Reports

You need to add box-sizing: border-box to your .input-box class:

See the documentation here for some visual examples: https://developer.mozilla.org/en-US/docs/Web/CSS/box-sizing

<!DOCTYPE html>
<html>
  <head>
    <title>
      Practice 
    </title>
    <style>
      .parent{
        display: flex;
        flex-direction: row;
      }

      .child-1{
        flex: 1;
      }

      .input-box{
        width: 100%;
        box-sizing: border-box;
      }

      .child-2{
        width: 100px;
      }
    </style>
  </head>
  <body>
    <div class="parent">
      <div class="child-1">
        <input class="input-box" placeholder="Search">
      </div>
      <div class="child-2">
        <button style="opacity: 0.8;">Click me</button>
      </div>
    </div>
  </body>
</html>

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