:root {
  --navy: #2c3e50;
  --softgray: #ecf0f1;
  --teal: #1abc9c;
  --gold: #f1c40f;
  --slate: #7f8c8d;
  --coral: #e74c3c;
  --white: #ffffff;
  --gray-100: #f7fafc;
  --gray-200: #edf2f7;
  --gray-300: #e2e8f0;
  --max-width: 1120px;
  --container-pad: 16px;
  --radius: 10px;
  --shadow-sm: 0 6px 18px rgba(0, 0, 0, 0.06);
  --shadow-xs: 0 1px 2px rgba(0, 0, 0, 0.06);
}

* {
  box-sizing: border-box;
}

html,
body {
  height: 100%;
}

body {
  margin: 0;
  font-family: "Montserrat", system-ui, -apple-system, "Segoe UI", Roboto,
    "Helvetica Neue", Arial;
  color: var(--navy);
  background: linear-gradient(135deg, #ecf0f1 0%, #d6dbdf 100%);
  padding: 16px;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

/* Responsive container */
.container {
  max-width: var(--max-width);
  margin: 0 auto;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  gap: 18px;
  padding: 18px;
}

li {
  line-height: 2; 
}

/* Page header */
.header-left{
  justify-self: start;
}

.page-header {
    text-align: center;
    color: var(--navy);
    padding: 1rem;
    position: relative;
}

.header-grid{
  display: grid;
  grid-template-columns: 1fr auto 1fr; 
  align-items: center;
  gap: 12px;
  padding: 8px 0;
}
.page-title{
  margin: 0;
  justify-self: center;   /* ensures exact centering of the title cell */
  font-size: 2rem;
  font-weight: 700;
  font-family: 'Montserrat', sans-serif;
}
.header-actions{
  justify-self: end;
}

/* mobile: stack nicely */
@media (max-width: 767px){
  .header-grid{
    grid-template-columns: 1fr;
    grid-auto-flow: row;
    text-align: left;
  }
  .page-title{ justify-self: start; }
  .header-actions{ justify-self: start; width:100%; margin-top:8px; }
  .quiz-btn{ width:100%; max-width:420px; }
}
.cols {
  display: flex;
  flex-direction: column;
  gap: 24px;
}

/* Button styles */
.teal-btn {
  background-color: rgba(26, 188, 156, 0.8);
  color: white;
}

.teal-btn:hover {
    background-color: rgba(26, 188, 156, 0.7);
    transform: translateY(-2px);
}

.gold-btn {
  background-color: rgba(241, 196, 15, 1);
  color: var(--navy); opacity:0.9;
}

.gold-btn:hover {
  background-color: rgba(241, 196, 15, 0.5);
  transform: translateY(-2px);
}

.coral-btn {
  background-color: rgba(231, 76, 60, 0.8);
  color: white;
}

.coral-btn:hover {
  background-color: rgba(231, 76, 60, 0.7);
  transform: translateY(-2px);
}

.navy-btn {
  background-color: rgba(44, 62, 80, 0.8);
  color: white;
}

.navy-btn:hover {
    background-color: rgba(44, 62, 80, 0.7);
    transform: translateY(-2px);
}

.teal-btn:active,
.gold-btn:active,
.coral-btn:active,
.navy-btn:active {
  transform: translateY(1px);
  opacity: 0.95;
}

.button-stack {
  display: flex;
  flex-direction: column;
  gap: 10px;
  margin-top: 24px;
}

.btn {
    display: inline-block;
    padding: 0.5rem 1rem;
    border-radius: 0.375rem;
    font-weight: 500;
    text-decoration: none;
    text-align: center;
    transition: all 0.2s;
    cursor: pointer;
    border: 1px solid transparent;
    width: auto;
    font-size: 16px;
    font-weight: 600;
    padding: 12px 24px;
}



