HTML API: Preserve ::set_modifiable_text() TEXTAREA leading newlines.

HTML specifies that a single newline is ignored at the start of a `TEXTAREA`. If `::set_modifiable_text()` is called with a leading newline, ensure it is preserved in the resulting HTML.

Developed in WordPress/wordpress-develop#11062.

Props jonsurrell, dmsnell.
See #64609.

Built from https://develop.svn.wordpress.org/trunk@61754


git-svn-id: http://core.svn.wordpress.org/trunk@61060 1a063a9b-81f0-0310-95a4-ce76da25c4cd

Author: sirreal

wp-includes/html-api/class-wp-html-tag-processor.php

@@ -3877,6 +3877,17 @@ static function ( $tag_match ) {
 					$plaintext_content
 				);
 
+				/*
+				 * HTML ignores a single leading newline in this context. If a leading newline
+				 * is intended, preserve it by adding an extra newline.
+				 */
+				if (
+					'TEXTAREA' === $this->get_tag() &&
+					1 === strspn( $plaintext_content, "\n\r", 0, 1 )
+				) {
+					$plaintext_content = "\n{$plaintext_content}";
+				}
+
 				/*
 				 * These don't _need_ to be escaped, but since they are decoded it's
 				 * safe to leave them escaped and this can prevent other code from

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '7.0-beta2-61753';
+$wp_version = '7.0-beta2-61754';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.