:root {
    --bg: #4b6542;
    --surface: #fffdf1;
    --surface-soft: #f7ffe9;
    --surface-lavender: #cda8ff;
    --surface-strong: #111111;
    --ink: #111111;
    --muted: #5f655c;
    --line: #dadccc;
    --teal: #6f3dff;
    --blue: #2f63ff;
    --amber: #c95100;
    --red: #b4423b;
    --lime: #c5ff32;
    --lime-soft: #ecffd2;
    --code: #111111;
    --radius: 8px;

    /* Surfaces and text */
    --surface-elevated: #ffffff;
    --surface-planned: #fffdf8;
    --muted-strong: #3d4947;
    --text-on-strong: #ffffff;
    --text-on-lime: #111111;
    --lime-faint: #fbfff3;
    --disabled-bg: #e7e8de;
    --input-placeholder: #687063;

    /* Accent (purple) tints */
    --accent-soft: #f0e9ff;
    --accent-tint: #eee7ff;
    --accent-text: #3b1a99;
    --lavender-text: #2b203b;

    /* Status colours and their soft backgrounds */
    --warn-soft: #fff4df;
    --warn-softer: #fff8ed;
    --warn-faint: #fffaf0;
    --warn-text: #5f430f;
    --danger-soft: #fff0ed;
    --danger-text: #6f201b;
    --stderr-bg: #2a1717;
    --stderr-text: #ffe8e3;

    /* Top navigation and text sitting directly on the page background */
    --nav-bg: #111111;
    --nav-text: #f4f7ef;
    --nav-text-strong: #ffffff;
    --nav-muted: #c9d0c5;
    --on-bg-text: #fffdf1;
    --on-bg-muted: #dce7d7;
    --on-bg-lede: #e5ecdf;

    /* Code panels (always dark) */
    --code-text: #eef7f0;
    --code-line: #cbd7dc;
    --code-muted: #81919a;
    --code-active-text: #ffffff;
    --code-active-num: #ecdcff;

    --focus-ring: #2f63ff;

    color-scheme: light;
}

/*
 * Dark theme. The same values appear twice: once for an explicit data-theme="dark" and once for
 * the OS preference when the page has not forced the light theme. Keep both blocks in sync.
 */
:root[data-theme="dark"] {
    --bg: #121611;
    --surface: #1b2019;
    --surface-soft: #222a1f;
    --surface-lavender: #2e2450;
    --surface-strong: #e9eee3;
    --ink: #eef2e8;
    --muted: #aab3a4;
    --line: #3a4336;
    --teal: #b79dff;
    --blue: #8fa8ff;
    --amber: #ffa462;
    --red: #ff8f86;
    --lime: #c5ff32;
    --lime-soft: #1f2b12;
    --code: #0c0f0b;
    --surface-elevated: #262e23;
    --surface-planned: #1d221b;
    --muted-strong: #c3ccbd;
    --text-on-strong: #111111;
    --text-on-lime: #111111;
    --lime-faint: #1c2416;
    --disabled-bg: #2c3329;
    --input-placeholder: #8d9688;
    --accent-soft: #2c2347;
    --accent-tint: #2a2246;
    --accent-text: #cdb8ff;
    --lavender-text: #e4d9ff;
    --warn-soft: #33270f;
    --warn-softer: #2c2413;
    --warn-faint: #282218;
    --warn-text: #ffd89a;
    --danger-soft: #3a1d1a;
    --danger-text: #ffc4bd;
    --stderr-bg: #2a1717;
    --stderr-text: #ffe8e3;
    --nav-bg: #0a0c09;
    --nav-text: #f4f7ef;
    --nav-text-strong: #ffffff;
    --nav-muted: #c9d0c5;
    --on-bg-text: #fffdf1;
    --on-bg-muted: #dce7d7;
    --on-bg-lede: #e5ecdf;
    --code-text: #eef7f0;
    --code-line: #cbd7dc;
    --code-muted: #8d9ca5;
    --code-active-text: #ffffff;
    --code-active-num: #ecdcff;
    --focus-ring: #c5ff32;
    --bs-code-color: #ff9ccb;

    color-scheme: dark;
}

@media (prefers-color-scheme: dark) {
    /* Keep in sync with :root[data-theme="dark"] above. */
    :root:not([data-theme="light"]) {
        --bg: #121611;
        --surface: #1b2019;
        --surface-soft: #222a1f;
        --surface-lavender: #2e2450;
        --surface-strong: #e9eee3;
        --ink: #eef2e8;
        --muted: #aab3a4;
        --line: #3a4336;
        --teal: #b79dff;
        --blue: #8fa8ff;
        --amber: #ffa462;
        --red: #ff8f86;
        --lime: #c5ff32;
        --lime-soft: #1f2b12;
        --code: #0c0f0b;
        --surface-elevated: #262e23;
        --surface-planned: #1d221b;
        --muted-strong: #c3ccbd;
        --text-on-strong: #111111;
        --text-on-lime: #111111;
        --lime-faint: #1c2416;
        --disabled-bg: #2c3329;
        --input-placeholder: #8d9688;
        --accent-soft: #2c2347;
        --accent-tint: #2a2246;
        --accent-text: #cdb8ff;
        --lavender-text: #e4d9ff;
        --warn-soft: #33270f;
        --warn-softer: #2c2413;
        --warn-faint: #282218;
        --warn-text: #ffd89a;
        --danger-soft: #3a1d1a;
        --danger-text: #ffc4bd;
        --stderr-bg: #2a1717;
        --stderr-text: #ffe8e3;
        --nav-bg: #0a0c09;
        --nav-text: #f4f7ef;
        --nav-text-strong: #ffffff;
        --nav-muted: #c9d0c5;
        --on-bg-text: #fffdf1;
        --on-bg-muted: #dce7d7;
        --on-bg-lede: #e5ecdf;
        --code-text: #eef7f0;
        --code-line: #cbd7dc;
        --code-muted: #8d9ca5;
        --code-active-text: #ffffff;
        --code-active-num: #ecdcff;
        --focus-ring: #c5ff32;
        --bs-code-color: #ff9ccb;

        color-scheme: dark;
    }
}

/* Regions that sit on the dark nav / page background use the lime ring for contrast. */
.app-nav,
.workspace-header,
.lesson-header {
    --focus-ring: var(--lime);
}

/* Consistent, visible keyboard focus. */
a:focus-visible,
button:focus-visible,
input:focus-visible,
select:focus-visible,
textarea:focus-visible,
summary:focus-visible,
[role="tab"]:focus-visible,
[tabindex]:not([tabindex="-1"]):focus-visible {
    outline: 2px solid var(--focus-ring);
    outline-offset: 2px;
    box-shadow: none;
}

