-bottom: 0.5rem;
    border: none;
    padding: 0;
}

.drawing {
    min-height: 140px;
    display: flex;
    align-items: center;
    justify-content: center;
    margin-bottom: 0.75rem;
    border-bottom: 1px solid #eee;
    padding-bottom: 0.75rem;
}

.drawing svg {
    max-width: 100%;
    height: auto;
}

.evaluation {
    font-family: ui-monospace, Menlo, Consolas, monospace;
    font-size: 0.85rem;
    background: #f3f3f3;
    padding: 0.4rem 0.6rem;
    border-radius: 4px;
    overflow-x: auto;
}

.invariants {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 0.15rem 0.5rem;
    font-size: 0.82rem;
    margin-bottom: 0.75rem;
}

.card details {
    margin-top: 0.5rem;
}

.card summary {
    cursor: pointer;
    color: var(--color-link);
    font-size: 0.82rem;
}

.card pre {
    white-space: pre-wrap;
    background: #f5f5f5;
    padding: 0.5rem;
    border-radius: 4px;
    font-size: 0.78rem;
    overflow-x: auto;
    margin-top: 0.35rem;
}

/* ── Responsive ──────────────────────────────────────────── */

@media (max-width: 768px) {
    .sidebar {
        display: none;
        position: fixed;
        top: var(--topbar-height);
        left: 0;
        height: calc(100vh - var(--topbar-height));
        z-index: 99;
        box-shadow: 2px 0 8px rgba(0, 0, 0, 0.12);
    }

    .sidebar.open {
        display: block;
    }

    .menu-toggle {
        display: block;
    }

    .main-content {
        padding: var(--gap);
    }
}