WordPress · sirreal · Feb 5, 2026 · Feb 5, 2026 · Feb 5, 2026 · Feb 5, 2026
diff --git a/src/wp-includes/html-api/class-wp-html-tag-processor.php b/src/wp-includes/html-api/class-wp-html-tag-processor.php
@@ -3770,6 +3770,17 @@ public function get_modifiable_text(): string {
 	 */
 	public function set_modifiable_text( string $plaintext_content ): bool {
 		if ( self::STATE_TEXT_NODE === $this->parser_state ) {
+			/*
+			 * HTML ignores a single leading newline in this context. If a leading newline
+			 * is intended, preserve it by adding an extra newline.
+			 */
+			if (
+				$this->skip_newline_at === $this->text_starts_at &&
+				1 === strspn( $plaintext_content, "\n\r", 0, 1 )
+			) {
+				$plaintext_content = "\n{$plaintext_content}";
+			}
+
 			$this->lexical_updates['modifiable text'] = new WP_HTML_Text_Replacement(
 				$this->text_starts_at,
 				$this->text_length,
@@ -3877,6 +3888,17 @@ 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

diff --git a/tests/phpunit/tests/html-api/wpHtmlProcessorModifiableText.php b/tests/phpunit/tests/html-api/wpHtmlProcessorModifiableText.php
@@ -0,0 +1,199 @@
+<?php
+/**
+ * Unit tests covering WP_HTML_Processor modifiable text functionality.
+ *
+ * @package WordPress
+ * @subpackage HTML-API
+ * @group html-api
+ *
+ * @coversDefaultClass WP_HTML_Processor
+ */
+class Tests_HtmlApi_WpHtmlProcessorModifiableText extends WP_UnitTestCase {
+	/**
+	 * TEXTAREA elements ignore the first newline in their content.
+	 * Setting the modifiable text with a leading newline (or carriage return variants)
+	 * should ensure that the leading newline is present in the resulting TEXTAREA.
+	 *
+	 * TEXTAREA are treated as atomic tags by the tag processor, so `set_modifiable_text()`
+	 * is called directly on the TEXTAREA token, making them different from PRE and LISTING
+	 * tags that also have special newline handling in HTML.
+	 *
+	 * @ticket 64609
+	 *
+	 * @dataProvider data_modifiable_text_special_textarea
+	 *
+	 * @param string $set_text         Text to set.
+	 * @param string $expected_html    Expected HTML output.
+	 */
+	public function test_modifiable_text_special_textarea( string $set_text, string $expected_html ) {
+		$processor = WP_HTML_Processor::create_fragment( '<textarea></textarea>' );
+		$processor->next_token();
+		$processor->set_modifiable_text( $set_text );
+		$this->assertSame(
+			strtr(
+				$set_text,
+				array(
+					"\r\n" => "\n",
+					"\r"   => "\n",
+				)
+			),
+			$processor->get_modifiable_text(),
+			'Should have preserved or normalized the leading newline in the TEXTAREA content.'
+		);
+		$this->assertEqualHTML(
+			$expected_html,
+			$processor->get_updated_html(),
+			'<body>',
+			'Should have correctly output the TEXTAREA HTML.'
+		);
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public static function data_modifiable_text_special_textarea() {
+		return array(
+			'Leading newline'                   => array(
+				"\nAFTER NEWLINE",
+				"<textarea>\n\nAFTER NEWLINE</textarea>",
+			),
+			'Leading carriage return'           => array(
+				"\rCR",
+				"<textarea>\n\nCR</textarea>",
+			),
+			'Leading carriage return + newline' => array(
+				"\r\nCR-N",
+				"<textarea>\n\nCR-N</textarea>",
+			),
+		);
+	}
+
+	/**
+	 * PRE and LISTING elements ignore the first newline in their content.
+	 * Leading whitespace may split into multiple text nodes in the HTML Processor.
+	 * Setting the modifiable text with a leading newline should ensure that the
+	 * leading newline is present in the resulting element.