* {
    box-sizing: border-box;
}

html,
body {
    min-height: 100%;
    margin: 0;
    background: var(--bg);
    color: var(--ink);
    font-family: "DM Sans", Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
}

body {
    overflow-x: hidden;
}

a {
    color: inherit;
}

button,
textarea {
    font: inherit;
}

.app-shell {
    min-height: 100vh;
}

body:has(.lesson-screen) {
    overflow: hidden;
}

body:has(.lesson-screen) .app-shell {
    display: grid;
    grid-template-columns: 200px minmax(0, 1fr);
    grid-template-rows: minmax(0, 1fr);
    height: 100dvh;
    min-height: 0;
}

body:has(.lesson-screen) .app-main {
    grid-column: 2;
    grid-row: 1;
    width: 100%;
    max-width: none;
    min-height: 0;
    overflow: hidden;
    padding: clamp(0.4rem, 0.9vw, 0.75rem) clamp(0.65rem, 1.4vw, 1.35rem);
}

body:has(.lesson-screen) .app-nav {
    grid-column: 1;
    grid-row: 1;
    position: relative;
    top: auto;
    flex-direction: column;
    justify-content: flex-start;
    align-items: stretch;
    height: 100%;
    min-height: 0;
    padding: 1rem 0.55rem;
    border-right: 1px solid rgba(255, 255, 255, 0.08);
    border-bottom: 0;
}

body:has(.lesson-screen) .brand-mark {
    width: 36px;
    height: 36px;
}

body:has(.lesson-screen) .brand {
    padding: 0 0.4rem 0.75rem;
}

body:has(.lesson-screen) .brand strong {
    font-size: 0.95rem;
    line-height: 1.08;
}

body:has(.lesson-screen) .brand small {
    margin-top: 0.08rem;
    font-size: 0.78rem;
}

body:has(.lesson-screen) .nav-links {
    align-items: stretch;
    justify-content: flex-start;
    flex-direction: column;
    gap: 0.35rem;
    width: 100%;
    margin-top: 1rem;
}

body:has(.lesson-screen) .nav-link {
    min-height: 36px;
    padding: 0.5rem 0.7rem;
    color: var(--nav-text);
}

body:has(.lesson-screen) .nav-link.active,
body:has(.lesson-screen) .nav-link:hover {
    border-color: var(--lime);
    background: var(--lime);
    color: var(--text-on-lime);
}

.app-nav {
    position: sticky;
    top: 0;
    z-index: 20;
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 1rem;
    min-height: 72px;
    padding: 0.85rem clamp(1rem, 3vw, 2.5rem);
    background: var(--nav-bg);
    border-bottom: 1px solid rgba(255, 255, 255, 0.08);
    color: var(--nav-text-strong);
    backdrop-filter: blur(16px);
}

.brand {
    display: inline-flex;
    align-items: center;
    gap: 0.75rem;
    min-width: 0;
    text-decoration: none;
}

.brand-mark {
    display: grid;
    place-items: center;
    width: 42px;
    height: 42px;
    border-radius: var(--radius);
    background: var(--lime);
    color: var(--text-on-lime);
    font-family: "Cascadia Code", Consolas, monospace;
    font-weight: 800;
}

.brand strong,
.brand small {
    display: block;
    line-height: 1.15;
}

.brand small {
    margin-top: 0.2rem;
    color: var(--nav-muted);
}

.nav-links {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    flex-wrap: wrap;
    justify-content: flex-end;
}

.nav-link {
    min-height: 40px;
    padding: 0.55rem 0.85rem;
    border: 1px solid transparent;
    border-radius: var(--radius);
    color: var(--nav-text);
    text-decoration: none;
}

.nav-link.active,
.nav-link:hover {
    border-color: var(--lime);
    background: var(--lime);
    color: var(--text-on-lime);
}

.auth-widget {
    display: flex;
    align-items: center;
    justify-content: flex-end;
    gap: 0.35rem;
    min-width: 0;
    flex-wrap: wrap;
}

.auth-input,
.auth-button,
.auth-pill {
    min-height: 40px;
    border: 1px solid rgba(255, 255, 255, 0.2);
    border-radius: var(--radius);
    font-size: 0.84rem;
    font-weight: 800;
}

.auth-input {
    width: 138px;
    min-width: 0;
    padding: 0.45rem 0.6rem;
    background: var(--surface);
    color: var(--ink);
}

.auth-input::placeholder {
    color: var(--input-placeholder);
}

.auth-button {
    padding: 0.45rem 0.72rem;
    background: var(--lime);
    color: var(--text-on-lime);
    cursor: pointer;
}

.auth-button.ghost,
.auth-pill {
    background: transparent;
    color: var(--nav-text);
}

.auth-button:disabled {
    cursor: wait;
    opacity: 0.65;
}

.auth-pill {
    display: inline-flex;
    align-items: center;
    max-width: 220px;
    padding: 0.45rem 0.7rem;
    overflow: hidden;
    text-overflow: ellipsis;
    white-space: nowrap;
}

.auth-pill.signed-in {
    border-color: var(--lime);
    color: var(--lime);
}

.auth-message {
    flex-basis: 100%;
    color: var(--on-bg-muted);
    font-size: 0.72rem;
    font-weight: 700;
    line-height: 1.2;
    text-align: right;
}

body:has(.lesson-screen) .auth-widget {
    display: none;
}

.app-main {
    width: min(1400px, 100%);
    margin: 0 auto;
    padding: clamp(1rem, 3vw, 2.5rem);
}

.lesson-screen {
    display: grid;
    grid-template-rows: auto minmax(0, 1fr) auto;
    gap: 0.45rem;
    height: 100%;
    min-height: 0;
    overflow: hidden;
}

.lesson-screen .lesson-header {
    display: grid;
    grid-template-columns: minmax(0, 1fr) minmax(230px, 300px);
    align-items: center;
    gap: 0.85rem;
    width: min(1880px, 100%);
    margin: 0 auto;
    min-height: 0;
    padding: 0;
    color: var(--on-bg-text);
}

.lesson-screen .lesson-header h1 {
    margin-bottom: 0.2rem;
    font-size: clamp(1.45rem, 1.9vw, 2rem);
    line-height: 1;
}

.lesson-screen .lesson-header .lede {
    color: var(--on-bg-muted);
    font-size: 0.86rem;
    line-height: 1.25;
}

.lesson-screen .lesson-header .eyebrow {
    margin-bottom: 0.18rem;
    font-size: 0.72rem;
}

.lesson-screen .progress-meter {
    gap: 0.36rem;
    min-width: 0;
}

.lesson-screen .progress-meter div {
    height: 7px;
}

.lesson-screen .back-link {
    margin-bottom: 0.18rem;
    font-size: 0.86rem;
}

.workspace-header,
.lesson-header {
    display: flex;
    align-items: flex-start;
    justify-content: space-between;
    gap: 2rem;
    padding: clamp(1.25rem, 4vw, 2.5rem) 0;
    color: var(--on-bg-text);
}

.eyebrow {
    margin: 0 0 0.55rem;
    color: var(--lime);
    font-size: 0.78rem;
    font-weight: 800;
    letter-spacing: 0;
    text-transform: uppercase;
}

h1,
h2,
h3,
p {
    margin-top: 0;
}

h1:focus,
h2:focus,
h3:focus {
    outline: none;
}

h1 {
    max-width: 780px;
    margin-bottom: 0.7rem;
    font-size: clamp(2rem, 4vw, 4.2rem);
    line-height: 0.98;
}

h2 {
    font-size: clamp(1.35rem, 2vw, 2rem);
    line-height: 1.1;
}

h3 {
    font-size: 1rem;
}

.lede {
    max-width: 720px;
    margin-bottom: 0;
    color: var(--on-bg-lede);
    font-size: clamp(1rem, 1.4vw, 1.18rem);
    line-height: 1.55;
}

.status-strip {
    display: flex;
    flex-wrap: wrap;
    gap: 0.5rem;
    justify-content: flex-end;
    max-width: 360px;
}

.status-strip span,
.status-strip a,
.progress-meter span {
    display: inline-flex;
    min-height: 34px;
    align-items: center;
    padding: 0.35rem 0.65rem;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface);
    color: var(--ink);
    font-size: 0.86rem;
    font-weight: 700;
    text-decoration: none;
}

.status-strip a {
    border-color: var(--surface-strong);
    background: var(--surface-strong);
    color: var(--text-on-strong);
}

.dashboard-grid {
    display: grid;
    grid-template-columns: minmax(0, 1.65fr) minmax(300px, 0.75fr);
    gap: 1.25rem;
    align-items: start;
}

