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Html - Remove gap between lines of flex blocks?

How to Html - Remove gap between lines of flex blocks

how can I remove the gap between the lines of flex blocks?

I have a certain number of red blocks, which may vary. When the number of blocks is small, the distance between the lines is large; when there is a large number, there is none. How to remove a gap with a small quantity.

.container {
  width: 100%;
  display: flex;
  border: solid 3px black;
  flex-wrap: wrap;
  height: 500px;
  overflow-y: scroll;
}

.container-red-box {
  width: 25%;
}

.red-box {
  margin: 2px;
  height: 100px;
  background: red;
  border: solid 3px black;
}

.container-boxs {
  width: 100%;
  display: flex;
  flex-wrap: wrap;
}

<!doctype html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
</head>

<body>
  <div class="container">
    <div class="container-boxs">
      <div class="container-red-box">
        <div class="red-box"></div>
      </div>
      <div class="container-red-box">
        <div class="red-box"></div>
      </div>
      <div class="container-red-box">
        <div class="red-box"></div>
      </div>
      <div class="container-red-box">
        <div class="red-box"></div>
      </div>
      <div class="container-red-box">
        <div class="red-box"></div>
      </div>
      <div class="container-red-box">
        <div class="red-box"></div>
      </div>
      <div class="container-red-box">
        <div class="red-box"></div>
      </div>
      <div class="container-red-box">
        <div class="red-box"></div>
      </div>

    </div>

  </div>
</body>

</html>

10 blocks:

enter image description here

40 blocks: enter image description here

Solution:

Consider adding align-items: flex-start to the .container. By default in the flex layout, align-items is behaving as stretch, which will make its children try to occupy the entire cross-axis. By setting it to flex-start (or start), it will make them occupy only the necessary space.

.container {
  width: 100%;
  display: flex;
  border: solid 3px black;
  flex-wrap: wrap;
  height: 500px;
  overflow-y: scroll;
  
  /* added */
  align-items: flex-start;
}

.container-red-box {
  width: 25%;
}

.red-box {
  margin: 2px;
  height: 100px;
  background: red;
  border: solid 3px black;
}

.container-boxs {
  width: 100%;
  display: flex;
  flex-wrap: wrap;
}

<!doctype html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
</head>

<body>
  <div class="container">
    <div class="container-boxs">
      <div class="container-red-box">
        <div class="red-box"></div>
      </div>
      <div class="container-red-box">
        <div class="red-box"></div>
      </div>
      <div class="container-red-box">
        <div class="red-box"></div>
      </div>
      <div class="container-red-box">
        <div class="red-box"></div>
      </div>
      <div class="container-red-box">
        <div class="red-box"></div>
      </div>
      <div class="container-red-box">
        <div class="red-box"></div>
      </div>
      <div class="container-red-box">
        <div class="red-box"></div>
      </div>
      <div class="container-red-box">
        <div class="red-box"></div>
      </div>

    </div>

  </div>
</body>

</html>