/* Simulation Area */
.simulation-area {
    margin-bottom: 2rem;
}

.simulation-container {
    position: relative;
    background-color: rgba(255, 255, 255, 0.6);
    height: clamp(160px, 25vw, 200px);
    border: 2px solid var(--teal);
    border-radius: var(--border-radius);
    overflow: hidden;
  margin-top:20px;
}

.track {
    position: absolute;
    bottom: 0;
    width: 100%;
    height: 10px;
    background: linear-gradient(to bottom, #7F8C8D, #95A5A6);
}

.track::after {
    content: "";
    position: absolute;
    top: 10px;
    left: 0;
    right: 0;
    height: 4px;
    background: repeating-linear-gradient(90deg, #7F8C8D, #7F8C8D 20px, transparent 20px, transparent 30px);
}

.trolley {
    transition: transform 0.5s linear;
    position: absolute;
    bottom: 7px;
    transform-origin: center bottom;
}

.velocity-indicator {
    position: absolute;
    bottom: 60px;
    transform: translateX(-50%);
    display: flex;
    flex-direction: column;
    align-items: center;
    font-weight: bold;
    text-shadow: 0px 0px 3px white;
}

.velocity-value {
    font-size: 0.75rem;
    margin-bottom: 0.25rem;
}

.velocity-arrow {
    position: relative;
    height: 4px;
    transition: width 0.3s ease, background-color 0.3s ease;
}

/* Arrowhead (triangle) */
.velocity-arrow::after {
    content: "";
    position: absolute;
    top: 50%;
    right: 0;
    transform: translateY(-50%);
    width: 0;
    height: 0;
    border-style: solid;
    border-width: 6px 0 6px 8px;
    border-color: transparent transparent transparent currentColor;
}

/* Flip arrowhead for negative velocity (left direction) */
.velocity-arrow.left-direction::after {
    left: 0;
    right: auto;
    border-width: 6px 8px 6px 0;
    border-color: transparent currentColor transparent transparent;
}

/* Position markers */
.position-markers {
    position: absolute;
    bottom: 4rem;
    width: 100%;
    padding: 0 1rem;
}

.marker-container {
    position: relative;
    height: 1rem;
}

.marker-lines {
    position: absolute;
    width: 100%;
    height: 100%;
    display: flex;
    justify-content: space-between;
}

.marker-line {
    height: 100%;
    border-left: 1px solid var(--slate);
}

.marker-labels {
    position: absolute;
    width: 100%;
    display: flex;
    justify-content: space-between;
    font-size: 0.75rem;
    color: var(--slate);
    margin-top: -1.25rem;
}

/* Boundary indicators */
.boundary-left {
    position: absolute;
    top: 0;
    left: 0;
    height: 100%;
    border-left: 2px solid var(--coral);
}

.boundary-right {
    position: absolute;
    top: 0;
    right: 0;
    height: 100%;
    border-right: 2px solid var(--coral);
}

/* Edge notification */
.edge-notification {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    background-color: rgba(44, 62, 80, 0.9);
    color: white;
    padding: 10px 20px;
    border-radius: 8px;
    font-weight: bold;
    opacity: 0;
    transition: opacity 0.3s ease;
    pointer-events: none;
}

/* Control Panel */
.control-panel {
    background: var(--white);
    border-radius: var(--border-radius);
    box-shadow: 0 10px 25px var(--shadow);
    padding: 1.5rem;
    margin-bottom: 2rem;
}

.control-grid {
    display: grid;
    grid-template-columns: 1fr;
    gap: 1.5rem;
}

.control-section {
    display: flex;
    flex-direction: column;
    gap: 1rem;
}

.control-section h2 {
    color: var(--coral);
    font-family: 'Montserrat', sans-serif;
}

.control-section:nth-child(2) h2 {
    color: var(--teal);
}

.control-group {
    display: flex;
    flex-direction: column;
}

.control-group label {
    display: block;
    font-size: 0.875rem;
    font-weight: 500;
    color: var(--navy);
    margin-bottom: 0.25rem;
}