.book-hub-grid {
    grid-template-columns: minmax(0, 1.55fr) minmax(320px, 0.65fr);
}

.lesson-board,
.practice-panel,
.lesson-section,
.lesson-toc {
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface);
    box-shadow: 0 18px 42px rgba(17, 17, 17, 0.12);
}

.lesson-board,
.practice-panel {
    padding: clamp(1rem, 2vw, 1.5rem);
}

.chapter-board {
    max-height: calc(100dvh - 20rem);
    min-height: 520px;
    overflow-x: hidden;
    overflow-y: auto;
    scrollbar-gutter: stable;
}

.section-heading {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 1rem;
    margin-bottom: 1rem;
    color: var(--muted);
    font-size: 0.9rem;
    font-weight: 800;
}

.section-heading strong {
    color: var(--ink);
}

.lesson-list {
    display: grid;
    grid-template-columns: repeat(2, minmax(0, 1fr));
    gap: 0.8rem;
}

.lesson-card {
    display: grid;
    grid-template-columns: 3rem minmax(0, 1fr);
    gap: 0.85rem;
    min-height: 112px;
    padding: 1rem;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    color: inherit;
    text-decoration: none;
}

.lesson-card:hover {
    border-color: rgba(111, 61, 255, 0.5);
    box-shadow: 0 14px 34px rgba(22, 32, 41, 0.08);
}

.lesson-card.complete {
    border-color: rgba(197, 255, 50, 0.95);
    background: var(--lime-soft);
}

.lesson-card.in-progress {
    border-color: rgba(184, 121, 25, 0.45);
    background: var(--warn-faint);
}

.lesson-card.basics.not-started {
    border-color: rgba(197, 255, 50, 0.65);
    background: var(--lime-faint);
}

.lesson-order {
    display: grid;
    place-items: center;
    width: 3rem;
    height: 3rem;
    border-radius: var(--radius);
    background: var(--lime);
    color: var(--text-on-lime);
    font-weight: 900;
}

.lesson-card strong,
.lesson-card small {
    display: block;
}

.lesson-card strong {
    display: flex;
    align-items: flex-start;
    justify-content: space-between;
    gap: 0.6rem;
}

.lesson-card em,
.test-row em {
    flex: 0 0 auto;
    padding: 0.18rem 0.4rem;
    border-radius: 999px;
    background: var(--surface-soft);
    color: var(--muted);
    font-size: 0.7rem;
    font-style: normal;
    font-weight: 800;
}

.lesson-card small {
    margin-top: 0.4rem;
    color: var(--muted);
    line-height: 1.35;
}

.lesson-type {
    display: inline-flex;
    width: fit-content;
    margin-top: 0.6rem;
    padding: 0.22rem 0.45rem;
    border-radius: 999px;
    background: var(--surface-soft);
    color: var(--muted);
    font-size: 0.72rem;
    font-weight: 900;
}

.lesson-type.basics {
    background: var(--lime);
    color: var(--text-on-lime);
}

.lesson-type.problem {
    background: var(--accent-soft);
    color: var(--teal);
}

.chapter-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
    gap: 0.8rem;
}

.chapter-card {
    display: grid;
    grid-template-columns: 3.1rem minmax(0, 1fr);
    gap: 0.85rem;
    min-height: 150px;
    padding: 1rem;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface-elevated);
    color: inherit;
    text-decoration: none;
}

.chapter-card:hover {
    border-color: rgba(111, 61, 255, 0.5);
    box-shadow: 0 14px 34px rgba(22, 32, 41, 0.08);
}

.chapter-card.live {
    border-color: rgba(197, 255, 50, 0.95);
    background: var(--lime-soft);
}

.chapter-card.planned {
    background: var(--surface-planned);
}

.chapter-order {
    display: grid;
    place-items: center;
    width: 3.1rem;
    height: 3.1rem;
    border-radius: var(--radius);
    background: var(--lime);
    color: var(--text-on-lime);
    font-weight: 900;
}

.chapter-card.planned .chapter-order {
    background: var(--surface-soft);
}

.chapter-card-body,
.chapter-card-body strong,
.chapter-card-body small,
.chapter-meta,
.part-label {
    display: block;
}

.part-label {
    margin-bottom: 0.35rem;
    color: var(--teal);
    font-size: 0.72rem;
    font-weight: 900;
    text-transform: uppercase;
}

.chapter-card-body strong {
    line-height: 1.18;
}

.chapter-card-body strong em {
    display: inline-flex;
    margin-left: 0.3rem;
    padding: 0.14rem 0.38rem;
    border-radius: 999px;
    background: var(--surface-soft);
    color: var(--muted);
    font-size: 0.68rem;
    font-style: normal;
    font-weight: 900;
    vertical-align: middle;
}

.chapter-card.live strong em {
    background: var(--surface-strong);
    color: var(--text-on-strong);
}

.chapter-card-body small {
    margin-top: 0.45rem;
    color: var(--muted);
    line-height: 1.35;
}

.chapter-meta {
    margin-top: 0.65rem;
    color: var(--ink);
    font-size: 0.82rem;
    font-weight: 900;
}

.roadmap-panel {
    display: grid;
    gap: 1.1rem;
}

.current-focus {
    display: grid;
    grid-template-columns: 3.1rem minmax(0, 1fr);
    gap: 0.85rem;
    padding: 1rem;
    border: 1px solid rgba(197, 255, 50, 0.95);
    border-radius: var(--radius);
    background: var(--lime-soft);
}

.current-focus strong,
.current-focus p {
    display: block;
}

.current-focus p {
    margin: 0.35rem 0 0.9rem;
    color: var(--muted);
    line-height: 1.45;
}

.flow-steps.compact {
    gap: 0.65rem;
}

.chapter-empty {
    margin-bottom: 1rem;
    padding: 1rem;
    border: 1px solid rgba(111, 61, 255, 0.28);
    border-radius: var(--radius);
    background: var(--surface-lavender);
}

.chapter-empty strong,
.chapter-empty p {
    display: block;
}

.chapter-empty p {
    margin: 0.4rem 0 0;
    color: var(--lavender-text);
    line-height: 1.45;
}

.section-outline {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(230px, 1fr));
    gap: 0.75rem;
}

.section-outline-item {
    display: grid;
    grid-template-columns: auto minmax(0, 1fr);
    gap: 0.65rem;
    align-items: start;
    min-height: 78px;
    padding: 0.85rem;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface-elevated);
}

.section-outline-item.basics {
    border-color: rgba(197, 255, 50, 0.7);
    background: var(--lime-soft);
}

.section-outline-item span {
    display: inline-grid;
    place-items: center;
    min-width: 3.1rem;
    min-height: 2.1rem;
    border-radius: var(--radius);
    background: var(--surface-soft);
    color: var(--teal);
    font-weight: 900;
}

.section-outline-item strong {
    display: block;
    line-height: 1.25;
}

.section-outline-item em {
    display: inline-flex;
    width: fit-content;
    margin-top: 0.45rem;
    padding: 0.18rem 0.42rem;
    border-radius: 999px;
    background: var(--surface-soft);
    color: var(--muted);
    font-size: 0.68rem;
    font-style: normal;
    font-weight: 900;
}

.section-outline-item.basics em {
    background: var(--surface-strong);
    color: var(--text-on-strong);
}

.section-outline-item.problem em {
    background: var(--accent-soft);
    color: var(--teal);
}

.flow-steps {
    display: grid;
    gap: 0.8rem;
}

.flow-steps div {
    padding-bottom: 0.8rem;
    border-bottom: 1px solid var(--line);
}

