我编写了一个HTML编辑器，但是它不工作了。不久前它还能工作，但是可能是我姐姐编辑了代码（她实际上对HTML一无所知）
现在的问题是，每当我按下回车键，而不是简单地移动到下一行，一个矩形创建，为什么？
下面是代码：

const first = document.querySelector(".first");
const iframe = document.querySelector("iframe");
const btn = document.querySelector("button");

btn.addEventListener("click", () => {
  var html = first.textContent;
  iframe.src = "data:text/html;charset=utf-8," + encodeURI(html);
});

first.addEventListener('keyup',()=>{
  var html = first.textContent;
  iframe.src = "data:text/html;charset=utf-8," + encodeURI(html);
})

first.addEventListener("paste", function(e) {
        e.preventDefault();
        var text = e.clipboardData.getData("text/plain");
        document.execCommand("insertText", false, text);
    });

* {
 box-shadow: 0 2px 3px black;
 position: fixed;
 height: 100vh;
 justify-content: center;
 align-items: center;
 border: 7px solid #36383f;
}

.first {
  background-color: #ffffff;
  width: 50%;
  overflow-x: hidden;
  overflow-y: auto;
  white-space: pre;
  box-shadow: 0 1px 1px rgb(22, 22, 22);
  outline: none;
  padding: 0.4rem;
  height: 90vh;
}

.second {
  background-color: rgb(255, 255, 255);
  width: 50%;
  overflow-y: auto;
  white-space: pre;
  right: 0;
  box-shadow: 0 1px 1px rgb(22, 22, 22);
  padding: 0.4rem;
  height: 90vh;
}

<div class="main-editor">
  <button class="btn">Run</button>
  <div  class="first" contenteditable>writecode</div>
  <iframe class="second" > </iframe>
</div>