Filter the Backbone.js template for Elementor Section and Column elements, so that editing the custom spacing controls shows up in the page editor preview pane.

filter-elementor-backbone-template.php

<?php
/*
 * Change the Editor Preview Template for the section and column elements so that each
 * element's mld spacing settings are reflected in the preview window whenever the user makes a 
 * change.
 */
add_action( 'elementor/element/print_template', 'add_spacing_to_section_and_column_preview_template', 10, 2);

/**
 * 
 * @param $template string The JavaScript template output
 * @param $widget	Widget_Base	The widget instance
 */
function add_spacing_to_section_and_column_preview_template( $template, $widget ) {
	$class_name = get_class( $widget );
	
	// By default, new template is the same as the old template
	$new_template = $template;

	/*
	 * Create the Javascript data processing part of the Backbone preview template which
	 * tells Backbone to construct a string of mld spacing classes out of a repeater control 
	 * whenever the controls are updated.
	 */ 
	ob_start();
	?>
		<# 
			if ( settings.mld_spacing_items && (settings.spacing_items_override !== 'yes') ) {
				var spacing_classes_array = [];
				_.each( settings.mld_spacing_items, function( item ) { 
						var spacing_class = item.property + "-" + item.direction + "-" + item.heading_level;
						spacing_classes_array.push(spacing_class);
						
						if (item.zero_xs === 'yes') {
							spacing_classes_array.push(item.property + "-" + item.direction + "-none" + "-xs");
						}

						if (item.zero_sm === 'yes') {
							spacing_classes_array.push(item.property + "-" + item.direction + "-none" + "-sm");
						}

						if (item.zero_md === 'yes') {
							spacing_classes_array.push(item.property + "-" + item.direction + "-none" + "-md");
						}

						if (item.zero_lg === 'yes') {
							spacing_classes_array.push(item.property + "-" + item.direction + "-none" + "-lg");
						}

						if (item.zero_xl === 'yes') {
							spacing_classes_array.push(item.property + "-" + item.direction + "-none" + "-xl");
						}
				});
				
				var spacing_classes_string = spacing_classes_array.join(' ');
			}
		#>
	<?php
	// Get the output buffer contents as a string and turn off the output buffer
	// We will prepend the processing script to the template further down in this function
	$mld_spacing_processing_script = ob_get_clean();

	if ( $class_name === 'Elementor\\Element_Section' ) {
		/*
		 * Replace the original elementor-row with a row that has our spacing class variable
		 * which Backbone will dynamically update.
		 */
		$row_string = '<div class="elementor-row">';
		$modified_row_string = '<div class="elementor-row {{ spacing_classes_string }}">';

		// mixed str_replace ( mixed $search , mixed $replace , mixed $subject [, int &$count ] )
		$new_template = str_replace( $row_string, $modified_row_string, $template );
	} elseif (  $class_name === 'Elementor\\Element_Column' ) {
		/*
		 * Replace the original elementor-column with a column that has our spacing class variable
		 * which Backbone will dynamically update.
		 */
		$column_string = '<div class="elementor-column-wrap">';
		$modified_column_string = '<div class="elementor-column-wrap {{ spacing_classes_string }}">';

		// mixed str_replace ( mixed $search , mixed $replace , mixed $subject [, int &$count ] )
		$new_template = str_replace( $column_string, $modified_column_string, $template );
	}

	// Add the processing script to the beginning of the template
	$new_template = $mld_spacing_processing_script . "\n" .$new_template;

	return $new_template;
}