.flow-steps div:last-child {
    padding-bottom: 0;
    border-bottom: 0;
}

.flow-steps strong,
.flow-steps span {
    display: block;
}

.flow-steps span {
    margin-top: 0.35rem;
    color: var(--muted);
    line-height: 1.45;
}

.back-link {
    display: inline-flex;
    margin-bottom: 0.45rem;
    color: var(--lime);
    font-weight: 800;
    text-decoration: none;
}

.progress-meter {
    display: grid;
    gap: 0.65rem;
    min-width: 220px;
}

.progress-meter div {
    height: 10px;
    overflow: hidden;
    border-radius: 999px;
    background: rgba(255, 255, 255, 0.25);
}

.progress-meter i {
    display: block;
    height: 100%;
    border-radius: inherit;
    background: var(--lime);
}

.completion-panel {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 1rem;
    margin-bottom: 0;
    padding: 0.75rem 1rem;
    border: 1px solid rgba(197, 255, 50, 0.65);
    border-radius: var(--radius);
    background: var(--lime-soft);
}

.completion-panel span,
.completion-panel strong {
    display: block;
}

.completion-panel span {
    margin-bottom: 0.25rem;
    color: var(--surface-strong);
    font-size: 0.78rem;
    font-weight: 900;
    text-transform: uppercase;
}

.primary-link {
    display: inline-flex;
    align-items: center;
    min-height: 42px;
    padding: 0.65rem 0.9rem;
    border-radius: var(--radius);
    background: var(--surface-strong);
    color: var(--text-on-strong);
    font-weight: 900;
    text-decoration: none;
}

.lesson-layout {
    display: block;
    width: min(1880px, 100%);
    height: 100%;
    margin: 0 auto;
    min-height: 0;
    overflow: hidden;
}

.lesson-toc {
    position: sticky;
    top: 96px;
    display: grid;
    gap: 0.35rem;
    padding: 0.75rem;
}

.lesson-toc a {
    display: block;
    padding: 0.65rem 0.75rem;
    border-radius: var(--radius);
    color: var(--muted);
    text-decoration: none;
}

.lesson-toc a:hover {
    background: var(--surface-soft);
    color: var(--ink);
}

.lesson-flow {
    display: grid;
    gap: 1.25rem;
}

.lesson-reader {
    display: grid;
    grid-template-rows: auto minmax(0, 1fr) auto;
    gap: 0.55rem;
    width: 100%;
    height: 100%;
    min-height: 0;
    max-width: none;
    margin: 0 auto;
    padding: clamp(0.55rem, 0.9vw, 0.8rem);
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface-soft);
    overflow: hidden;
    box-shadow: 0 24px 52px rgba(17, 17, 17, 0.18);
}

.reader-toolbar,
.reader-footer {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 0.65rem;
    flex-wrap: wrap;
}

.reader-toolbar span,
.reader-footer span {
    display: block;
    color: var(--muted);
    font-size: 0.78rem;
    font-weight: 800;
}

.reader-toolbar strong {
    display: block;
    margin-top: 0.15rem;
    color: var(--ink);
    line-height: 1.2;
}

.reader-rail {
    display: flex;
    align-items: center;
    gap: 0.34rem;
    flex-wrap: wrap;
}

.reader-tab {
    display: grid;
    place-items: center;
    width: 2rem;
    height: 2rem;
    border: 1px solid var(--line);
    border-radius: 999px;
    background: var(--surface-elevated);
    color: var(--muted);
    font-weight: 900;
    cursor: pointer;
}

.reader-tab.active {
    border-color: var(--surface-strong);
    background: var(--lime);
    color: var(--text-on-lime);
}

.reader-window {
    display: grid;
    grid-template-columns: 2.35rem minmax(0, 1fr) 2.35rem;
    gap: 0.5rem;
    align-items: stretch;
    min-height: 0;
}

.reader-arrow {
    display: grid;
    place-items: center;
    min-width: 2.35rem;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface);
    color: var(--ink);
    font-size: 1.28rem;
    font-weight: 900;
    cursor: pointer;
}

.reader-arrow:disabled {
    cursor: not-allowed;
    opacity: 0.38;
}

.lesson-section {
    padding: clamp(0.7rem, 1.1vw, 1rem);
    scroll-margin-top: 96px;
}

.reader-page {
    min-height: 0;
    max-height: none;
    overflow: hidden;
    animation: reader-page-in 180ms ease;
}

@keyframes reader-page-in {
    from {
        opacity: 0;
        transform: translateX(8px);
    }

    to {
        opacity: 1;
        transform: translateX(0);
    }
}

.block-heading {
    max-width: 820px;
    margin-bottom: 1.1rem;
}

.block-heading span {
    display: inline-block;
    margin-bottom: 0.35rem;
    color: var(--teal);
    font-size: 0.78rem;
    font-weight: 900;
    text-transform: uppercase;
}

.block-heading p {
    color: var(--muted);
    line-height: 1.55;
}

.problem-guide {
    display: grid;
    grid-template-columns: repeat(2, minmax(0, 1fr));
    gap: 0.8rem;
    min-height: 0;
    overflow: hidden;
    animation: reader-page-in 180ms ease;
}

.problem-workspace {
    display: grid;
    grid-template-rows: auto minmax(0, 1fr);
    gap: 0.75rem;
    height: 100%;
    min-height: 0;
    overflow: hidden;
}

.problem-workspace-head {
    display: grid;
    grid-template-columns: minmax(0, 1fr) auto;
    gap: 1rem;
    align-items: end;
}

.problem-heading {
    margin-bottom: 0;
}

.problem-heading h2 {
    margin-bottom: 0.35rem;
}

.problem-heading p {
    margin-bottom: 0;
}

.problem-tabs {
    display: flex;
    justify-content: flex-end;
    gap: 0.4rem;
    flex-wrap: wrap;
}

.problem-tab {
    min-height: 36px;
    padding: 0.45rem 0.75rem;
    border: 1px solid var(--line);
    border-radius: 999px;
    background: var(--surface-elevated);
    color: var(--muted);
    font-weight: 900;
    cursor: pointer;
}

.problem-tab.active {
    border-color: var(--surface-strong);
    background: var(--lime);
    color: var(--text-on-lime);
}

.problem-hero,
.problem-panel {
    min-width: 0;
    min-height: 0;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface);
    overflow: hidden;
}

.problem-hero,
.problem-panel.full {
    grid-column: 1 / -1;
}

.problem-hero {
    padding: 1rem;
    border-color: rgba(111, 61, 255, 0.35);
    background: var(--surface-lavender);
}

.problem-hero span,
.section-kicker,
.metric-tile span,
.test-brief span,
.expected-output span,
.case-tile span {
    display: block;
    margin-bottom: 0.35rem;
    color: var(--teal);
    font-size: 0.74rem;
    font-weight: 900;
    text-transform: uppercase;
}

.problem-hero h3 {
    max-width: 880px;
    margin: 0;
    font-size: clamp(1.2rem, 2.2vw, 1.65rem);
    line-height: 1.3;
}

.problem-panel {
    padding: 0.9rem;
}

.problem-guide p {
    margin: 0;
    color: var(--muted);
    line-height: 1.45;
    overflow-wrap: anywhere;
}

.approach-flow,
.complexity-grid,
.case-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(180px, 1fr));
    gap: 0.65rem;
}

.flow-step,
.metric-tile,
.case-tile,
.test-brief,
.expected-output {
    min-width: 0;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface-elevated);
}

.flow-step {
    display: grid;
    grid-template-rows: auto 1fr;
    gap: 0.55rem;
    padding: 0.8rem;
}

