FixIt/assets/css/_partials/_widgets/_reading-progress.scss

.reading-progress-bar {
  height: var(--#{$prefix}progress-h, 2px);
  background-color: var(--#{$prefix}progress-bg, var(--#{$prefix}info));
  position: fixed;
  left: 0;
  right: 0;
  transform-origin: var(--#{$prefix}progress-start, left) 50%;
  animation: grow-progress 3s linear;
  animation-timeline: scroll();
  @include z-index(2);
  @extend .print-d-none;

  &[data-reversed=true] {
    animation-name: grow-progress-reversed;
  }
  
  [data-theme="dark"] & {
    background-color: var(--#{$prefix}progress-bg-dark, var(--#{$prefix}info-dark));
  }

  @keyframes grow-progress {
    from { transform: scaleX(0); }
    to { transform: scaleX(1); }
  }
  @keyframes grow-progress-reversed {
    from { transform: scaleX(1); }
    to { transform: scaleX(0); }
  }

  /**
   * After the CSS property `animation-timeline: scroll()` exceeds 90% browser support in the future, this code can be removed
   * See https://caniuse.com/mdn-css_properties_animation-timeline_scroll
   */
  @supports not (animation-timeline: scroll()) {
    display: none;
  }
}