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
index 69e3e5d2c7557..7843a83b3932c 100644
--- 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
new file mode 100644
index 0000000000000..5a4bcfe606459
--- /dev/null
+++ b/tests/phpunit/tests/html-api/wpHtmlProcessorModifiableText.php
@@ -0,0 +1,199 @@
+' );
+ $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(),
+ '',
+ '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",
+ "",
+ ),
+ 'Leading carriage return' => array(
+ "\rCR",
+ "",
+ ),
+ 'Leading carriage return + newline' => array(
+ "\r\nCR-N",
+ "",
+ ),
+ );
+ }
+
+ /**
+ * 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(),
+ '',
+ '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(
+ "<{$tag_name}>REPLACEME",
+ 1,
+ 'REPLACEME',
+ "\nAFTER NEWLINE.",
+ "<{$tag_name}>\n\nAFTER NEWLINE.",
+ );
+
+ yield "<{$tag_name}> with leading newline, first text node" => array(
+ "<{$tag_name}>\nREPLACEME",
+ 1,
+ '',
+ "\nAFTER NEWLINE.",
+ "<{$tag_name}>\n\nAFTER NEWLINE.REPLACEME",
+ );
+
+ yield "<{$tag_name}> with leading newline, second text node" => array(
+ "<{$tag_name}>\nREPLACEME",
+ 2,
+ 'REPLACEME',
+ "\nAFTER NEWLINE.",
+ "<{$tag_name}>\n\nAFTER NEWLINE.",
+ );
+
+ yield "<{$tag_name}> with leading space, first text node" => array(
+ "<{$tag_name}> REPLACEME",
+ 1,
+ ' ',
+ "\nAFTER NEWLINE.",
+ "<{$tag_name}>\n\nAFTER NEWLINE.REPLACEME",
+ );
+
+ yield "<{$tag_name}> with leading space, second text node" => array(
+ "<{$tag_name}> REPLACEME",
+ 2,
+ 'REPLACEME',
+ "\nAFTER NEWLINE.",
+ "<{$tag_name}>\n \nAFTER NEWLINE.",
+ );
+
+ yield "<{$tag_name}> insert with leading carriage return" => array(
+ "<{$tag_name}>REPLACEME",
+ 1,
+ 'REPLACEME',
+ "\rCR",
+ "<{$tag_name}>\n\nCR",
+ );
+
+ yield "<{$tag_name}> insert with leading carriage return + newline" => array(
+ "<{$tag_name}>REPLACEME",
+ 1,
+ 'REPLACEME',
+ "\r\nCR-N",
+ "<{$tag_name}>\n\nCR-N",
+ );
+
+ yield "<{$tag_name}> clear text" => array(
+ "<{$tag_name}>REPLACEME",
+ 1,
+ 'REPLACEME',
+ '',
+ "<{$tag_name}>",
+ );
+ }
+ }
+}
diff --git a/tests/phpunit/tests/html-api/wpHtmlTagProcessorModifiableText.php b/tests/phpunit/tests/html-api/wpHtmlTagProcessorModifiableText.php
index 9e0d94aecd17e..b4262f80ea3d9 100644
--- a/tests/phpunit/tests/html-api/wpHtmlTagProcessorModifiableText.php
+++ b/tests/phpunit/tests/html-api/wpHtmlTagProcessorModifiableText.php
@@ -636,4 +636,74 @@ 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( '' );
+ $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.'
+ );
+ }
+
+ /**
+ * 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( 'REPLACEME
' );
+ $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() );
+ $this->assertEqualHTML(
+ <<
+ {$set_text}
+ HTML,
+ $processor->get_updated_html(),
+ '',
+ '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( 'REPLACEME' );
+ $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() );
+ $this->assertEqualHTML(
+ <<
+ {$set_text}
+ HTML,
+ $processor->get_updated_html(),
+ '',
+ 'Should have preserved the leading newline in the content.'
+ );
+ }
}