[解決済み] ウィンドウサイズを小さくすると、Flex-wrapがラップされない

body {
  font-family: arial;
}

p {
  color: white;
}

.container {
  background-color: #666;
  width: 800px;
  height: 200px;
  margin: 0 auto;
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
}

.item {
  padding: 10px;
  box-sizing: border-box;
}

.item1 {
  flex: 1;
  background-color: red;
}

.item2 {
  flex: 1;
  background-color: blue;
}

.item3 {
  flex: 1;
  background-color: green;
}

<div class="container">

  <div class="item item1">
    <h1>ITEM1</h1>
    <p>flex: 1</p>
  </div>

  <div class="item item2">
    <h1>ITEM2</h1>
    <p>flex: 1</p>
  </div>

  <div class="item item3">
    <h1>ITEM3</h1>
    <p>flex: 1</p>
  </div>

</div>

body {
  font-family: arial;
}

p {
  color: white;
}

.container {
  background-color: #666;
  max-width: 800px;
  height: 200px;
  margin: 0 auto;
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
}

.item {
  padding: 10px;
  box-sizing: border-box;
}

.item1 {
  flex: 1;
  background-color: red;
}

.item2 {
  flex: 1;
  background-color: blue;
}

.item3 {
  flex: 1;
  background-color: green;
}

<div class="container">

  <div class="item item1">
    <h1>ITEM1</h1>
    <p>flex: 1</p>
  </div>

  <div class="item item2">
    <h1>ITEM2</h1>
    <p>flex: 1</p>
  </div>

  <div class="item item3">
    <h1>ITEM3</h1>
    <p>flex: 1</p>
  </div>

</div>