/* =========================================================
   layout.css — page structure: topbar, no-scroll app shell,
                info sections, footer.

   The page is laid out as a "pro app" shell:

       ┌──────────────────────────────────────────────────────┐
       │  TOPBAR  brand · nav · …actions… · [Train SVM]       │  60px sticky
       ├────────────┬───────────────────────────┬─────────────┤
       │ left rail  │      canvas (square)      │ right rail  │
       │ (controls) │      + legend below       │ (live stats)│  100vh − topbar
       └────────────┴───────────────────────────┴─────────────┘
       (educational sections live below this shell, reachable
        only by intentional scrolling)
   ========================================================= */

/* ---------- Topbar ---------- */
.topbar {
    position: sticky;
    top: 0;
    z-index: 20;
    height: var(--topbar-h);
    display: flex;
    align-items: center;
    gap: 18px;
    padding: 0 20px;
    background: var(--surface);
    border-bottom: 1px solid var(--border);
    box-shadow: var(--shadow-sm);
}

.brand {
    display: flex;
    align-items: center;
    gap: 12px;
    flex-shrink: 0;
}
.logo {
    width: 36px;
    height: 36px;
    border-radius: var(--radius);
    background: var(--accent);
    color: #fff;
    display: grid;
    place-items: center;
    font-weight: 700;
    font-size: 0.78rem;
    letter-spacing: 0.04em;
    box-shadow: var(--shadow-sm);
}
.brand-text h1 {
    font-size: 1.05rem;
    line-height: 1;
}

.topbar-actions {
    margin-left: auto;
    display: flex;
    align-items: center;
    gap: 8px;
}
.action-group {
    display: flex;
    gap: 6px;
}

/* ---------- App shell ---------- */
.app-shell {
    height: calc(100vh - var(--topbar-h));
    display: grid;
    grid-template-columns: var(--rail-left-w) minmax(0, 1fr) var(--rail-right-w);
    gap: var(--shell-gap);
    padding: var(--shell-pad);
    overflow: hidden;
}

.rail {
    display: flex;
    flex-direction: column;
    gap: var(--shell-gap);
    min-height: 0;
    min-width: 0;
    overflow-y: auto;
    padding-right: 4px;
    scrollbar-width: thin;
    scrollbar-color: var(--scrollbar-thumb) var(--scrollbar-track);
}
.rail::-webkit-scrollbar {
    width: 8px;
}
.rail::-webkit-scrollbar-track {
    background: var(--scrollbar-track);
    border-radius: 4px;
}
.rail::-webkit-scrollbar-thumb {
    background: var(--scrollbar-thumb);
    border-radius: 4px;
}
.rail::-webkit-scrollbar-thumb:hover {
    background: var(--scrollbar-thumb-hover);
}

/* canvas-area is its own column; never scrolls */
.canvas-area {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-md);
    padding: 14px;
    display: grid;
    grid-template-rows: minmax(0, 1fr) auto;
    gap: 10px;
    min-width: 0;
    min-height: 0;
    overflow: hidden;
}

/* ---------- Responsive ---------- */
@media (max-width: 1180px) {
    :root {
        --rail-left-w: 270px;
        --rail-right-w: 210px;
        --shell-gap: 14px;
        --shell-pad: 14px;
    }
}

/* Hard fallback for genuinely small screens only — stack and let the page
   scroll. We deliberately do NOT trigger this on short heights (e.g. typical
   1366×768 laptops with browser chrome have ~680px of usable height) because
   the 3-column shell handles low heights gracefully: the rails scroll
   internally and the canvas (square via container queries) shrinks to fit. */
@media (max-width: 900px), (max-height: 480px) {
    .app-shell {
        height: auto;
        grid-template-columns: 1fr;
        overflow: visible;
        padding: 16px;
    }
    .rail { overflow-y: visible; }
    .canvas-area { min-height: min(72vw, 480px); }
    .topbar { flex-wrap: wrap; height: auto; padding: 10px 16px; }
    .topbar-actions { flex-wrap: wrap; }
}

/* ---------- Info sections (below the app shell) ---------- */
.info-section {
    max-width: 1100px;
    margin: 36px auto;
    padding: 0 32px;
}
.info-section h2 { margin-bottom: 8px; font-size: 1.15rem; }
.info-section p  { color: var(--text-muted); }

.kernel-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
    gap: 14px;
    margin-top: 14px;
}
.kernel-grid article {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 14px;
    box-shadow: var(--shadow-sm);
}
.kernel-grid h3 { margin-bottom: 6px; color: var(--accent-strong); }
.kernel-grid p  { font-size: 0.88rem; }

/* ---------- Metrics explanation grid ---------- */
.metrics-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(260px, 1fr));
    gap: 14px;
    margin: 14px 0;
}
.metrics-grid article {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 14px;
    box-shadow: var(--shadow-sm);
}
.metrics-grid h3 { margin-bottom: 6px; color: var(--accent-strong); }
.metrics-grid p  { font-size: 0.88rem; margin: 6px 0; }
.metric-warn {
    border-left: 3px solid #f59e0b;
    padding-left: 10px;
    background: rgba(245, 158, 11, 0.06);
    border-radius: 0 6px 6px 0;
}
.metric-summary {
    margin-top: 14px;
    padding: 12px 14px;
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    font-size: 0.92rem;
}
.formula-inline {
    font-family: ui-monospace, SFMono-Regular, monospace;
    font-size: 0.85rem;
    color: var(--text);
}

ol li { margin: 6px 0; color: var(--text-muted); }