在父容器内实现可拖拽、可调整大小且边界受限的HTML元素

<div class="container">
  <div class="draggable">
    <div class="move">draggable handle</div>
    Non-draggable content
    <div class="resize"></div>
  </div>
  <div class="draggable" style="left:230px;">
    <div class="move">draggable handle</div>
    Non-draggable content
    <div class="resize"></div>
  </div>
</div>

html,body{
  height:100%;
  margin:0;
  padding:0;
}
*{
  box-sizing: border-box; /* 确保padding和border计入元素的总宽度和高度 */
}

.draggable{
  position: absolute; /* 绝对定位，便于拖拽 */
  padding:45px 15px 15px 15px; /* 留出move手柄的空间 */
  border-radius:4px;
  background:#ddd;
  user-select: none; /* 防止拖拽时选中内部文本 */
  left: 15px;
  top: 15px;
  min-width:200px; /* 最小宽度 */
}
.draggable>.move{
  line-height: 30px;
  padding: 0 15px;
  background:#bbb;
  border-bottom: 1px solid #777;
  cursor:move; /* 拖拽手柄鼠标样式 */
  position:absolute;
  left:0;
  top:0;
  height:30px;
  width:100%;
  border-radius: 4px 4px 0;
}
.draggable>.resize{
  cursor:nw-resize; /* 调整大小手柄鼠标样式 */
  position:absolute;
  right:0;
  bottom:0;
  height:16px;
  width:16px;
  border-radius: 0 0 4px 0;
  background: linear-gradient(to left top, #777 50%, transparent 50%) /* 视觉提示 */
}
.container{
  left:15px;
  top:15px;
  background: #111;
  border-radius:4px;
  width:calc(100% - 30px); /* 占据大部分视口宽度 */
  height:calc(100% - 30px); /* 占据大部分视口高度 */
  position: relative; /* 相对定位，作为draggable元素的定位上下文 */
}

const container = document.querySelector('.container');
const draggables = document.querySelectorAll('.draggable');

draggables.forEach(elem => {
  makeDraggableResizable(elem);
  // 当元素被点击时，提升其z-index，使其置于最顶层
  elem.addEventListener('mousedown', () => {
     const maxZ = Math.max(...[...draggables].map(elem => parseInt(getComputedStyle(elem)['z-index']) || 0));
     elem.style['z-index'] = maxZ + 1;
  });
});

function makeDraggableResizable(draggable){

  // 移动元素函数
  const move = (x, y) => {
    // 计算相对于容器的新位置
    x = state.fromX + (x - state.startX);
    y = state.fromY + (y - state.startY);

    // 边界限制：不允许移动到容器外部
    if (x < 0) x = 0; // 左边界
    else if (x + draggable.offsetWidth > container.offsetWidth) x = container.offsetWidth - draggable.offsetWidth; // 右边界

    if (y < 0) y = 0; // 上边界
    else if (y + draggable.offsetHeight > container.offsetHeight) y = container.offsetHeight - draggable.offsetHeight; // 下边界

    draggable.style.left = x + 'px';
    draggable.style.top = y + 'px';
  };

  // 调整大小元素函数
  const resize = (x, y) => {
    // 计算相对于容器的新尺寸
    x = state.fromWidth + (x - state.startX);
    y = state.fromHeight + (y - state.startY);

    // 边界限制：不允调整大小超出容器
    if (state.fromX + x > container.offsetWidth) x = container.offsetWidth - state.fromX; // 右边界
    if (state.fromY + y > container.offsetHeight ) y = container.offsetHeight - state.fromY; // 下边界

    draggable.style.width = x + 'px';
    draggable.style.height = y + 'px';
  };

  // 添加/移除全局事件监听器
  const listen = (op = 'add') => 
    Object.entries(listeners).slice(1) // 排除mousedown，因为它是局部监听
      .forEach(([name, listener]) => document[op + 'EventListener'](name, listener));

  // 使用Proxy管理状态，确保操作顺序和逻辑分离
  const state = new Proxy({}, {
    set(state, prop, val){
      const out = Reflect.set(...arguments); // 执行默认的属性设置
      const ops = {
        // 当startY被设置时，初始化起始位置和尺寸，并添加全局mousemove/mouseup监听器
        startY: () => {
          listen();
          const style = getComputedStyle(draggable);
          [state.fromX, state.fromY] = [parseInt(style.left), parseInt(style.top)];
          [state.fromWidth, state.fromHeight] = [parseInt(style.width), parseInt(style.height)];
        },
        // 当dragY被设置时（即mousemove事件发生），执行当前设定的action（move或resize）
        dragY: () => state.action(state.dragX, state.dragY),
        // 当stopY被设置时（即mouseup事件发生），执行action并移除全局监听器
        stopY: () => listen('remove') + state.action(state.stopX, state.stopY),
      };
      // 使用Promise.resolve().then()将操作作为微任务推迟，确保所有相关状态已更新
      ops[prop] && Promise.resolve().then(ops[prop]);
      return out;
    }
  });

  // 定义事件监听器，但不直接绑定到document
  const listeners = {
    mousedown: e => Object.assign(state, {startX: e.pageX, startY: e.pageY}),
    mousemove: e => Object.assign(state, {dragY: e.pageY, dragX: e.pageX}), // 确保dragY后设置dragX
    mouseup: e => Object.assign(state, {stopX: e.pageX, stopY: e.pageY}),
  };

  // 为move和resize手柄绑定mousedown事件
  for(const [name, action] of Object.entries({move, resize})){
    draggable.querySelector(`.${name}`).addEventListener('mousedown', e => {
      state.action = action; // 设置当前要执行的动作
      listeners.mousedown(e); // 触发mousedown逻辑
    }); 
  }
}