/* ==========================================================================
   힘의 합성과 분해 (Vector Addition & Resolution of Forces)
   Modern Dark Theme Design System & Responsive Layout
   ========================================================================== */

/* --- 1. CSS Variables & Design System --- */
:root {
    /* Background & Surfaces */
    --bg-dark: #090d16;
    --bg-card: rgba(17, 24, 39, 0.88);
    --bg-card-hover: rgba(30, 41, 59, 0.95);
    --bg-input: rgba(15, 23, 42, 0.85);
    --border-color: rgba(255, 255, 255, 0.12);
    --border-glow: rgba(56, 189, 248, 0.35);
    
    /* Vector Color Palette */
    --vector-f1: #38bdf8;          /* F1 (Sky Blue) */
    --vector-f1-glow: rgba(56, 189, 248, 0.4);
    --vector-f2: #f43f5e;          /* F2 (Rose Red) */
    --vector-f2-glow: rgba(244, 63, 94, 0.4);
    --vector-net: #a855f7;         /* Fnet (Purple / Violet) */
    --vector-net-glow: rgba(168, 85, 247, 0.45);
    --vector-f3: #fbbf24;          /* F3 / Equilibrant (Amber Gold) */
    --vector-f3-glow: rgba(251, 191, 36, 0.4);
    --vector-fx: #34d399;          /* Fx (Emerald Green) */
    --vector-fy: #60a5fa;          /* Fy (Soft Blue) */
    --vector-fn: #06b6d4;          /* Normal Force (Cyan) */
    --vector-fric: #f97316;        /* Friction (Orange) */
    
    /* Status & UI Accents */
    --primary: #38bdf8;
    --success: #10b981;
    --warning: #f59e0b;
    --danger: #ef4444;
    --text-main: #f8fafc;
    --text-muted: #94a3b8;
    --text-dim: #64748b;

    /* Typography */
    --font-sans: 'Inter', 'Noto Sans KR', sans-serif;
    --font-display: 'Outfit', sans-serif;
    --font-mono: 'JetBrains Mono', monospace;

    /* Shadows & Radii */
    --shadow-sm: 0 2px 6px rgba(0, 0, 0, 0.35);
    --shadow-md: 0 6px 18px rgba(0, 0, 0, 0.45);
    --shadow-glow: 0 0 20px rgba(56, 189, 248, 0.2);
    --radius-sm: 6px;
    --radius-md: 10px;
    --radius-lg: 14px;
    --radius-full: 9999px;
    --transition: all 0.2s cubic-bezier(0.4, 0, 0.2, 1);
}

/* --- 2. Global Reset & Base Layout --- */
*, *::before, *::after {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

html, body {
    width: 100%;
    height: 100%;
    max-height: 100vh;
    overflow: hidden;
    background-color: var(--bg-dark);
    color: var(--text-main);
    font-family: var(--font-sans);
    font-size: 13.5px;
    line-height: 1.4;
    -webkit-font-smoothing: antialiased;
}

/* Ambient Background Glow */
body::before {
    content: '';
    position: fixed;
    top: -20%;
    left: -10%;
    width: 60vw;
    height: 60vw;
    background: radial-gradient(circle, rgba(56, 189, 248, 0.08) 0%, rgba(168, 85, 247, 0.05) 50%, transparent 70%);
    pointer-events: none;
    z-index: 0;
}

body::after {
    content: '';
    position: fixed;
    bottom: -20%;
    right: -10%;
    width: 60vw;
    height: 60vw;
    background: radial-gradient(circle, rgba(244, 63, 94, 0.06) 0%, rgba(251, 191, 36, 0.04) 50%, transparent 70%);
    pointer-events: none;
    z-index: 0;
}

/* Main Container */
.app-container {
    position: relative;
    z-index: 1;
    display: flex;
    flex-direction: column;
    height: 100vh;
    max-height: 100vh;
    max-width: 1680px;
    margin: 0 auto;
    padding: 10px 16px;
    gap: 8px;
}

/* --- 3. App Header --- */
.app-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 8px 16px;
    background: var(--bg-card);
    backdrop-filter: blur(16px);
    -webkit-backdrop-filter: blur(16px);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-sm);
    flex-shrink: 0;
    gap: 12px;
}

.header-left {
    display: flex;
    align-items: center;
    gap: 12px;
}

.icon-btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: 6px;
    padding: 7px 12px;
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    background: var(--bg-input);
    color: var(--text-main);
    font-family: var(--font-sans);
    font-size: 13px;
    font-weight: 500;
    cursor: pointer;
    transition: var(--transition);
}

.icon-btn:hover {
    background: rgba(255, 255, 255, 0.1);
    border-color: var(--primary);
    transform: translateY(-1px);
}

.title-container {
    display: flex;
    align-items: center;
    gap: 8px;
}

.pulse-dot {
    width: 9px;
    height: 9px;
    border-radius: 50%;
    background: var(--primary);
    box-shadow: 0 0 10px var(--primary);
    animation: pulse 2s infinite;
}

@keyframes pulse {
    0%, 100% { opacity: 1; transform: scale(1); }
    50% { opacity: 0.5; transform: scale(0.85); }
}