+	 *
+	 * The HTML Processor has special behavior when a text node starts with whitespace.
+	 * Test that PRE and LISTING `::set_modifiable_text()` handling works correctly
+	 * with leading whitespace.
+	 *
+	 * @ticket 64609
+	 *
+	 * @dataProvider data_modifiable_text_special_leading_whitespace
+	 *
+	 * @param string $html             HTML containing the element to test.
+	 * @param int    $advance_n_tokens Count of times to run `next_token()` after `next_tag()`.
+	 * @param string $stopped_on_text  Expected modifiable text before the update.
+	 * @param string $set_text         Text to set.
+	 * @param string $expected_html    Expected HTML output after setting modifiable text.
+	 */
+	public function test_modifiable_text_special_leading_whitespace(
+		string $html,
+		int $advance_n_tokens,
+		string $stopped_on_text,
+		string $set_text,
+		string $expected_html
+	) {
+		$processor = WP_HTML_Processor::create_fragment( $html );
+		$processor->next_tag();
+		while ( --$advance_n_tokens >= 0 ) {
+			$processor->next_token();
+		}
+		$this->assertSame( '#text', $processor->get_token_type() );
+		$this->assertSame( $stopped_on_text, $processor->get_modifiable_text() );
+		$processor->set_modifiable_text( $set_text );
+
+		// Newline normalization transforms \r and \r\n into \n.
+		$this->assertSame(
+			strtr(
+				$set_text,
+				array(
+					"\r\n" => "\n",
+					"\r"   => "\n",
+				)
+			),
+			$processor->get_modifiable_text()
+		);
+		$this->assertEqualHTML(
+			$expected_html,
+			$processor->get_updated_html(),
+			'<body>',
+			'Should have preserved the leading newline in the element content.'
+		);
+	}
+
+	/**
+	 * Data provider.
+	 */
+	public static function data_modifiable_text_special_leading_whitespace() {
+		$tags = array( 'pre', 'listing' );
+		foreach ( $tags as $tag_name ) {
+			yield "<{$tag_name}> with no leading newline" => array(
+				"<{$tag_name}>REPLACEME<!--x--></{$tag_name}>",
+				1,
+				'REPLACEME',
+				"\nAFTER NEWLINE.",
+				"<{$tag_name}>\n\nAFTER NEWLINE.<!--x--></{$tag_name}>",
+			);
+
+			yield "<{$tag_name}> with leading newline, first text node" => array(
+				"<{$tag_name}>\nREPLACEME<!--x--></{$tag_name}>",
+				1,
+				'',
+				"\nAFTER NEWLINE.",
+				"<{$tag_name}>\n\nAFTER NEWLINE.REPLACEME<!--x--></{$tag_name}>",
+			);
+
+			yield "<{$tag_name}> with leading newline, second text node" => array(
+				"<{$tag_name}>\nREPLACEME<!--x--></{$tag_name}>",
+				2,
+				'REPLACEME',
+				"\nAFTER NEWLINE.",
+				"<{$tag_name}>\n\nAFTER NEWLINE.<!--x--></{$tag_name}>",
+			);
+
+			yield "<{$tag_name}> with leading space, first text node" => array(
+				"<{$tag_name}> REPLACEME<!--x--></{$tag_name}>",
+				1,
+				' ',
+				"\nAFTER NEWLINE.",
+				"<{$tag_name}>\n\nAFTER NEWLINE.REPLACEME<!--x--></{$tag_name}>",
+			);
+
+			yield "<{$tag_name}> with leading space, second text node" => array(
+				"<{$tag_name}> REPLACEME<!--x--></{$tag_name}>",
+				2,
+				'REPLACEME',
+				"\nAFTER NEWLINE.",
+				"<{$tag_name}>\n \nAFTER NEWLINE.<!--x--></{$tag_name}>",
+			);
+
+			yield "<{$tag_name}> insert with leading carriage return" => array(
+				"<{$tag_name}>REPLACEME<!--x--></{$tag_name}>",
+				1,
+				'REPLACEME',
+				"\rCR",
+				"<{$tag_name}>\n\nCR<!--x--></{$tag_name}>",
+			);
+
+			yield "<{$tag_name}> insert with leading carriage return + newline" => array(
+				"<{$tag_name}>REPLACEME<!--x--></{$tag_name}>",
+				1,
+				'REPLACEME',
+				"\r\nCR-N",
+				"<{$tag_name}>\n\nCR-N<!--x--></{$tag_name}>",
+			);
+
+			yield "<{$tag_name}> clear text" => array(
+				"<{$tag_name}>REPLACEME<!--x--></{$tag_name}>",
+				1,
+				'REPLACEME',
+				'',
+				"<{$tag_name}><!--x--></{$tag_name}>",
+			);
+		}
+	}
+}
diff --git a/tests/phpunit/tests/html-api/wpHtmlTagProcessorModifiableText.php b/tests/phpunit/tests/html-api/wpHtmlTagProcessorModifiableText.php
@@ -636,4 +636,84 @@ public function test_json_auto_escaping() {
 			$decoded_json_from_html
 		);
 	}
+
+	/**
+	 * TEXTAREA elements ignore the first newline in their content.