.slider {
    -webkit-appearance: none;
    height: 8px;
    border-radius: 4px;
    background: var(--softgray);
    outline: none;
    width: 100%;
    touch-action: none;
}

.slider::-webkit-slider-thumb {
    -webkit-appearance: none;
    width: 28px;
    height: 28px;
    border-radius: 50%;
    background: gray;
    cursor: pointer;
    border: 2px solid white;
    box-shadow: 0 2px 5px var(--shadow);
    margin-top: -10px;
}

.slider::-moz-range-thumb {
    width: 28px;
    height: 28px;
    border-radius: 50%;
    background: var(--teal);
    cursor: pointer;
    border: 2px solid white;
    box-shadow: 0 2px 5px var(--shadow);
}

/* Trolley 1 (Red theme) */
#mass1::-webkit-slider-thumb,
#velocity1Input::-webkit-slider-thumb {
    background: var(--coral);
    border: 2px solid white;
    box-shadow: 0 2px 5px var(--shadow);
}

#mass1::-moz-range-thumb,
#velocity1Input::-moz-range-thumb {
    background: var(--coral);
    border: 2px solid white;
    box-shadow: 0 2px 5px var(--shadow);
}

/* Trolley 2 (Teal theme) */
#mass2::-webkit-slider-thumb,
#velocity2Input::-webkit-slider-thumb {
    background: var(--teal);
    border: 2px solid white;
    box-shadow: 0 2px 5px var(--shadow);
}

#mass2::-moz-range-thumb,
#velocity2Input::-moz-range-thumb {
    background: var(--teal);
    border: 2px solid white;
    box-shadow: 0 2px 5px var(--shadow);
}

.slider-labels {
    display: flex;
    justify-content: space-between;
    margin-top: 0.25rem;
}

.slider-labels span {
    font-size: 0.75rem;
    color: var(--slate);
}

.value-display {
    font-size: 0.875rem;
    font-weight: 500;
}

.control-section:nth-child(1) .value-display {
    color: var(--coral);
}

.control-section:nth-child(2) .value-display {
    color: var(--teal);
}

/* Collision Controls */
.collision-controls {
    margin-top: 1.5rem;
    display: flex;
    flex-direction: column;
    gap: 1rem;
}

.collision-type {
    display: flex;
    flex-direction: column;
    gap: 0.5rem;
}

.collision-type label {
    display: block;
    font-size: 0.875rem;
    font-weight: 500;
    color: var(--navy);
}

.radio-group {
    display: flex;
    flex-wrap: wrap;
    gap: 1rem;
}

.radio-option {
    display: inline-flex;
    align-items: center;
}

.radio-option input[type="radio"] {
    margin-right: 0.5rem;
}

/* Buttons */
.action-buttons {
    display: flex;
    flex-wrap: wrap;
    gap: 1rem;
    justify-content: center;
    padding-top: 1rem;
}


/* Instructions + Controls two-column row */
.instructions-controls-row {
  display: flex;
  flex-direction: column;
  gap: 1.5rem;
  margin-bottom: 2rem;
}

@media (min-width: 768px) {
  .instructions-controls-row {
    flex-direction: row;
    align-items: flex-start;
  }

  .instructions-controls-row .instructions-wrapper {
    flex: 1;
  }

  .instructions-controls-row .control-panel {
    flex: 1;
    margin-bottom: 0;
    height: auto;
  }

  .instructions-controls-row .instructions-wrapper {
    align-self: stretch;
  }
}

/* Instructions section */
.instructions-wrapper {
  background: var(--white);
  color: var(--navy);
  border-radius: var(--border-radius);
  padding: 1.5rem;
  box-shadow: 0 10px 25px var(--shadow);
}

.instructions-title {
  text-align: center;
  margin: 0 0 12px 0;
  font-weight: 700;
  font-size: 1.125rem;
  color: var(--navy);
  font-family: 'Montserrat', sans-serif;
}