.title-container h1 {
    font-family: var(--font-display);
    font-size: 17px;
    font-weight: 700;
    letter-spacing: -0.02em;
    background: linear-gradient(135deg, #ffffff 40%, var(--primary) 100%);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
}

/* Nav Tabs */
.nav-tabs {
    display: flex;
    align-items: center;
    gap: 6px;
    background: rgba(0, 0, 0, 0.35);
    padding: 4px;
    border-radius: var(--radius-md);
    border: 1px solid rgba(255, 255, 255, 0.08);
}

.tab-btn {
    display: inline-flex;
    align-items: center;
    gap: 7px;
    padding: 6px 14px;
    border: none;
    border-radius: var(--radius-sm);
    background: transparent;
    color: var(--text-muted);
    font-family: var(--font-sans);
    font-size: 12.5px;
    font-weight: 600;
    cursor: pointer;
    transition: var(--transition);
}

.tab-btn i {
    font-size: 13px;
}

.tab-btn:hover {
    color: var(--text-main);
    background: rgba(255, 255, 255, 0.05);
}

.tab-btn.active {
    color: #fff;
    background: linear-gradient(135deg, rgba(56, 189, 248, 0.25) 0%, rgba(168, 85, 247, 0.25) 100%);
    border: 1px solid rgba(56, 189, 248, 0.4);
    box-shadow: 0 2px 10px rgba(56, 189, 248, 0.15);
}

.principle-btn {
    background: linear-gradient(135deg, rgba(56, 189, 248, 0.15), rgba(168, 85, 247, 0.15));
    border-color: rgba(56, 189, 248, 0.3);
    color: var(--primary);
    font-weight: 600;
}

.principle-btn:hover {
    background: linear-gradient(135deg, rgba(56, 189, 248, 0.25), rgba(168, 85, 247, 0.25));
    color: #fff;
}

/* --- 4. Main Content Layout --- */
.main-content {
    flex: 1;
    position: relative;
    min-height: 0;
    display: flex;
    overflow: hidden;
}

.tab-pane {
    display: none;
    width: 100%;
    height: 100%;
}

.tab-pane.active {
    display: block;
}

.simulation-layout {
    display: grid;
    grid-template-columns: 1fr 370px;
    gap: 12px;
    width: 100%;
    height: 100%;
    min-height: 0;
}

/* --- 5. Canvas Container Wrapper --- */
.canvas-container-wrapper {
    display: flex;
    flex-direction: column;
    background: var(--bg-card);
    backdrop-filter: blur(16px);
    -webkit-backdrop-filter: blur(16px);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-lg);
    padding: 10px 14px;
    gap: 8px;
    min-height: 0;
    box-shadow: var(--shadow-sm);
}

.canvas-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 10px;
    flex-shrink: 0;
    flex-wrap: wrap;
}

.status-indicator {
    display: flex;
    align-items: center;
    gap: 8px;
    flex-wrap: wrap;
}

.badge {
    display: inline-flex;
    align-items: center;
    gap: 5px;
    padding: 4px 10px;
    border-radius: var(--radius-full);
    font-size: 12px;
    font-weight: 600;
    font-family: var(--font-mono);
    background: rgba(255, 255, 255, 0.07);
    color: var(--text-main);
    border: 1px solid rgba(255, 255, 255, 0.12);
}

.badge.net-badge {
    background: rgba(168, 85, 247, 0.2);
    color: #d8b4fe;
    border-color: rgba(168, 85, 247, 0.4);
}

.badge.dir-badge {
    background: rgba(56, 189, 248, 0.15);
    color: #7dd3fc;
    border-color: rgba(56, 189, 248, 0.35);
}

.badge.angle-badge {
    background: rgba(251, 191, 36, 0.15);
    color: #fcd34d;
    border-color: rgba(251, 191, 36, 0.35);
}

.badge.purple {
    background: rgba(168, 85, 247, 0.2);
    color: #d8b4fe;
    border-color: rgba(168, 85, 247, 0.4);
}

.badge.green, .badge.success {
    background: rgba(16, 185, 129, 0.2);
    color: #6ee7b7;
    border-color: rgba(168, 85, 247, 0.4);
}

.badge.warning {
    background: rgba(245, 158, 11, 0.2);
    color: #fcd34d;
    border-color: rgba(245, 158, 11, 0.4);
}

.canvas-actions {
    display: flex;
    align-items: center;
    gap: 6px;
}

.sm-btn {
    display: inline-flex;
    align-items: center;
    gap: 5px;
    padding: 5px 10px;
    border-radius: var(--radius-sm);
    background: var(--bg-input);
    border: 1px solid var(--border-color);
    color: var(--text-muted);
    font-size: 12px;
    font-weight: 500;
    cursor: pointer;
    transition: var(--transition);
}

.sm-btn:hover {
    background: rgba(255, 255, 255, 0.1);
    color: var(--text-main);
    border-color: var(--primary);
}

.sm-btn.active {
    background: rgba(56, 189, 248, 0.2);
    border-color: var(--primary);
    color: #fff;
}