.flow-step span,
.track-item span {
    display: grid;
    place-items: center;
    width: 2.25rem;
    height: 2.25rem;
    border-radius: var(--radius);
    background: var(--lime);
    color: var(--text-on-lime);
    font-weight: 900;
}

.code-map {
    display: grid;
    gap: 0.55rem;
}

.code-map div {
    display: grid;
    grid-template-columns: auto minmax(0, 1fr);
    gap: 0.65rem;
    align-items: start;
    padding-bottom: 0.65rem;
    border-bottom: 1px solid var(--line);
}

.code-map div:last-child {
    padding-bottom: 0;
    border-bottom: 0;
}

.code-map i {
    width: 0.8rem;
    height: 0.8rem;
    margin-top: 0.35rem;
    border: 2px solid var(--teal);
    border-radius: 50%;
    background: var(--surface-elevated);
}

.metric-tile {
    padding: 0.9rem;
    background: var(--surface-soft);
}

.metric-tile strong,
.expected-output strong {
    display: block;
    color: var(--ink);
    line-height: 1.45;
    overflow-wrap: anywhere;
}

.step-track {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(190px, 1fr));
    gap: 0.65rem;
}

.track-item {
    display: grid;
    grid-template-columns: auto minmax(0, 1fr);
    gap: 0.7rem;
    align-items: start;
    padding: 0.8rem;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface-elevated);
}

.test-visual {
    display: grid;
    grid-template-columns: minmax(0, 1.1fr) minmax(220px, 0.9fr);
    gap: 0.65rem;
}

.test-brief,
.expected-output,
.case-tile {
    padding: 0.85rem;
}

.expected-output {
    border-color: rgba(111, 61, 255, 0.28);
    background: var(--surface-soft);
}

.expected-output.output-large {
    min-height: 120px;
}

.test-case-label {
    margin-top: 0.8rem;
}

.list-diagram {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(76px, 1fr));
    gap: 0.6rem;
    max-width: 760px;
}

.list-diagram.compact {
    max-width: 680px;
}

.list-slot {
    display: grid;
    grid-template-rows: auto 1fr;
    min-height: 82px;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface-soft);
    overflow: hidden;
}

.list-slot span {
    padding: 0.3rem 0.45rem;
    border-bottom: 1px solid var(--line);
    color: var(--muted);
    font-size: 0.74rem;
    font-weight: 800;
}

.list-slot strong {
    display: grid;
    place-items: center;
    min-width: 0;
    padding: 0.55rem;
    font-family: "Cascadia Code", Consolas, monospace;
    font-size: 1.15rem;
    word-break: break-word;
}

.list-slot.accent,
.list-slot.active {
    border-color: rgba(111, 61, 255, 0.55);
    background: var(--accent-tint);
}

.list-slot.warn {
    border-color: rgba(184, 121, 25, 0.5);
    background: var(--warn-soft);
}

.edge-list {
    display: flex;
    flex-wrap: wrap;
    gap: 0.5rem;
    margin-top: 0.9rem;
}

.edge-list span {
    padding: 0.5rem 0.65rem;
    border-radius: var(--radius);
    background: var(--surface-soft);
    color: var(--muted);
}

.theory-block {
    display: grid;
    grid-template-rows: auto minmax(0, 1fr);
    gap: 0.65rem;
    height: 100%;
    min-height: 0;
    overflow: hidden;
}

.theory-card-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(290px, 1fr));
    gap: 0.75rem;
    min-height: 0;
    overflow: auto;
    padding-right: 0.2rem;
}

.theory-card {
    display: grid;
    grid-template-rows: auto minmax(0, auto);
    gap: 0.75rem;
    min-width: 0;
    padding: 0.9rem;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface);
}

.theory-card h3 {
    margin-bottom: 0.5rem;
    font-size: clamp(1rem, 1.3vw, 1.18rem);
}

.theory-card ul {
    display: grid;
    gap: 0.35rem;
    margin: 0;
    padding-left: 1.1rem;
    color: var(--muted);
    line-height: 1.45;
}

.theory-note {
    margin: 0.65rem 0 0;
    padding: 0.6rem 0.7rem;
    border-radius: var(--radius);
    background: var(--lime-soft);
    color: var(--ink);
    font-weight: 800;
    line-height: 1.4;
}

.theory-code-pair {
    display: grid;
    grid-template-columns: minmax(0, 1fr) minmax(0, 0.9fr);
    gap: 0.6rem;
    min-width: 0;
}

.theory-code-pair div {
    min-width: 0;
}

.theory-code-pair span {
    display: block;
    margin-bottom: 0.3rem;
    color: var(--teal);
    font-size: 0.72rem;
    font-weight: 900;
    text-transform: uppercase;
}

.theory-code-pair pre {
    max-height: 190px;
    min-height: 74px;
    padding: 0.75rem;
    border-radius: var(--radius);
    background: var(--code);
    color: var(--code-text);
    font-size: 0.82rem;
    line-height: 1.45;
    white-space: pre-wrap;
}

.stepper-shell {
    display: grid;
    grid-template-columns: minmax(0, 1.15fr) minmax(320px, 0.85fr);
    gap: 1rem;
    align-items: stretch;
}

.stepper-visual,
.stepper-code,
.editor-panel,
.test-panel {
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface);
}

.stepper-visual {
    display: grid;
    align-content: start;
    gap: 1rem;
    padding: 1rem;
}

.stepper-visual p {
    margin-bottom: 0;
    color: var(--muted);
    line-height: 1.5;
}

.stepper-code {
    display: grid;
    gap: 0.85rem;
    padding: 1rem;
}

pre {
    margin: 0;
    overflow: auto;
}

code,
.code-fallback {
    font-family: "Cascadia Code", Consolas, monospace;
}

.stepper-code pre {
    padding: 1rem;
    border-radius: var(--radius);
    background: var(--code);
    color: var(--code-text);
}

.stepper-controls,
.run-bar {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 0.75rem;
    flex-wrap: wrap;
}

.icon-button,
.primary-action,
.quiet-action {
    min-height: 40px;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface);
    color: var(--ink);
    font-weight: 800;
    cursor: pointer;
}

.icon-button {
    min-width: 72px;
}

.primary-action {
    padding: 0.65rem 1rem;
    border-color: var(--surface-strong);
    background: var(--surface-strong);
    color: var(--text-on-strong);
}

.quiet-action {
    width: 100%;
    padding: 0.6rem 0.75rem;
    text-align: left;
}

.quiet-action.inline,
.quiet-action.compact {
    width: auto;
    text-align: center;
}

.quiet-action.inline {
    padding: 0.65rem 0.9rem;
}

.quiet-action.compact {
    min-height: 34px;
    padding: 0.35rem 0.65rem;
}

button:disabled {
    cursor: not-allowed;
    opacity: 0.55;
}

.primary-action:disabled {
    border-color: var(--line);
    background: var(--disabled-bg);
    color: var(--muted);
}

.exercise-grid {
    display: grid;
    grid-template-rows: auto minmax(0, 1fr);
    gap: 0.55rem;
    height: 100%;
    min-height: 0;
    overflow: hidden;
}

.exercise-grid .block-heading {
    margin-bottom: 0;
}

.exercise-topline {
    display: flex;
    align-items: flex-start;
    justify-content: space-between;
    gap: 0.85rem;
    min-width: 0;
}

.exercise-topline .block-heading h2 {
    margin-bottom: 0.22rem;
    font-size: clamp(1.35rem, 1.6vw, 1.85rem);
    line-height: 1.06;
}

.exercise-topline .block-heading p {
    display: -webkit-box;
    margin-bottom: 0;
    overflow: hidden;
    color: var(--muted);
    line-height: 1.25;
    -webkit-box-orient: vertical;
    -webkit-line-clamp: 2;
}

