discourse/app/assets/stylesheets/common/components/svg.scss

.svg-icon,
.svg-icon-title {
  height: 1em;
  width: 1em;
  line-height: 1;
  display: inline-flex;
  position: relative;
  vertical-align: -0.125em;
  fill: currentColor;
  flex-shrink: 0; // Prevent the icon from shrinking if it's in a flexbox
  overflow: visible;
}

// Fixes Edge bug with SVG elements not triggering click event
svg > use {
  pointer-events: none;
}

// Stacked Icons
// Usage:
// <span class="fa-stack">
//    <svg class="... d-icon-icon1 fa-stack-1x ..." xmlns="http://www.w3.org/2000/svg"><use xlink:href="#icon1"></use></svg>
//    <svg class="... d-icon-icon2 fa-stack-2x ..." xmlns="http://www.w3.org/2000/svg"><use xlink:href="#icon2"></use></svg>
// </span>
// -------------------------

.fa-stack {
  position: relative;
  width: 1em;
  height: 1em;
  display: inline-block;

  .fa-stack-1x,
  .fa-stack-2x {
    left: 0px;
    top: 0px;
    position: absolute;
  }

  .fa-stack-2x {
    height: 1.5em;
    width: 1.5em;
    margin-top: -0.25em;
    margin-left: -0.25em;
  }
}