.sm-btn.auto-balance-btn {
    background: linear-gradient(135deg, rgba(251, 191, 36, 0.2), rgba(16, 185, 129, 0.2));
    border-color: rgba(251, 191, 36, 0.4);
    color: #fde047;
    font-weight: 600;
}

.sm-btn.auto-balance-btn:hover {
    background: linear-gradient(135deg, rgba(251, 191, 36, 0.35), rgba(16, 185, 129, 0.35));
    color: #fff;
}

.sm-btn.play-btn {
    background: rgba(16, 185, 129, 0.2);
    border-color: rgba(16, 185, 129, 0.4);
    color: #6ee7b7;
    font-weight: 600;
}

.sm-btn.play-btn:hover {
    background: rgba(16, 185, 129, 0.35);
    color: #fff;
}

.sm-btn.play-btn.playing {
    background: rgba(244, 63, 94, 0.2);
    border-color: rgba(244, 63, 94, 0.4);
    color: #fda4af;
}

/* Sub-mode Tab Buttons in Tab 4 */
.sub-mode-tabs {
    display: flex;
    align-items: center;
    gap: 4px;
    background: rgba(0, 0, 0, 0.4);
    padding: 3px;
    border-radius: var(--radius-sm);
    border: 1px solid rgba(255, 255, 255, 0.08);
}

.sub-tab-btn {
    display: inline-flex;
    align-items: center;
    gap: 5px;
    padding: 4px 10px;
    border: none;
    border-radius: 4px;
    background: transparent;
    color: var(--text-muted);
    font-size: 11.5px;
    font-weight: 600;
    cursor: pointer;
    transition: var(--transition);
}

.sub-tab-btn:hover {
    color: #fff;
    background: rgba(255, 255, 255, 0.06);
}

.sub-tab-btn.active {
    color: #fff;
    background: var(--primary);
    background: linear-gradient(135deg, #0284c7 0%, #38bdf8 100%);
    box-shadow: 0 2px 6px rgba(56, 189, 248, 0.3);
}

/* Canvas Viewport */
.canvas-viewport {
    position: relative;
    flex: 1;
    width: 100%;
    min-height: 0;
    background: radial-gradient(circle at center, #0f172a 0%, #080d1a 100%);
    border-radius: var(--radius-md);
    overflow: hidden;
    border: 1px solid rgba(255, 255, 255, 0.08);
}

.canvas-viewport canvas {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    display: block;
    cursor: default;
    touch-action: none;
    user-select: none;
    -webkit-user-select: none;
}

/* Canvas Data Chips Overlay */
.canvas-overlay-data {
    position: absolute;
    top: 10px;
    left: 10px;
    display: flex;
    flex-direction: column;
    gap: 6px;
    pointer-events: none;
    z-index: 10;
}

.data-chip {
    display: inline-flex;
    align-items: center;
    gap: 8px;
    padding: 5px 10px;
    border-radius: var(--radius-sm);
    background: rgba(15, 23, 42, 0.85);
    backdrop-filter: blur(10px);
    -webkit-backdrop-filter: blur(10px);
    border: 1px solid rgba(255, 255, 255, 0.1);
    font-size: 12px;
    font-family: var(--font-mono);
    color: var(--text-main);
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.3);
}

.color-dot {
    width: 8px;
    height: 8px;
    border-radius: 50%;
    flex-shrink: 0;
}

.f1-dot { background: var(--vector-f1); box-shadow: 0 0 6px var(--vector-f1); }
.f2-dot { background: var(--vector-f2); box-shadow: 0 0 6px var(--vector-f2); }
.f3-dot { background: var(--vector-f3); box-shadow: 0 0 6px var(--vector-f3); }
.net-dot { background: var(--vector-net); box-shadow: 0 0 8px var(--vector-net); }

.data-chip.f1-chip { border-left: 3px solid var(--vector-f1); }
.data-chip.f2-chip { border-left: 3px solid var(--vector-f2); }
.data-chip.fnet-chip { border-left: 3px solid var(--vector-net); background: rgba(168, 85, 247, 0.15); }
.data-chip.eq1-chip { border-left: 3px solid var(--vector-f1); }
.data-chip.eq2-chip { border-left: 3px solid var(--vector-f2); }
.data-chip.eq3-chip { border-left: 3px solid var(--vector-f3); }
.data-chip.fmain-chip { border-left: 3px solid var(--vector-net); }
.data-chip.fx-chip { border-left: 3px solid var(--vector-fx); }
.data-chip.fy-chip { border-left: 3px solid var(--vector-fy); }

.data-chip strong {
    color: #fff;
    font-weight: 700;
}

/* Interactive Tip */
.interactive-tip {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 7px 12px;
    background: rgba(56, 189, 248, 0.08);
    border: 1px dashed rgba(56, 189, 248, 0.3);
    border-radius: var(--radius-sm);
    color: #bae6fd;
    font-size: 12px;
    flex-shrink: 0;
}

.interactive-tip i {
    color: var(--primary);
    font-size: 14px;
    flex-shrink: 0;
}