.exercise-tabs {
    display: flex;
    align-items: center;
    justify-content: flex-end;
    gap: 0.36rem;
    flex-wrap: wrap;
    flex: 0 0 auto;
}

.exercise-tab {
    min-height: 34px;
    padding: 0.4rem 0.7rem;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface-elevated);
    color: var(--muted);
    font-weight: 900;
    cursor: pointer;
}

.exercise-tab.active {
    border-color: var(--surface-strong);
    background: var(--lime);
    color: var(--text-on-lime);
}

.exercise-pane {
    min-height: 0;
    overflow-x: hidden;
    overflow-y: auto;
    scrollbar-gutter: stable;
}

.exercise-pane.code-pane {
    overflow: hidden;
}

.exercise-code-pane {
    height: 100%;
}

.exercise-code-pane .code-editor-wrap,
.exercise-code-pane .code-editor-host,
.exercise-code-pane .code-fallback {
    min-height: 0;
    height: 100%;
}

.exercise-tests-pane {
    height: 100%;
    overflow-x: hidden;
    overflow-y: auto;
    scrollbar-gutter: stable;
}

.editor-panel {
    display: grid;
    grid-template-rows: minmax(0, 1fr) auto;
    min-height: 0;
    overflow: hidden;
}

.code-editor-wrap {
    position: relative;
    min-height: 320px;
    background: var(--code);
}

.code-editor-host {
    position: absolute;
    inset: 0;
    min-height: 320px;
}

.code-fallback {
    position: absolute;
    inset: 0;
    width: 100%;
    min-height: 320px;
    padding: 1rem;
    border: 0;
    resize: vertical;
    background: var(--code);
    color: var(--code-text);
    line-height: 1.5;
}

.run-bar {
    padding: 0.85rem;
    border-top: 1px solid var(--line);
    background: var(--surface);
}

.test-panel {
    display: grid;
    align-content: start;
    gap: 0.48rem;
    padding: 0.8rem;
}

.test-panel-head {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 0.75rem;
}

.test-panel-head h3 {
    margin: 0;
}

.solution-walkthrough {
    display: grid;
    grid-template-rows: auto minmax(0, 1fr) auto;
    gap: 0.62rem;
    height: 100%;
    min-height: 0;
    min-width: 0;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface);
    overflow: hidden;
}

.solution-reveal {
    text-align: center;
}

.solution-head {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 1rem;
    padding: 1rem 1rem 0;
}

.solution-head span,
.solution-frame-copy span,
.solution-visual-row > span,
.solution-state-chip span {
    display: block;
    margin-bottom: 0.35rem;
    color: var(--teal);
    font-size: 0.74rem;
    font-weight: 900;
    text-transform: uppercase;
}

.solution-head h3,
.solution-frame-copy h4 {
    margin: 0;
}

.solution-grid {
    display: grid;
    grid-template-columns: minmax(0, 1.1fr) minmax(300px, 0.9fr);
    gap: 0.75rem;
    min-height: 0;
    padding: 0 0.75rem;
    overflow-x: hidden;
    overflow-y: auto;
    scrollbar-gutter: stable;
}

.solution-code-card,
.solution-stage {
    min-width: 0;
    min-height: 0;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface-elevated);
    overflow: hidden;
}

.solution-code-card {
    padding: 0.6rem;
    background: var(--code);
}

.solution-code-line {
    display: grid;
    grid-template-columns: 2rem minmax(0, 1fr);
    gap: 0.65rem;
    align-items: start;
    padding: 0.35rem 0.45rem;
    border-radius: 6px;
    color: var(--code-line);
    transition: background-color 180ms ease, color 180ms ease;
}

.solution-code-line span {
    color: var(--code-muted);
    text-align: right;
    user-select: none;
}

.solution-code-line code {
    white-space: pre-wrap;
    overflow-wrap: anywhere;
}

.solution-code-line.active {
    background: rgba(111, 61, 255, 0.38);
    color: var(--code-active-text);
}

.solution-code-line.active span {
    color: var(--code-active-num);
}

.solution-stage {
    display: grid;
    align-content: start;
    gap: 0.65rem;
    padding: 0.75rem;
    overflow-x: hidden;
    overflow-y: auto;
    scrollbar-gutter: stable;
    animation: solution-frame-in 220ms ease;
}

.solution-frame-copy p {
    margin: 0.45rem 0 0;
    color: var(--muted);
    line-height: 1.5;
}

.solution-visual-stack {
    display: grid;
    gap: 0.65rem;
}

.solution-visual-row,
.solution-state-chip {
    min-width: 0;
    padding: 0.75rem;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface-soft);
}

.solution-visual-row.accent,
.solution-state-chip.accent {
    border-color: rgba(111, 61, 255, 0.35);
    background: var(--accent-tint);
}

.solution-visual-row.warn,
.solution-state-chip.warn {
    border-color: rgba(184, 121, 25, 0.35);
    background: var(--warn-softer);
}

.solution-visual-row.success,
.solution-state-chip.success {
    border-color: rgba(197, 255, 50, 0.95);
    background: var(--lime-soft);
}

.solution-slot-row {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(44px, 1fr));
    gap: 0.45rem;
}

.solution-slot-row strong,
.solution-state-chip strong {
    display: grid;
    place-items: center;
    min-height: 34px;
    padding: 0.45rem;
    border-radius: 6px;
    background: var(--surface-elevated);
    color: var(--ink);
    font-family: "Cascadia Code", Consolas, monospace;
    overflow-wrap: anywhere;
}

.solution-controls {
    display: flex;
    align-items: center;
    justify-content: flex-end;
    gap: 0.65rem;
    padding: 0 0.75rem 0.75rem;
    flex-wrap: wrap;
}

.trace-playground {
    display: flex;
    flex-direction: column;
    gap: 0.6rem;
    margin: 0 0.75rem 0.75rem;
    padding: 0.75rem;
    min-height: 0;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface-elevated);
    overflow-x: hidden;
    overflow-y: auto;
    scrollbar-gutter: stable;
}

.trace-head {
    display: flex;
    align-items: flex-start;
    justify-content: space-between;
    gap: 0.75rem;
    align-items: center;
}

.trace-head span,
.trace-state-card > span,
.variable-chip span,
.trace-return span {
    display: block;
    margin-bottom: 0.35rem;
    color: var(--teal);
    font-size: 0.74rem;
    font-weight: 900;
    text-transform: uppercase;
}

.trace-head h3,
.trace-state-card h4 {
    margin: 0;
}

.trace-head p {
    margin: 0.18rem 0 0;
    color: var(--muted);
    line-height: 1.3;
}

.trace-call-row {
    display: grid;
    grid-template-columns: minmax(0, 1fr) auto;
    gap: 0.55rem;
    align-items: end;
}

.trace-call-row label {
    grid-column: 1 / -1;
    color: var(--muted);
    font-weight: 800;
    font-size: 0.86rem;
}

.trace-call {
    display: grid;
    grid-template-columns: auto minmax(0, 1fr) auto;
    align-items: center;
    min-width: 0;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface-soft);
    overflow: hidden;
}

.trace-call code {
    padding: 0.6rem 0.68rem;
    color: var(--muted);
    white-space: nowrap;
}

.trace-call input {
    min-width: 0;
    min-height: 38px;
    border: 0;
    border-left: 1px solid var(--line);
    border-right: 1px solid var(--line);
    background: var(--surface-elevated);
    color: var(--ink);
    font: inherit;
    font-family: "Cascadia Code", Consolas, monospace;
    padding: 0.55rem 0.68rem;
}

.trace-call input:focus {
    outline: 2px solid var(--focus-ring);
    outline-offset: -2px;
}

