javascript - Is a see-through child div possible?

Question

The image is the grandparent div, the black translucent overlay is the parent div, and the cropped section is the child div. User will see the grandparent image and the parent overlay, then he can crop through it using the child cropper div. I tried and failed with opacity and rgba background.

These crazy approaches do seem to work for me -

1. Set the grandparent image in the background of the child div as well and then change the x/y of the background-position.
2. Combine child and parent into one single div, and use rgba border as the overlay (my friend's suggestion).
3. Found this on stackoverflow, which uses box-shadow instead of borders and seems like a similar approach to #2.

My minor gripe with #2 and #3 is that I'll need to add another div for the dashed borders so the user clearly knows what he's cropping. But my bigger gripe with all of them is that none of these looks like the right approach.

Is there a proper / better / 2018-ish / "its so obvious, you idiot" way to do this?

Update: Here's the basic markup (I am okay with a different markup too if that helps in solving this)

#grandparentImage {
  background: url(https://9to5mac.com/wp-content/uploads/sites/6/2018/07/Desert-2.jpg) no-repeat;
  background-size: cover;
  position: relative;
  height: 500px;
}

#parentOverlay {
  background: rgba(0,0,0,0.5);
  height: 100%;
  position: relative;
}

#childCropper {
  border: 1px dashed #ccc;
  left: 50px;
  height: 100px;
  width: 100px;
  position: absolute;
  top: 50px;
}

<div id="grandparentImage">
  <div id="parentOverlay">
    <div id="childCropper"></div>
  </div>
</div>

Answer 1

You can set box-shadow with 100vmax spread radius on the #childCropper. In this way it will always cover the screen:

#grandparentImage {
  background: url(https://9to5mac.com/wp-content/uploads/sites/6/2018/07/Desert-2.jpg) no-repeat;
  background-size: cover;
  position: relative;
  height: 500px;
}

#childCropper {
  position: absolute;  
  top: 50px;
  left: 50px;
  height: 200px;
  width: 200px;
  border: 1px dashed #ccc;
  box-shadow: 0 0 0 100vmax rgba(0,0,0,0.5);
}

body {
  margin: 0;
}

<div id="grandparentImage">
  <div id="childCropper"></div>
</div>