.interactive-tip strong {
    color: #fff;
    font-weight: 600;
}

/* --- 6. Control Sidebar --- */
.control-sidebar {
    display: flex;
    flex-direction: column;
    gap: 10px;
    overflow-y: auto;
    padding-right: 4px;
    min-height: 0;
}

/* Custom Scrollbar for Sidebar */
.control-sidebar::-webkit-scrollbar {
    width: 5px;
}

.control-sidebar::-webkit-scrollbar-track {
    background: rgba(0, 0, 0, 0.2);
    border-radius: 4px;
}

.control-sidebar::-webkit-scrollbar-thumb {
    background: rgba(255, 255, 255, 0.15);
    border-radius: 4px;
}

.control-sidebar::-webkit-scrollbar-thumb:hover {
    background: rgba(255, 255, 255, 0.3);
}

/* Control Card */
.control-card {
    background: var(--bg-card);
    backdrop-filter: blur(16px);
    -webkit-backdrop-filter: blur(16px);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-lg);
    padding: 12px 14px;
    display: flex;
    flex-direction: column;
    gap: 10px;
    box-shadow: var(--shadow-sm);
    transition: var(--transition);
}

.control-card:hover {
    border-color: rgba(255, 255, 255, 0.2);
}

.control-card.compact-card {
    padding: 10px 12px;
    gap: 8px;
}

.card-title {
    display: flex;
    align-items: center;
    justify-content: space-between;
    font-size: 13px;
    font-weight: 700;
    color: #fff;
    border-bottom: 1px solid rgba(255, 255, 255, 0.08);
    padding-bottom: 6px;
}

.card-title span i {
    margin-right: 6px;
    color: var(--primary);
}

.card-title.f1-title span i { color: var(--vector-f1); }
.card-title.f2-title span i { color: var(--vector-f2); }
.card-title.f3-title span i { color: var(--vector-f3); }

.val-badge {
    font-family: var(--font-mono);
    font-size: 11px;
    padding: 2px 8px;
    border-radius: var(--radius-full);
    background: rgba(255, 255, 255, 0.1);
    color: var(--text-main);
}

.val-badge.f1-bg { background: rgba(56, 189, 248, 0.2); color: #7dd3fc; border: 1px solid rgba(56, 189, 248, 0.3); }
.val-badge.f2-bg { background: rgba(244, 63, 94, 0.2); color: #fda4af; border: 1px solid rgba(244, 63, 94, 0.3); }
.val-badge.f3-bg { background: rgba(251, 191, 36, 0.2); color: #fde047; border: 1px solid rgba(251, 191, 36, 0.3); }
.val-badge.purple { background: rgba(168, 85, 247, 0.2); color: #d8b4fe; border: 1px solid rgba(168, 85, 247, 0.3); }
.val-badge.gold { background: rgba(251, 191, 36, 0.2); color: #fde047; border: 1px solid rgba(251, 191, 36, 0.3); }

/* Control Group & Sliders */
.control-group {
    display: flex;
    flex-direction: column;
    gap: 5px;
}

.control-label {
    display: flex;
    align-items: center;
    justify-content: space-between;
    font-size: 12px;
    color: var(--text-muted);
}

.val-sub {
    font-family: var(--font-mono);
    font-size: 11.5px;
    color: #fff;
    font-weight: 600;
}

.val-sub.gold { color: #fcd34d; }

.slider-container {
    width: 100%;
    display: flex;
    align-items: center;
}

input[type="range"] {
    -webkit-appearance: none;
    appearance: none;
    width: 100%;
    height: 5px;
    background: rgba(255, 255, 255, 0.15);
    border-radius: 3px;
    outline: none;
    cursor: pointer;
    transition: var(--transition);
}

input[type="range"]:hover {
    background: rgba(255, 255, 255, 0.25);
}

input[type="range"]::-webkit-slider-thumb {
    -webkit-appearance: none;
    appearance: none;
    width: 15px;
    height: 15px;
    border-radius: 50%;
    background: #fff;
    border: 2px solid var(--primary);
    box-shadow: 0 0 8px var(--primary);
    cursor: pointer;
    transition: transform 0.1s ease;
}

input[type="range"]::-webkit-slider-thumb:hover {
    transform: scale(1.2);
}

/* Dual Slider Row for Compact Cards */
.dual-slider-row {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 10px;
}

.slider-item {
    display: flex;
    flex-direction: column;
    gap: 4px;
}

.slider-item label {
    font-size: 11px;
    color: var(--text-muted);
}

/* Preset Buttons Grid */
.preset-grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 6px;
}

.preset-btn {
    padding: 6px 8px;
    background: var(--bg-input);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-sm);
    color: var(--text-muted);
    font-size: 11.5px;
    font-weight: 500;
    cursor: pointer;
    text-align: center;
    transition: var(--transition);
    white-space: nowrap;
    overflow: hidden;
    text-overflow: ellipsis;
}

.preset-btn:hover {
    background: rgba(255, 255, 255, 0.08);
    color: #fff;
    border-color: rgba(255, 255, 255, 0.3);
}