.trace-status {
    margin: 0;
    padding: 0.8rem;
    border-radius: var(--radius);
    background: var(--surface-soft);
    color: var(--muted);
    line-height: 1.45;
}

.trace-error {
    max-height: 220px;
    padding: 0.85rem;
    border: 1px solid rgba(180, 66, 59, 0.35);
    border-radius: var(--radius);
    background: var(--danger-soft);
    color: var(--danger-text);
}

.trace-result-grid {
    display: grid;
    grid-template-columns: minmax(520px, 1.2fr) minmax(360px, 0.8fr);
    gap: 0.75rem;
    align-items: start;
    flex: 0 0 auto;
    min-height: auto;
}

.trace-code-card,
.trace-state-card {
    min-width: 0;
    min-height: 0;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    overflow: hidden;
}

.trace-code-card {
    padding: 0.52rem;
    background: var(--code);
}

.trace-code-line {
    display: grid;
    grid-template-columns: 2rem minmax(0, 1fr);
    gap: 0.65rem;
    padding: 0.28rem 0.4rem;
    border-radius: 6px;
    color: var(--code-line);
    transition: background-color 180ms ease, color 180ms ease;
}

.trace-code-line span {
    color: var(--code-muted);
    text-align: right;
}

.trace-code-line code {
    white-space: pre-wrap;
    overflow-wrap: anywhere;
}

.trace-code-line.active {
    background: rgba(184, 121, 25, 0.34);
    color: var(--code-active-text);
}

.trace-state-card {
    display: grid;
    align-content: start;
    gap: 0.6rem;
    padding: 0.75rem;
    background: var(--surface);
    animation: solution-frame-in 220ms ease;
}

.variable-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(112px, 1fr));
    gap: 0.5rem;
}

.variable-grid p {
    margin: 0;
    color: var(--muted);
}

.variable-chip,
.trace-return {
    min-width: 0;
    padding: 0.58rem;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface-elevated);
}

.variable-chip strong,
.trace-return strong {
    display: block;
    font-family: "Cascadia Code", Consolas, monospace;
    overflow-wrap: anywhere;
}

.trace-return {
    border-color: rgba(111, 61, 255, 0.35);
    background: var(--accent-tint);
}

.trace-controls {
    display: flex;
    justify-content: flex-end;
    gap: 0.65rem;
    flex-wrap: wrap;
}

.solution-pane .solution-walkthrough,
.trace-pane .solution-walkthrough {
    border-color: var(--line);
}

.solution-pane .solution-stage,
.trace-pane .trace-state-card {
    opacity: 1;
    animation: none;
}

.trace-pane .solution-walkthrough {
    grid-template-rows: minmax(0, 1fr);
}

.trace-pane .trace-playground {
    height: calc(100% - 1.5rem);
    margin: 0.75rem;
}

@keyframes solution-frame-in {
    from {
        opacity: 0;
        transform: translateY(6px);
    }

    to {
        opacity: 1;
        transform: translateY(0);
    }
}

.feedback-card {
    display: grid;
    gap: 0.25rem;
    padding: 0.5rem 0.65rem;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface-soft);
}

.feedback-card span {
    color: var(--muted);
    line-height: 1.25;
}

.feedback-card.passed {
    border-color: rgba(197, 255, 50, 0.95);
    background: var(--lime-soft);
}

.feedback-card.failed {
    border-color: rgba(180, 66, 59, 0.38);
    background: var(--danger-soft);
}

.test-row {
    display: grid;
    gap: 0.15rem;
    padding: 0.45rem 0.65rem;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface);
}

.test-row strong {
    line-height: 1.25;
}

.test-row span,
.test-row small {
    color: var(--muted);
    line-height: 1.25;
}

.test-row.passed {
    border-color: rgba(197, 255, 50, 0.95);
    background: var(--lime-soft);
}

.test-row.failed {
    border-color: rgba(180, 66, 59, 0.38);
    background: var(--danger-soft);
}

.result-pass,
.result-idle {
    color: var(--teal);
    font-weight: 900;
}

.result-fail {
    color: var(--red);
    font-weight: 900;
}

.stderr {
    max-height: 220px;
    padding: 0.75rem;
    border-radius: var(--radius);
    background: var(--stderr-bg);
    color: var(--stderr-text);
    white-space: pre-wrap;
}

.hint-stack {
    display: grid;
    gap: 0.55rem;
    margin-top: 0.25rem;
}

.hint-stack p {
    margin: 0;
    padding: 0.7rem;
    border-radius: var(--radius);
    background: var(--warn-soft);
    color: var(--warn-text);
    line-height: 1.4;
}

.loading-band {
    display: grid;
    place-items: center;
    min-height: 40vh;
    color: var(--muted);
}

.status-panel {
    gap: 0.75rem;
    align-content: center;
    justify-self: center;
    width: min(560px, 100%);
    min-height: 0;
    margin: 3rem auto;
    padding: 2rem 1.5rem;
    border: 1px solid var(--line);
    border-radius: var(--radius);
    background: var(--surface);
    color: var(--ink);
    text-align: center;
}

.status-panel h1 {
    margin: 0;
    color: var(--ink);
    font-size: 1.6rem;
}

.status-panel p {
    margin: 0;
}

#blazor-error-ui {
    display: none;
    position: fixed;
    right: 1rem;
    bottom: 1rem;
    z-index: 1000;
    max-width: min(480px, calc(100vw - 2rem));
    padding: 0.85rem 2.4rem 0.85rem 1rem;
    border: 1px solid rgba(180, 66, 59, 0.4);
    border-radius: var(--radius);
    background: var(--danger-soft);
    color: var(--danger-text);
    box-shadow: 0 18px 44px rgba(22, 32, 41, 0.18);
}

#blazor-error-ui .dismiss {
    position: absolute;
    top: 0.55rem;
    right: 0.75rem;
    cursor: pointer;
}

@media (max-width: 980px) {
    .workspace-header,
    .lesson-header,
    .dashboard-grid,
    .lesson-layout,
    .problem-guide,
    .solution-grid,
    .trace-result-grid,
    .stepper-shell,
    .exercise-grid {
        grid-template-columns: 1fr;
    }

    .workspace-header,
    .lesson-header {
        display: grid;
    }

    .lesson-toc {
        position: static;
        display: flex;
        overflow-x: auto;
    }

    .lesson-toc a {
        white-space: nowrap;
    }

    .problem-hero,
    .problem-panel.full {
        grid-column: auto;
    }

    .problem-workspace-head {
        grid-template-columns: 1fr;
        gap: 0.65rem;
    }

    .problem-tabs {
        justify-content: flex-start;
    }

    .test-visual {
        grid-template-columns: 1fr;
    }

    .trace-call-row {
        grid-template-columns: 1fr;
    }

    .reader-window {
        grid-template-columns: 1fr;
    }

    .reader-window > .reader-arrow {
        display: none;
    }

    .reader-page {
        min-height: 0;
        max-height: none;
    }
}

@media (max-width: 720px) {
    .app-nav {
        align-items: flex-start;
        flex-direction: column;
    }

