Drupal cTools/panels custom layout. Creating dynamic layout classes based on whether or not regions have content.

_l-3up.scss

//
// @file
// Sidebars After Layout.
// One main content area with two sidebars after.
// ----------------------------------------------------------------------------

.l--3up {
  @include breakpoint(41em, $no-query: '.lt-ie9') {
    .l-main {
      @include zen-grid-item(8, 1);
      padding-left: 0;
      padding-right: 30px;
    }

    .l-sidebar-first {
      @include zen-grid-item(8, 9);
      padding-left: 15px;
      padding-right: 15px;
    }

    .l-sidebar-second {
      @include zen-clear(right);
      @include zen-grid-item(8, 1, right);
      padding-left: 30px;
    }

    &.l--1-0-0 .l-main,
    &.l--0-1-0 .l-sidebar-first,
    &.l--0-0-1 .l-sidebar-second {
      @include zen-grid-item(8, 9);
      padding-left: 15px;
      padding-right: 15px;
    }

    &.l--1-1-0 .l-main,
    &.l--1-0-1 .l-main,
    &.l--0-1-1 .l-sidebar-first {
      @include zen-grid-item(8, 5);
      padding-left: 0;
      padding-right: 30px;
    }

    &.l--1-1-0 .l-sidebar-first,
    &.l--1-0-1 .l-sidebar-second,
    &.l--0-1-1 .l-sidebar-second {
      @include zen-grid-item(8, 5, right);
      padding-left: 30px;
      padding-right: 0;
    }
  }

  @include breakpoint($bp-two-sidebars, $no-query: '.lt-ie9') {
    .l-main {
      padding-right: 50px;
    }

    .l-sidebar-first {
      padding-left: 25px;
      padding-right: 25px;
    }

    .l-sidebar-second {
      padding-left: 50px;
    }
  }
}

resource.inc

<?php
// cTools Layout definition

// Plugin definition
$plugin = array(
  'title' => t('Resource'),
  'category' => t('WRI'),
  'icon' => 'resource.png',
  'theme' => 'wri_layouts_resource',
  'admin css' => 'resource.css',
  'regions' => array(
    'header' => t('Header'),
    'main' => t('Main'),
    'meta' => t('Meta'),
    'supplement_main' => t('Supplement Main'),
    'supplement_sidebar_first' => t('Supplement Sidebar First'),
    'supplement_sidebar_second' => t('Supplement Sidebar Second'),
  ),
);

/**
 * Implements template_preprocess_wri_layouts_project_level_2
 */

function wri_layouts_preprocess_wri_layouts_resource(&$vars) {
  $content = $vars['content'];

  // Create a dynamic class for the Supplement regions
  $vars['supplement_attributes_array']['class'][] = wri_layouts_create_region_collection_classes(
    array(
      $content['supplement_main'],
      $content['supplement_sidebar_first'],
      $content['supplement_sidebar_second'],
    )
  );
  $vars['supplement_classes'] = implode(' ', $vars['supplement_attributes_array']['class']);
}

wri-layouts-resource.tpl.php

<?php
/**
 * @file
 * Template for a resource layout.
 *
 */
?>
<article class="l--resource" <?php print $attributes; ?>>
  <?php if ($content['header']): ?>
    <header class="l--constrained resource-header">
      <?php print $content['header']; ?>
    </header>
  <?php endif; ?>

  <?php if ($content['main'] || $content['meta']): ?>
    <div class="l--sidebar-after l--constrained">
      <?php if ($content['main']): ?>
        <div class="l-main resource-main">
          <?php print $content['main']; ?>
        </div>
      <?php endif; ?>
      <?php if ($content['meta']): ?>
        <footer class="l-sidebar resource-meta">
          <?php print $content['meta']; ?>
        </footer>
      <?php endif; ?>
    </div>
  <?php endif; ?>

  <?php if ($content['supplement_main'] || $content['supplement_sidebar_first'] || $content['supplement_sidebar_second']): ?>
    <section class="l--3up l--constrained resource-supplement <?php print $supplement_classes; ?>">
      <?php if ($content['supplement_main']): ?>
        <aside class="l-main">
          <?php print $content['supplement_main']; ?>
        </aside>
      <?php endif; ?>
      <?php if ($content['supplement_sidebar_first']): ?>
        <aside class="l-sidebar-first">
          <?php print $content['supplement_sidebar_first']; ?>
        </aside>
      <?php endif; ?>
      <?php if ($content['supplement_sidebar_second']): ?>
        <aside class="l-sidebar-second">
          <?php print $content['supplement_sidebar_second']; ?>
        </aside>
      <?php endif; ?>
    </section>
  <?php endif; ?>
</article>

wri_layouts.module

<?php
/** ... */

/**
 * Creates a layout class based on the availability of regions in a given array.
 */

function wri_layouts_create_region_collection_classes($regions) {
  $class = 'l-';
  // Construct a class based on if regions are empty or not.
  foreach ($regions as $key => $region) {
    $class .= empty($region) ? '-0' : '-1';
  }
  return $class;
}