ScrollPos-Styler

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Adds CSS class to a HTML element depending on scroll position.

This module adds a format filter:

Enable ScrollPos-Styler adding (class="sps") to your elements.
Configure the scroll position in px to trigger the style adding (data-sps-offset="80") to your elements.
"sps--abv" or "sps--blw" classes will be added when the window is scrolled above/below the defined position.

Form element example:

$form['container'] = [
  '#type' => 'container',
  '#attributes' => [
    'class' => ['sps'],
    'data-sps-offset' => '80',
    '#attached' => [
      'library' => [
        'scrollpos_styler/library',
      ],
    ],
  ],
];