Resolve #3486079 "Pasting update clipboard"

Closes #3486079

src/Twig/FootnotesSpacelessTwig.php

+<?php
+
+namespace Drupal\footnotes\Twig;
+
+use Twig\Extension\AbstractExtension;
+use Twig\TwigFilter;
+
+/**
+ * Fallback for the Twig spaceless deprecation.
+ *
+ * This is needed because Footnotes truly does need to strip any whitespace
+ * around html tags to avoid extra spaces before and after inline citations.
+ */
+class FootnotesSpacelessTwig extends AbstractExtension {
+
+  /**
+   * Returns an array of twig filters to be added.
+   *
+   * @return array
+   *   The twig filter for footnotes spaceless.
+   */
+  public function getFilters(): array {
+    return [
+      new TwigFilter(
+        'footnotes_spaceless',
+        [self::class, 'footnotesSpaceless'],
+        ['is_safe' => ['html']],
+      ),
+    ];
+  }
+
+  /**
+   * Removes whitespaces between HTML tags.
+   *
+   * @param string|null $content
+   *   The content to remove whitespaces from.
+   *
+   * @return string
+   *   The duplication of the spaceless function prior to Twig 3.12.
+   */
+  public static function footnotesSpaceless(?string $content): string {
+    return trim(preg_replace('/>\s+</', '><', $content ?? ''));
+  }
+
+}