wagtail/client/scss/components/forms/_nested-panel.scss

$header-icon-size: theme('spacing.4');
$header-button-size-sm: theme('spacing.6');
$icon-center-offset: 2px;
$panel-bottom-margin: $form-field-spacing;
$guide-line-bottom-margin: calc($form-field-spacing / 3);

/**
 * Panel styles shared between StreamField and InlinePanel,
 * for repeating collapsible panels which can be reordered.
 * Top-level and nested panels have guiding borders to show their start and end,
 * as well as indentation for nested panels.
 */

// Styles for the top-level panel, and any panel within.
.w-panel--nested {
  --nesting-indent: #{$nested-field-indent-sm};

  @include media-breakpoint-up(sm) {
    --nesting-indent: #{$nested-field-indent};
  }

  .w-panel__content {
    @include guide-line-vertical();
    // Center the vertical line.
    margin-inline-start: calc(-1 * var(--nesting-indent));
    padding-inline-start: var(--nesting-indent);
    margin-bottom: $guide-line-bottom-margin;

    @include media-breakpoint-up(sm) {
      // Extra pixels for better alignment with center of icon.
      margin-inline-start: calc(
        -1 * var(--nesting-indent) + $icon-center-offset
      );
    }
  }

  .w-panel__anchor {
    // Mask the overlap with the parent panel’s guide line.
    background-color: theme('colors.surface-page');
  }

  .w-field__wrapper {
    // Reduced field spacing for nested panels.
    // Using both padding and margin so the field guide line extends below.
    padding-bottom: calc($form-field-spacing / 2);
    margin-bottom: $guide-line-bottom-margin;
  }
}

// Styles for nested panels at the top level only.
.w-panel--nested:not(.w-panel .w-panel) {
  > .w-panel__content {
    @include guide-line-vertical-stop();
  }
}

// Styles for nested panels excluding the top level.
.w-panel--nested .w-panel {
  @include guide-line-nested();
  margin-inline-start: var(--nesting-indent);
  margin-bottom: 0;

  .w-panel__content {
    margin-inline-start: calc(
      -1 * (var(--nesting-indent) + $icon-center-offset - var(--header-gap) / 2)
    );
  }

  // Tighter spacing for nested panel headers so their icons align
  // with parent panels’ guiding line.
  .w-panel__header {
    gap: 0;
    transform: translateX(
      calc(var(--w-direction-factor) * -1 * var(--nesting-indent))
    );

    @include media-breakpoint-up(sm) {
      transform: translateX(
        calc(var(--w-direction-factor) * theme('spacing.1'))
      );
    }
  }

  .w-panel__divider {
    @include guide-line-horizontal();
    // Slightly nicer text alignment.
    margin-top: 1px;
  }

  .w-panel__heading--label {
    // Use smaller labels within nested panels in InlinePanel.
    @apply w-label-2;
  }

  // For nested panels, there is always enough space for the prefix anchor.
  .w-panel__anchor--prefix {
    display: inline-grid;
  }

  // Nested panels never need the suffix anchor.
  .w-panel__anchor--suffix {
    display: none;
  }
}