wagtail/client/scss/tools/_mixins.guide-line.scss

/**
 * Vertical and horizontal divider lines to visualise nesting in
 * StreamField and InlinePanel.
 */

$guide-line-default-color: theme('colors.grey.150');

@mixin guide-line-vertical() {
  // 3px dash height, 3px space, 1px dash width.
  background-size: 1px 6px;
  background-repeat: repeat-y;
  background-image: linear-gradient(
    to bottom,
    var(--guide-line-color, $guide-line-default-color) 50%,
    rgba(255, 255, 255, 0) 0%
  );

  @media (forced-colors: active) {
    border-inline-start: 1px dashed var(--guide-line-color, CanvasText);
    background: none;
  }
}

@mixin guide-line-horizontal() {
  min-height: 1px;
  // 3px dash width, 3px space, 1px dash height.
  background-size: 6px 1px;
  background-repeat: repeat-x;
  background-image: linear-gradient(
    to right,
    var(--guide-line-color, $guide-line-default-color) 50%,
    rgba(255, 255, 255, 0) 0%
  );

  // Guide lines are always shown in forced-colors mode, as it’s
  // not possible to have transparent borders there.
  @media (forced-colors: active) {
    border-top: 1px dashed var(--guide-line-color, CanvasText);
    background: none;
  }
}

// More visible guide line for nested panels in the currently-active DOM tree.
@mixin guide-line-nested() {
  &:is(:hover, :focus-within) {
    --guide-line-color: theme('colors.primary.DEFAULT');

    @media (forced-colors: active) {
      --guide-line-color: Highlight;
    }
  }
}