WordPress: Use HiDPI thumbnails with the same aspect ratio as the original image in gallery shortcodes.

hidpi-gallery-thumbnails.php

<?php
// Width is 2x the max that should ever show.
// Height is really tall so it'll never be cropped vertically.
add_image_size( 'gallery-thumbnail', 920, 5000 );

/**
 * Set the default gallery size to 'gallery-thumbnail'.
 *
 * @param array $out Combined and sanitized attributes list.
 * @param array $pairs All supported attributes and their defaults.
 * @param array $atts User-defined attributes in the shortcode tag.
 * @return array
 */
function prefix_gallery_shortcode_attributes( $out, $pairs, $atts ) {
	if ( empty( $atts['size'] ) || 'thumbnail' == $atts['size'] ) {
		$out['size'] = 'gallery-thumbnail';
	}

	return $out;
}
add_filter( 'shortcode_atts_gallery', 'prefix_gallery_shortcode_attributes', 10, 3 );