[Solved] How can i fixed div inside relative div?

mordad Asks: How can i fixed div inside relative div?
My English is not good. If I made a typo, please edit it. And i am very beginner

Click for what I want to have

I want to have a fixed-div whose width and height are controlled, but responsive and not manually, inside a responsive parent.

I want the child inside the .parent to be reactively fixed, and that’s what I tried

#parent {
    position: relative;
    width: 50%;
    margin: 2rem auto;
    background-color: orange;
    padding: 1rem;

#middle {
  position: absolute;
  width: 100%;
  height: 200px;
  background: blue;

#child {
    position: fixed;
    width: 100%;
    height: 100px;
    background: red;


    <div id="parent">
        <div id="middle">
            <div id="child"></div>

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