.preset-btn.active {
    background: rgba(56, 189, 248, 0.18);
    border-color: var(--primary);
    color: #fff;
    font-weight: 600;
    box-shadow: 0 0 10px rgba(56, 189, 248, 0.15);
}

/* Formula & Math Calculation Cards */
.formula-card {
    background: linear-gradient(180deg, rgba(17, 24, 39, 0.95) 0%, rgba(15, 23, 42, 0.95) 100%);
    border-color: rgba(168, 85, 247, 0.25);
}

.vector-calc-table, .trig-formula-box, .equil-sum-table, .rw-calc-box {
    display: flex;
    flex-direction: column;
    gap: 5px;
    font-size: 12px;
}

.calc-row, .trig-row, .sum-row, .rw-row {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 4px 6px;
    border-radius: 4px;
    background: rgba(0, 0, 0, 0.25);
}

.calc-row.highlight, .trig-row.check-row, .sum-row.total, .rw-row.total {
    background: rgba(168, 85, 247, 0.15);
    border: 1px solid rgba(168, 85, 247, 0.3);
    font-weight: 600;
}

.calc-row code, .trig-row code {
    font-family: var(--font-mono);
    color: #a5f3fc;
    font-size: 11.5px;
}

.calc-formula {
    text-align: center;

    margin-top: 4px;
    padding-top: 4px;
    border-top: 1px dashed rgba(255, 255, 255, 0.1);
    color: var(--text-muted);
    font-family: var(--font-mono);
}

.fx-name { color: var(--vector-fx); }
.fy-name { color: var(--vector-fy); }

.red { color: #f87171 !important; }
.blue { color: #60a5fa !important; }
.green { color: #34d399 !important; }
.gold { color: #fbbf24 !important; }
.purple { color: #c084fc !important; }

.tip-text {
    font-size: 10.5px;
    color: #fb923c;
    margin-top: 4px;
    display: block;
}

/* Reset Button */
.reset-btn {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 6px;
    padding: 8px 12px;
    background: rgba(255, 255, 255, 0.05);
    border: 1px solid rgba(255, 255, 255, 0.1);
    border-radius: var(--radius-md);
    color: var(--text-muted);
    font-size: 12px;
    font-weight: 600;
    cursor: pointer;
    transition: var(--transition);
    margin-top: auto;
}

.reset-btn:hover {
    background: rgba(255, 255, 255, 0.1);
    color: #fff;
    border-color: rgba(255, 255, 255, 0.25);
}

/* Toggle Mode Group (Pull / Push) */
.toggle-mode-group {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 4px;
    background: rgba(0, 0, 0, 0.3);
    padding: 3px;
    border-radius: var(--radius-sm);
}

.mode-select-btn {
    padding: 5px 8px;
    border: none;
    border-radius: 4px;
    background: transparent;
    color: var(--text-muted);
    font-size: 12px;
    font-weight: 600;
    cursor: pointer;
    transition: var(--transition);
}

.mode-select-btn.active {
    background: var(--primary);
    color: #fff;
}

/* Sub Panels in Tab 4 */
.sub-panel {
    display: none;
    flex-direction: column;
    gap: 10px;
}

.sub-panel.active {
    display: flex;
}

/* --- 7. Theory / Principle Modal --- */
.modal-backdrop {
    position: fixed;
    top: 0;
    left: 0;
    width: 100vw;
    height: 100vh;
    background: rgba(0, 0, 0, 0.75);
    backdrop-filter: blur(8px);
    -webkit-backdrop-filter: blur(8px);
    z-index: 1000;
    display: flex;
    align-items: center;
    justify-content: center;
    opacity: 0;
    visibility: hidden;
    transition: opacity 0.25s ease, visibility 0.25s ease;
    padding: 20px;
}

.modal-backdrop.open {
    opacity: 1;
    visibility: visible;
}

.modal-container {
    background: #0f172a;
    border: 1px solid rgba(255, 255, 255, 0.15);
    border-radius: var(--radius-lg);
    box-shadow: 0 20px 50px rgba(0, 0, 0, 0.6), 0 0 30px rgba(56, 189, 248, 0.15);
    width: 100%;
    max-width: 820px;
    max-height: 88vh;
    display: flex;
    flex-direction: column;
    overflow: hidden;
    transform: scale(0.95);
    transition: transform 0.25s ease;
}

.modal-backdrop.open .modal-container {
    transform: scale(1);
}

.modal-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 14px 20px;
    background: rgba(17, 24, 39, 0.95);
    border-bottom: 1px solid var(--border-color);
}

.modal-title {
    display: flex;
    align-items: center;
    gap: 10px;
    color: #fff;
}

.modal-title i {
    color: var(--primary);
    font-size: 18px;
}

.modal-title h2 {
    font-size: 17px;
    font-weight: 700;
    font-family: var(--font-display);
}

.close-modal-btn {
    padding: 6px;
    border-radius: 50%;
    width: 32px;
    height: 32px;
}

.modal-body {
    flex: 1;
    overflow-y: auto;
    padding: 20px 24px;
    display: flex;
    flex-direction: column;
    gap: 20px;
    color: #cbd5e1;
    font-size: 13.5px;
    line-height: 1.6;
}