+	 * Setting the modifiable text with a leading newline should ensure that the leading newline
+	 * is present in the resulting element.
+	 *
+	 * @ticket 64609
+	 */
+	public function test_modifiable_text_special_textarea() {
+		$processor = new WP_HTML_Tag_Processor( '<textarea></textarea>' );
+		$processor->next_token();
+		$processor->set_modifiable_text( "\nAFTER NEWLINE" );
+		$this->assertSame(
+			"\nAFTER NEWLINE",
+			$processor->get_modifiable_text(),
+			'Should have preserved the leading newline in the content.'
+		);
+		$this->assertEqualHTML(
+			<<<'HTML'
+			<textarea>
+
+			AFTER NEWLINE</textarea>
+			HTML,
+			$processor->get_updated_html(),
+			'<body>',
+			'Should have preserved the leading newline in the content.'
+		);
+	}
+
+	/**
+	 * PRE elements ignore the first newline in their content.
+	 * Setting the modifiable text with a leading newline should ensure that the leading newline
+	 * is present in the resulting element.
+	 *
+	 * @ticket 64609
+	 */
+	public function test_modifiable_text_special_pre() {
+		$set_text  = "\nAFTER NEWLINE";
+		$processor = new WP_HTML_Tag_Processor( '<pre>REPLACEME<!--x--></pre>' );
+		$processor->next_tag();
+		$processor->next_token();
+		$this->assertSame( '#text', $processor->get_token_type() );
+		$processor->set_modifiable_text( $set_text );
+		$this->assertSame( $set_text, $processor->get_modifiable_text() );
+		$this->assertEqualHTML(
+			<<<HTML
+			<pre>
+			{$set_text}<!--x--></pre>
+			HTML,
+			$processor->get_updated_html(),
+			'<body>',
+			'Should have preserved the leading newline in the content.'
+		);
+	}
+
+	/**
+	 * LISTING elements ignore the first newline in their content.
+	 * Setting the modifiable text with a leading newline should ensure that the leading newline
+	 * is present in the resulting element.
+	 *
+	 * @ticket 64609
+	 */
+	public function test_modifiable_text_special_listing() {
+		$set_text  = "\nAFTER NEWLINE";
+		$processor = new WP_HTML_Tag_Processor( '<listing>REPLACEME<!--x--></listing>' );
+		$processor->next_tag();
+		$processor->next_token();
+		$this->assertSame( '#text', $processor->get_token_type() );
+		$processor->set_modifiable_text( $set_text );
+		$this->assertSame( $set_text, $processor->get_modifiable_text() );
+		$this->assertEqualHTML(
+			<<<HTML
+			<listing>
+			{$set_text}<!--x--></listing>
+			HTML,
+			$processor->get_updated_html(),
+			'<body>',
+			'Should have preserved the leading newline in the content.'
+		);
+	}
 }