.instructions-aim {
  margin: 0 0 12px 0;
  line-height: 1.6;
}

.instructions-list {
  margin: 0;
  padding-left: 20px;
  line-height: 2;
}

/* Results Container */
.results-container {
    background-color: var(--white);
    border-radius: var(--border-radius);
    box-shadow: 0 10px 25px var(--shadow);
    padding: 1.5rem;
    margin-bottom: 2rem;
}

.results-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 1rem;
}

.results-header h2 {
    color: var(--teal);
    font-family: 'Montserrat', sans-serif;
}

.results-actions {
    display: flex;
    gap: 0.5rem;
}

.results-actions .btn {
    padding: 0.25rem 1rem;
    font-size: 0.875rem;
}

/* Tables */
.table-section {
    margin-bottom: 1.5rem;
}

.results-table {
    overflow-x: auto;
}

.results-table table {
    width: 100%;
    border-collapse: collapse;
    min-width: 100%;
}

.results-table th,
.results-table td {
    border: 1px solid var(--slate);
    padding: 0.5rem 0.75rem;
    text-align: left;
}

.results-table thead tr {
    background-color: var(--softgray);
}

.results-table th {
    color: var(--navy);
}

.trolley1-bg {
    background-color: rgba(231, 76, 60, 0.2);
}

.trolley2-bg {
    background-color: rgba(26, 188, 156, 0.2);
}

.empty-message {
    text-align: center;
    color: var(--slate);
    padding: 1rem 0.75rem;
}

/* Table inputs */
table input {
    width: 100%;
    border: 1px solid var(--softgray);
    border-radius: 4px;
    padding: 0.25rem 0.5rem;
    transition: border-color 0.2s ease;
}

table input:focus {
    outline: none;
    border-color: var(--teal);
    box-shadow: 0 0 0 2px rgba(26, 188, 156, 0.2);
}

.student-input {
    background-color: #f8fbfc;
}

.student-input:focus {
    background-color: #e8f7f4;
}

.correct-answer {
    background-color: rgba(26, 188, 156, 0.1);
}

.incorrect-answer {
    background-color: rgba(231, 76, 60, 0.1);
}

/* Tooltips */
.tooltip {
    position: relative;
    display: inline-block;
    cursor: help;
    width: 100%;
}

.tooltip .tooltiptext {
    visibility: hidden;
    width: 200px;
    background-color: var(--navy);
    color: var(--white);
    text-align: center;
    border-radius: 6px;
    padding: 8px;
    position: absolute;
    z-index: 1;
    bottom: 125%;
    left: 50%;
    margin-left: -100px;
    opacity: 0;
    transition: opacity 0.3s;
    font-size: 0.75rem;
}

.tooltip .tooltiptext::after {
    content: "";
    position: absolute;
    top: 100%;
    left: 50%;
    margin-left: -5px;
    border-width: 5px;
    border-style: solid;
    border-color: var(--navy) transparent transparent transparent;
}

.tooltip:hover .tooltiptext {
    visibility: visible;
    opacity: 1;
}

/* Explanation Section */
.explanation {
    margin-top: 2rem;
    background-color: var(--softgray);
    padding: 1.5rem;
    border-radius: var(--border-radius);
}

.explanation h2 {
    color: var(--navy);
    font-family: 'Montserrat', sans-serif;
    margin-bottom: 1rem;
}

.explanation-content {
    display: flex;
    flex-direction: column;
    gap: 0.75rem;
    color: var(--slate);
    font-family: 'Lora', serif;
}

/* Responsive Design */
@media (min-width: 768px) {
    body {
        padding: 2rem;
    }
    
    .control-grid {
        grid-template-columns: 1fr 1fr;
    }
    
    .radio-group {
        gap: 1rem;
    }
}

@media (min-width: 1024px) {
    body {
        padding: 2rem;
    }
}