/* Custom Scrollbar for Modal Body */
.modal-body::-webkit-scrollbar {
    width: 6px;
}

.modal-body::-webkit-scrollbar-track {
    background: rgba(0, 0, 0, 0.2);
}

.modal-body::-webkit-scrollbar-thumb {
    background: rgba(255, 255, 255, 0.2);
    border-radius: 4px;
}

.theory-section {
    display: flex;
    flex-direction: column;
    gap: 10px;
}

.section-title {
    font-size: 15px;
    font-weight: 700;
    color: #fff;
    display: flex;
    align-items: center;
    gap: 8px;
    border-bottom: 1px solid rgba(255, 255, 255, 0.1);
    padding-bottom: 6px;
}

.section-title i {
    color: var(--primary);
}

.theory-grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 12px;
}

.theory-card {
    background: rgba(255, 255, 255, 0.04);
    border: 1px solid rgba(255, 255, 255, 0.08);
    border-radius: var(--radius-md);
    padding: 12px 14px;
}

.theory-card h4 {
    font-size: 13.5px;
    font-weight: 700;
    color: var(--primary);
    margin-bottom: 6px;
}

.theory-card ul {
    margin-left: 18px;
    margin-top: 6px;
}

.formula-highlight-box {
    background: linear-gradient(135deg, rgba(56, 189, 248, 0.08) 0%, rgba(168, 85, 247, 0.08) 100%);
    border: 1px solid rgba(56, 189, 248, 0.25);
    border-radius: var(--radius-md);
    padding: 12px 16px;
}

.formula-highlight-box ul {
    margin-left: 18px;
    margin-top: 8px;
}

.math-eq {
    font-family: var(--font-mono);
    font-size: 14px;
    color: #38bdf8;
    background: rgba(0, 0, 0, 0.4);
    padding: 8px 12px;
    border-radius: 6px;
    margin: 8px 0;
    text-align: center;
    font-weight: 600;
    border: 1px solid rgba(56, 189, 248, 0.2);
}

.modal-footer {
    display: flex;
    align-items: center;
    justify-content: flex-end;
    padding: 12px 20px;
    background: rgba(17, 24, 39, 0.95);
    border-top: 1px solid var(--border-color);
}

.primary-modal-btn {
    padding: 8px 18px;
    background: linear-gradient(135deg, #0284c7 0%, #38bdf8 100%);
    border: none;
    border-radius: var(--radius-md);
    color: #fff;
    font-family: var(--font-sans);
    font-size: 13.5px;
    font-weight: 600;
    cursor: pointer;
    box-shadow: 0 4px 12px rgba(56, 189, 248, 0.3);
    transition: var(--transition);
}

.primary-modal-btn:hover {
    transform: translateY(-1px);
    box-shadow: 0 6px 16px rgba(56, 189, 248, 0.45);
}

/* --- 8. Mobile & Responsive Design Optimization --- */

/* Base touch tap optimization */
button, 
.icon-btn, 
.tab-btn, 
.sub-tab-btn, 
.sm-btn, 
.preset-btn, 
.mode-select-btn, 
.reset-btn {
    touch-action: manipulation;
    -webkit-tap-highlight-color: transparent;
}

/* --- Tablet & Medium Screens (<= 1024px) --- */
@media (max-width: 1024px) {
    html, body {
        height: auto !important;
        min-height: 100% !important;
        max-height: none !important;
        overflow-x: hidden !important;
        overflow-y: auto !important;
    }

    .app-container {
        height: auto !important;
        max-height: none !important;
        min-height: 100vh;
        overflow: visible !important;
        padding: 8px max(12px, env(safe-area-inset-right)) max(32px, env(safe-area-inset-bottom)) max(12px, env(safe-area-inset-left));
        gap: 10px;
    }

    .main-content {
        height: auto !important;
        min-height: 0;
        overflow: visible !important;
        flex: none !important;
    }

    .tab-pane {
        height: auto !important;
    }

    .simulation-layout {
        display: flex !important;
        flex-direction: column !important;
        grid-template-columns: none !important;
        grid-template-rows: none !important;
        height: auto !important;
        min-height: 0 !important;
        overflow: visible !important;
        gap: 12px;
    }

    .canvas-container-wrapper {
        width: 100%;
        height: 440px;
        min-height: 380px;
        flex-shrink: 0;
    }

    .control-sidebar {
        width: 100% !important;
        height: auto !important;
        max-height: none !important;
        overflow: visible !important;
        padding-right: 0;
    }
}

/* --- Mobile Devices (<= 768px) --- */
@media (max-width: 768px) {
    /* 2-row header: Row 1 = Left(Back+Title) & Right(설명), Row 2 = Nav tabs */
    .app-header {
        display: grid;
        grid-template-areas:
            "left right"
            "nav nav";
        grid-template-columns: 1fr auto;
        padding: 8px 12px;
        gap: 8px;
        align-items: center;
    }

    .header-left {
        grid-area: left;
        min-width: 0;
        gap: 8px;
    }

