body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
    line-height: 1.6;
    background-color: #f4f7f6;
    color: #333;
    margin: 0;
    padding: 20px;
    display: flex;
    justify-content: center;
    align-items: flex-start;
    touch-action: manipulation; /* reduce double-tap zoom / gesture interference */
}

.container {
    max-width: 800px;
    margin: 0 auto;
    background: #fff;
    padding: 30px;
    border-radius: 8px;
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.1);
}

h1 {
    text-align: center;
    color: #2c3e50;
    margin-bottom: 10px;
}

p {
    text-align: center;
    color: #555;
    margin-bottom: 12px; /* reduce extra space under subtitle */
}

.controls {
    display: flex;
    flex-direction: column;
    gap: 14px; /* tighten vertical spacing to remove extra line feel */
    margin-bottom: 18px;
}

.input-group {
    display: flex;
    align-items: center;
    gap: 10px;
    margin: 0; /* prevent extra vertical spacing from user agents */
}

.input-group label {
    font-weight: 600;
    color: #34495e;
    flex-shrink: 0;
}

.input-group input {
    width: 100%;
    padding: 10px;
    border: 1px solid #ccc;
    border-radius: 4px;
    font-size: 1em;
}

.input-group input:focus {
    outline: none;
    border-color: #3498db;
    box-shadow: 0 0 0 2px rgba(52, 152, 219, 0.2);
}

/* grid-settings removed */

.button-row {
    display: flex;
    gap: 10px;
    flex-wrap: wrap;
}

/* initial condition inputs removed */

button {
    padding: 12px 20px;
    font-size: 1.1em;
    font-weight: 600;
    color: #fff;
    background-color: #3498db;
    border: none;
    border-radius: 5px;
    cursor: pointer;
    transition: background-color 0.3s;
}

button:hover {
    background-color: #2980b9;
}

#vector-field-canvas {
    box-sizing: content-box;
    width: 100%;
    max-width: 480px;
    height: auto;
    aspect-ratio: 1 / 1; /* rely on CSS to keep square; no JS resizing */
    border: 1px solid #ccc;
    border-radius: 5px;
    background-color: #fff;
    -webkit-tap-highlight-color: transparent;
    display: block;
    margin: 0 auto;
}

/* Modal styles */
.modal-overlay {
    position: fixed;
    inset: 0;
    background: rgba(0,0,0,0.4);
    display: none;
    align-items: center;
    justify-content: center;
    padding: 16px;
    z-index: 1000;
}
.modal-overlay[aria-hidden="false"] {
    display: flex;
}
.modal {
    background: #fff;
    color: #333;
    max-width: 560px;
    width: 100%;
    padding: 20px;
    border-radius: 8px;
    box-shadow: 0 10px 24px rgba(0,0,0,0.25);
}
.modal h2 {
    margin-top: 0;
}

/* Mobile-friendly layout */
@media (max-width: 480px) {
    .container {
        padding: 12px;
        display: flex;
        flex-direction: column;
    }
    /* Reorder so canvas stays visible above the input while typing */
    h1 { order: 1; }
    .container > p { order: 2; }
    #vector-field-canvas { order: 3; }
    .controls { order: 4; }
    .input-group input {
        font-size: 16px; /* prevent iOS zoom */
        padding: 10px;
    }
    .button-row {
        display: grid;
        grid-template-columns: 1fr 1fr;
        gap: 10px;
    }
    .button-row button {
        min-height: 44px; /* touch target */
        font-size: 15px;
    }
    #vector-field-canvas {
        max-width: 340px; /* slightly smaller to fit better */
    }
}

.error {
    color: #e74c3c;
    background-color: #fbeae5;
    border: 1px solid #e74c3c;
    padding: 10px;
    border-radius: 4px;
    margin-top: 8px;
    display: none; /* Hidden by default */
}

.success {
    color: #1e7e34;
    background-color: #e6f4ea;
    border: 1px solid #1e7e34;
    padding: 10px;
    border-radius: 4px;
    margin-top: 8px;
    display: none;
}

.notice {
    color: #555;
    background-color: #f2f2f2;
    border: 1px solid #cfcfcf;
    padding: 10px;
    border-radius: 4px;
    margin-top: 8px;
    display: none;
}