HTML API: Preserve newlines when normalizing special elements.

Ensures normalization preserves content in `PRE`, `LISTING`, and `TEXTAREA` elements. These elements ignore a single leading newline during parsing. Normalization now injects a newline after the tag opener to trigger this behavior, preventing significant newlines from being incorrectly stripped.

Developed in WordPress/wordpress-develop#10871.

Props jonsurrell, dmsnell, mukesh27.
Fixes #64607.

Built from https://develop.svn.wordpress.org/trunk@61747


git-svn-id: http://core.svn.wordpress.org/trunk@61053 1a063a9b-81f0-0310-95a4-ce76da25c4cd

Author: sirreal

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

@@ -1414,6 +1414,32 @@ public function serialize_token(): string {
 
 		$html .= '>';
 
+		/*
+		 * The HTML parser strips a leading newline immediately after the start
+		 * tag of TEXTAREA, PRE, and LISTING elements. When serializing, prepend
+		 * a leading newline to ensure the semantic HTML content is preserved.
+		 *
+		 * For example, `<pre>\n\nX</pre>` must not become `<pre>\nX</pre>` because its content
+		 * has changed. However, `<pre>X</pre>` and `<pre>\nX</pre>` are _equivalent_.
+		 *
+		 * > A start tag whose tag name is "textarea"
+		 * >   …
+		 * >   If the next token is a U+000A LINE FEED (LF) character token, then ignore
+		 * >   that token and move on to the next one. (Newlines at the start of textarea
+		 * >   elements are ignored as an authoring convenience.)
+		 *
+		 * > A start tag whose tag name is one of: "pre", "listing"
+		 * >   …
+		 * >   If the next token is a U+000A LINE FEED (LF) character token, then ignore
+		 * >   that token and move on to the next one. (Newlines at the start of pre blocks
+		 * >   are ignored as an authoring convenience.)
+		 *
+		 * @see https://html.spec.whatwg.org/multipage/parsing.html
+		 */
+		if ( 'TEXTAREA' === $tag_name || 'PRE' === $tag_name || 'LISTING' === $tag_name ) {
+			$html .= "\n";
+		}
+
 		// Flush out self-contained elements.
 		if ( $in_html && in_array( $tag_name, array( 'IFRAME', 'NOEMBED', 'NOFRAMES', 'SCRIPT', 'STYLE', 'TEXTAREA', 'TITLE', 'XMP' ), true ) ) {
 			$text = $this->get_modifiable_text();

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '7.0-beta1-61746';
+$wp_version = '7.0-beta1-61747';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.