Pseudo element with z-index: -1 is hidden by background-color

I have a .link class with an ::after pseudo element underneath it (as a box-shadow on hover).

.bg {
    background-color: aqua;
    height: 100vh;
}

.link {
    position: relative;
    font-weight: bold;
}

.link::after {
    content: '';
    position: absolute;
    z-index: -1;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    box-shadow: 0 0 0 1em #888888 inset;
    opacity: 0;
}

.link:hover::after {
    opacity: 1;
}

<div class="bg">
  <p class="link">Link</p>
</div>

However, when I add a background-colour to the encompassing div element, it covers up the box-shadow. I have tried adding

position: absolute;
z-index: -2;

to the div but that causes a resizing issue with the div, although the stacking is correct. I wonder if there's a preferred method to place a pseudo element above the bg layer but behind the original link class? Thanks!

Solution

Add z-index:0 (or any value) to the background element in order to create a stacking content and avoid the pseudo element moving behind:

.bg {
    background-color: aqua;
    height: 100vh;
    position:relative;
    z-index:0; /* added */
}

.link {
    position: relative;
    font-weight: bold;
}

.link::after {
    content: '';
    position: absolute;
    z-index: -999; /*you can put any negative value now*/
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    box-shadow: 0 0 0 1em #888888 inset;
    opacity: 0;
}

.link:hover::after {
    opacity: 1;
}

<div class="bg">
  <p class="link">Link</p>
</div>