Function updateBackdrop

docs/assets/script.js:28–32

* 更新高亮层

(textareaId, backdropId)

 * 更新高亮层
 */
function updateBackdrop(textareaId, backdropId) {
    const ta = document.getElementById(textareaId);
    const bd = document.getElementById(backdropId);
    if (ta && bd) bd.innerHTML = applyHighlight(ta.value) + '\n';
}

/**
 * 同步滚动