    .nav-links {
        width: 100%;
        justify-content: flex-start;
    }

    .auth-widget {
        justify-content: flex-start;
        width: 100%;
    }

    .auth-input {
        width: auto;
        flex: 1 1 9rem;
    }

    .auth-message {
        text-align: left;
    }

    body:has(.lesson-screen) .app-shell {
        grid-template-columns: 1fr;
        grid-template-rows: auto minmax(0, 1fr);
    }

    body:has(.lesson-screen) .app-nav {
        grid-column: 1;
        grid-row: 1;
        align-items: center;
        flex-direction: row;
        height: auto;
        min-height: 56px;
        padding: 0.45rem 0.7rem;
        border-right: 0;
        border-bottom: 1px solid rgba(255, 255, 255, 0.08);
    }

    body:has(.lesson-screen) .app-main {
        grid-column: 1;
        grid-row: 2;
    }

    body:has(.lesson-screen) .brand-mark {
        width: 34px;
        height: 34px;
        font-size: 0.82rem;
    }

    body:has(.lesson-screen) .brand {
        padding: 0;
    }

    body:has(.lesson-screen) .brand small,
    body:has(.lesson-screen) .nav-links {
        display: none;
    }

    body:has(.lesson-screen) .brand strong {
        font-size: 0.9rem;
    }

    body:has(.lesson-screen) .app-main {
        padding: 0.55rem;
    }

    .lesson-screen {
        gap: 0.5rem;
    }

    .lesson-screen .lesson-header {
        grid-template-columns: 1fr;
        gap: 0.45rem;
    }

    .lesson-screen .back-link,
    .lesson-screen .lesson-header .eyebrow {
        display: none;
    }

    .lesson-screen .lesson-header h1 {
        margin-bottom: 0;
        font-size: 1.35rem;
    }

    .lesson-screen .lesson-header .lede {
        display: none;
    }

    .lesson-screen .progress-meter {
        grid-template-columns: minmax(0, 1fr) auto;
        align-items: center;
        min-width: 0;
        gap: 0.45rem;
    }

    .lesson-screen .progress-meter > div {
        grid-column: 1 / -1;
        height: 7px;
        order: 3;
    }

    .lesson-screen .progress-meter span {
        min-height: 30px;
        padding: 0.25rem 0.5rem;
        font-size: 0.78rem;
    }

    .lesson-screen .progress-meter .quiet-action {
        min-height: 30px;
        padding: 0.25rem 0.55rem;
        font-size: 0.78rem;
    }

    .lesson-reader {
        gap: 0.55rem;
        padding: 0.55rem;
    }

    .reader-toolbar {
        gap: 0.5rem;
    }

    .reader-toolbar strong {
        display: -webkit-box;
        overflow: hidden;
        -webkit-box-orient: vertical;
        -webkit-line-clamp: 1;
    }

    .reader-tab {
        width: 2rem;
        height: 2rem;
    }

    .reader-footer {
        gap: 0.5rem;
    }

    .reader-footer span {
        display: none;
    }

    .reader-footer .quiet-action,
    .reader-footer .primary-action {
        min-height: 36px;
        padding: 0.5rem 0.7rem;
        font-size: 0.84rem;
    }

    .lesson-section {
        padding: 0.65rem;
    }

    .problem-workspace {
        gap: 0.5rem;
    }

    .problem-heading h2 {
        margin-bottom: 0.2rem;
        font-size: 1.05rem;
    }

    .problem-heading p {
        display: none;
    }

    .problem-tab {
        min-height: 31px;
        padding: 0.32rem 0.55rem;
        font-size: 0.78rem;
    }

    .problem-guide {
        gap: 0.55rem;
    }

    .problem-hero,
    .problem-panel {
        padding: 0.65rem;
    }

    .problem-hero h3 {
        display: -webkit-box;
        overflow: hidden;
        font-size: 1rem;
        line-height: 1.25;
        -webkit-box-orient: vertical;
        -webkit-line-clamp: 3;
    }

    .approach-flow,
    .case-grid,
    .step-track {
        grid-template-columns: repeat(2, minmax(0, 1fr));
        gap: 0.45rem;
    }

    .flow-step,
    .track-item,
    .case-tile,
    .metric-tile,
    .test-brief,
    .expected-output {
        padding: 0.52rem;
    }

    .flow-step span,
    .track-item span {
        width: 1.85rem;
        height: 1.85rem;
        font-size: 0.78rem;
    }

    .problem-guide p {
        font-size: 0.8rem;
        line-height: 1.35;
    }

    .section-kicker,
    .problem-hero span,
    .metric-tile span,
    .test-brief span,
    .expected-output span,
    .case-tile span {
        margin-bottom: 0.22rem;
        font-size: 0.66rem;
    }

    .exercise-topline {
        display: grid;
        gap: 0.55rem;
    }

    .exercise-topline .block-heading h2 {
        font-size: 1rem;
    }

    .exercise-topline .block-heading p {
        display: none;
    }

    .exercise-topline .block-heading span {
        margin-bottom: 0.15rem;
    }

    .exercise-tabs {
        justify-content: flex-start;
    }

    .exercise-tab {
        min-height: 32px;
        padding: 0.35rem 0.55rem;
        font-size: 0.82rem;
    }

    .run-bar {
        gap: 0.45rem;
        padding: 0.55rem;
    }

    .run-bar .primary-action,
    .run-bar .quiet-action {
        min-height: 36px;
        padding: 0.5rem 0.65rem;
    }

    .solution-walkthrough {
        gap: 0.5rem;
    }

    .solution-head {
        padding: 0.7rem 0.7rem 0;
    }

    .solution-grid {
        grid-template-rows: auto minmax(0, 1fr);
        gap: 0.55rem;
        padding: 0 0.7rem;
    }

    .solution-code-card {
        min-height: 76px;
        padding: 0.45rem;
    }

    .solution-code-line {
        padding: 0.28rem 0.35rem;
        font-size: 0.82rem;
    }

    .solution-stage {
        gap: 0.55rem;
        padding: 0.7rem;
        background: var(--surface-elevated);
    }

    .solution-frame-copy span {
        color: var(--teal);
    }

    .solution-frame-copy h4 {
        color: var(--ink);
    }

    .solution-frame-copy p {
        display: -webkit-box;
        overflow: hidden;
        color: var(--muted-strong);
        -webkit-box-orient: vertical;
        -webkit-line-clamp: 2;
    }

    .solution-visual-stack {
        gap: 0.45rem;
    }

    .solution-visual-row,
    .solution-state-chip {
        padding: 0.55rem;
    }

    .solution-controls {
        justify-content: space-between;
        gap: 0.45rem;
        padding: 0 0.7rem 0.7rem;
    }

    .solution-controls .primary-action,
    .solution-controls .quiet-action {
        min-height: 36px;
        padding: 0.5rem 0.65rem;
        font-size: 0.84rem;
    }

    .trace-pane .trace-playground {
        height: calc(100% - 1.4rem);
        margin: 0.7rem;
        padding: 0.7rem;
    }

    .lesson-list {
        grid-template-columns: 1fr;
    }

    .status-strip {
        justify-content: flex-start;
    }

    h1 {
        font-size: 2.35rem;
    }

    .trace-call {
        grid-template-columns: 1fr;
    }

    .trace-call code {
        padding: 0.55rem 0.75rem;
    }

    .trace-call input {
        border: 0;
        border-top: 1px solid var(--line);
        border-bottom: 1px solid var(--line);
    }
}

@media (prefers-reduced-motion: reduce) {
    .solution-stage,
    .trace-state-card,
    .reader-page {
        animation: none;
    }

    .solution-code-line,
    .trace-code-line {
        transition: none;
    }

    *,
    *::before,
    *::after {
        animation-duration: 0.01ms !important;
        animation-iteration-count: 1 !important;
        transition-duration: 0.01ms !important;
        scroll-behavior: auto !important;
    }
}

/* Accessibility overrides (WCAG 2.1 AA contrast). */
.problem-hero > div > span,
.block-heading span {
    color: var(--accent-text); /* the accent purple is too light on tinted panels */
}

.solution-code-line code,
.trace-code-line code {
    color: inherit; /* Bootstrap's pink code color fails contrast on the dark code panel */
}