width: inherit/100% of a child

I want to put a side bar on top of an image, with the same width as the image. Neither width: inherit|100% work.

CSS

       body {
            display: flex;
        }

        #main-pic {
            width: 40%; 
            height: 700px;
        }

        #main-pic>img {
            width: 100%;
            height: 100%;
        } 

        #logo-holder {
            width: inherit; 
            height: 100px;
            background-color: brown;
            position: absolute;
            top: 50%;
        }

HTML

    <div id="main-pic">
        <img src="myFlower.JPG" alt="My Flower">
        <div id="logo-holder"></div>
    </div>

    <div id="main-text">
        <p>This is the part for text</p>
    </div> 

In this example, I use width: inherit, the #logo-holder is a bit longer than the image. I tried width: 100%, it's stretched across browser. Can someone explain and suggest a solution?

You need to assign position: relative to the #main-pic element, so its children could use its relative dimensions properly:

body {
  display: flex;
}

#main-pic {
  width: 40%;
  height: 700px;
  
  /* added */
  position: relative;
}

#main-pic>img {
  width: 100%;
  height: 100%;
}

#logo-holder {
  width: 100%;
  height: 100px;
  background-color: brown;
  position: absolute;
  top: 50%;
}

<div id="main-pic">
  <img src="https://picsum.photos/id/42/400/800" alt="My Flower">
  <div id="logo-holder"></div>
</div>

<div id="main-text">
  <p>This is the part for text</p>
</div>