    .header-left .title-container h1 {
        font-size: 15px;
    }

    .header-right {
        grid-area: right;
    }

    .principle-btn {
        padding: 6px 12px;
        font-size: 12px;
    }

    /* Full-width touch-scrollable navigation bar */
    .nav-tabs {
        grid-area: nav;
        width: 100%;
        display: flex;
        overflow-x: auto;
        -webkit-overflow-scrolling: touch;
        scrollbar-width: none;
        -ms-overflow-style: none;
        padding: 4px;
        gap: 5px;
    }

    .nav-tabs::-webkit-scrollbar {
        display: none;
    }

    .tab-btn {
        flex-shrink: 0;
        white-space: nowrap;
        padding: 7px 11px;
        font-size: 12px;
    }

    .tab-btn span {
        font-size: 11.5px;
    }

    /* Canvas wrapper & header */
    .canvas-container-wrapper {
        height: 420px;
        min-height: 360px;
        padding: 10px 12px;
    }

    .canvas-header {
        gap: 6px;
        margin-bottom: 4px;
        flex-shrink: 0;
    }

    .badge {
        font-size: 10.5px;
        padding: 3px 6px;
        min-height: 26px;
        box-sizing: border-box;
        display: inline-flex;
        align-items: center;
        justify-content: center;
        white-space: nowrap;
    }

    .sm-btn {
        font-size: 10.5px;
        padding: 3px 6px;
        min-height: 26px;
        box-sizing: border-box;
        display: inline-flex;
        align-items: center;
        justify-content: center;
        white-space: nowrap;
    }

    /* Tab 1: 두 힘의 합성 - 모바일 정확히 2줄 2열 그리드 */
    #tab-compose .canvas-header {
        display: grid;
        grid-template-columns: 1fr 1fr;
        gap: 6px;
        align-items: stretch;
    }

    #tab-compose .status-indicator {
        display: contents;
    }

    #tab-compose #c-net-badge {
        grid-column: 1;
        grid-row: 1;
        width: 100%;
    }

    #tab-compose #c-dir-badge {
        grid-column: 2;
        grid-row: 1;
        width: 100%;
    }

    #tab-compose #c-angle-badge {
        grid-column: 1;
        grid-row: 2;
        width: 100%;
    }

    #tab-compose .canvas-actions {
        grid-column: 2;
        grid-row: 2;
        display: flex;
        margin: 0;
        padding: 0;
    }

    #tab-compose .canvas-actions .sm-btn {
        width: 100%;
        justify-content: center;
    }

    /* Tab 2: 힘의 성분 분해 - 모바일 정확히 2줄 그리드 */
    #tab-decompose .canvas-header {
        display: grid;
        grid-template-columns: 1fr 1fr;
        gap: 6px;
        align-items: stretch;
    }

    #tab-decompose .status-indicator {
        display: contents;
    }

    #tab-decompose #d-mag-badge {
        grid-column: 1;
        grid-row: 1;
        width: 100%;
    }

    #tab-decompose #d-angle-badge {
        grid-column: 2;
        grid-row: 1;
        width: 100%;
    }

    #tab-decompose #d-ratio-badge {
        grid-column: 1 / -1;
        grid-row: 2;
        width: 100%;
        justify-content: center;
    }

    /* Tab 3: 세 힘의 평형 - 모바일 정확히 2줄 레이아웃 (상단 상태 2개, 하단 버튼 2개) */
    #tab-equil .canvas-header {
        display: flex;
        flex-direction: column;
        gap: 6px;
        width: 100%;
    }

    #tab-equil .status-indicator {
        display: grid;
        grid-template-columns: 1fr 1fr;
        gap: 6px;
        width: 100%;
    }

    #tab-equil .status-indicator .badge {
        width: 100%;
        justify-content: center;
        text-align: center;
    }

    #tab-equil .canvas-actions {
        display: grid;
        grid-template-columns: 1fr 1fr;
        gap: 6px;
        width: 100%;
    }

    #tab-equil .canvas-actions .sm-btn {
        width: 100%;
        justify-content: center;
        text-align: center;
    }

    /* Tab 4: 실생활 역학 응용 - 모바일 정확히 2줄 레이아웃 */
    #tab-realworld .canvas-header {
        display: flex;
        flex-direction: column;
        gap: 6px;
        width: 100%;
    }

    #tab-realworld #rw-canvas-actions {
        display: grid;
        grid-template-columns: 1fr 1fr;
        gap: 6px;
        width: 100%;
    }

    #tab-realworld #rw-canvas-actions .sm-btn {
        width: 100%;
        justify-content: center;
        text-align: center;
    }

    /* Sub-mode tabs in Tab 4 */
    .sub-mode-tabs {
        width: 100%;
        overflow-x: auto;
        -webkit-overflow-scrolling: touch;
        scrollbar-width: none;
        padding: 3px;
    }

    .sub-mode-tabs::-webkit-scrollbar {
        display: none;
    }

    .sub-tab-btn {
        flex-shrink: 0;
        white-space: nowrap;
        padding: 6px 10px;
        font-size: 11.5px;
    }

    /* Interactive touch sliders */
    input[type="range"] {
        height: 7px;
        padding: 6px 0;
    }

    input[type="range"]::-webkit-slider-thumb {
        width: 20px;
        height: 20px;
        box-shadow: 0 0 10px rgba(56, 189, 248, 0.6);
    }

    input[type="range"]::-moz-range-thumb {
        width: 20px;
        height: 20px;
        border-radius: 50%;
        background: #fff;
        border: 2px solid var(--primary);
    }

    /* Principle modal on mobile */
    .modal-backdrop {
        padding: 12px;
    }

    .modal-container {
        max-height: 90vh;
        border-radius: var(--radius-md);
    }

    .modal-header {
        padding: 12px 16px;
    }

    .modal-title h2 {
        font-size: 15px;
    }

    .modal-body {
        padding: 16px 16px;
        gap: 16px;
        font-size: 13px;
    }

    .theory-grid {
        grid-template-columns: 1fr;
        gap: 10px;
    }
}

/* --- Compact Mobile Phones (<= 480px) --- */
@media (max-width: 480px) {
    .app-container {
        padding: 6px max(6px, env(safe-area-inset-right)) max(28px, env(safe-area-inset-bottom)) max(6px, env(safe-area-inset-left));
        gap: 8px;
    }

    .app-header {
        padding: 6px 8px;
        gap: 6px;
        border-radius: var(--radius-md);
    }

    .header-left {
        gap: 6px;
    }

    .header-left .title-container h1 {
        font-size: 14px;
    }

    .back-btn {
        padding: 5px 8px;
        font-size: 11.5px;
    }

    .back-btn span {
        display: none; /* Icon-only on tiny screens */
    }

    .principle-btn {
        padding: 5px 8px;
        font-size: 11.5px;
    }

    .tab-btn {
        padding: 6px 9px;
        font-size: 11px;
        gap: 5px;
    }

    .tab-btn i {
        font-size: 11px;
    }

    .canvas-container-wrapper {
        height: 390px;
        min-height: 340px;
        padding: 8px 8px;
        border-radius: var(--radius-md);
    }

    /* 1. Hide top-left overlay chips in Tab 1 (두 힘의 합성), Tab 2 (힘의 성분 분해), Tab 4 on mobile */
    #tab-compose .canvas-overlay-data,
    #tab-decompose .canvas-overlay-data,
    #tab-realworld .canvas-overlay-data {
        display: none !important;
    }

    /* 2. Tab 3 (세 힘의 평형): Ultra-compact mini HUD card that NEVER covers the simulation */
    #tab-equil .canvas-overlay-data {
        display: flex !important;
        position: absolute;
        top: 6px;
        left: 6px;
        flex-direction: column;
        gap: 2px;
        padding: 3px 6px;
        background: rgba(15, 23, 42, 0.90);
        backdrop-filter: blur(8px);
        -webkit-backdrop-filter: blur(8px);
        border: 1px solid rgba(255, 255, 255, 0.12);
        border-radius: 5px;
        box-shadow: 0 2px 6px rgba(0, 0, 0, 0.4);
        pointer-events: none;
        z-index: 10;
        width: fit-content;
        max-width: none;
    }

    #tab-equil .data-chip {
        display: flex !important;
        align-items: center !important;
        gap: 3.5px !important;
        padding: 0 !important;
        background: transparent !important;
        border: none !important;
        box-shadow: none !important;
        font-size: 8.5px !important;
        font-family: var(--font-mono) !important;
        color: var(--text-main) !important;
        line-height: 1.2 !important;
        white-space: nowrap !important;
    }

    #tab-equil .data-chip .color-dot {
        width: 4px !important;
        height: 4px !important;
        flex-shrink: 0 !important;
    }

    #tab-equil .data-chip strong,
    #tab-equil .data-chip b {
        font-size: 8.5px !important;
    }

    .interactive-tip {
        font-size: 11px;
        padding: 6px 8px;
    }

    .control-card {
        padding: 10px 10px;
        border-radius: var(--radius-md);
    }

    .card-title {
        font-size: 12px;
    }

    /* Stack dual sliders vertically on small mobile for comfortable thumb dragging */
    .dual-slider-row {
        grid-template-columns: 1fr;
        gap: 8px;
    }

    .preset-grid {
        grid-template-columns: 1fr 1fr;
        gap: 5px;
    }

    .preset-btn {
        font-size: 11px;
        padding: 7px 4px;
        min-height: 34px;
    }

    .trig-formula-box {
        padding: 8px 10px;
        font-size: 11.5px;
    }

    .math-eq {
        font-size: 12px;
        padding: 6px 8px;
    }
}

/* --- Mobile Landscape (Height <= 520px) --- */
@media (max-height: 520px) and (orientation: landscape) {
    .app-container {
        padding: 4px 8px 20px;
    }

    .canvas-container-wrapper {
        height: 290px;
        min-height: 240px;
    }

    .modal-backdrop {
        padding: 6px;
    }

    .modal-container {
        max-height: 94vh;
    }

    .modal-body {
        padding: 10px 14px;
        gap: 12px;
    }
}