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 e41e1120550b5..615f2491d4d4a 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
@@ -3793,6 +3793,17 @@ 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->token_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,
diff --git a/tests/phpunit/tests/html-api/wpHtmlProcessorModifiableText.php b/tests/phpunit/tests/html-api/wpHtmlProcessorModifiableText.php
index 5d093ae05dd07..868e392387fd1 100644
--- a/tests/phpunit/tests/html-api/wpHtmlProcessorModifiableText.php
+++ b/tests/phpunit/tests/html-api/wpHtmlProcessorModifiableText.php
@@ -69,6 +69,102 @@ public static function data_modifiable_text_special_textarea(): array {
);
}
+ /**
+ * Ensures that text node updates preserve a leading newline where the HTML parser
+ * ignores one newline after the opening tag.
+ *
+ * @ticket 64609
+ *
+ * @dataProvider data_modifiable_text_ignores_leading_newline_text_nodes
+ *
+ * @param string $tag_name Expected tag name.
+ * @param string $set_text Text to set.
+ * @param string $expected_html Expected HTML output.
+ */
+ public function test_modifiable_text_preserves_ignored_leading_newline_in_text_nodes( string $tag_name, string $set_text, string $expected_html ): void {
+ $html = "<{$tag_name}>initial";
+ $processor = WP_HTML_Processor::create_fragment( $html );
+ $this->assertNotNull( $processor, 'Failed to create a processor.' );
+ $this->assertTrue( $processor->next_tag( $tag_name ), 'Failed to find target tag.' );
+ $this->assertTrue( $processor->next_token(), 'Failed to find text node.' );
+ $this->assertSame( '#text', $processor->get_token_name(), 'Should have found a text node.' );
+
+ $this->assertTrue( $processor->set_modifiable_text( $set_text ) );
+ $expected_text = strtr(
+ $set_text,
+ array(
+ "\r\n" => "\n",
+ "\r" => "\n",
+ )
+ );
+ $this->assertSame(
+ $expected_text,
+ $processor->get_modifiable_text(),
+ 'Should have preserved the leading newline when reading the updated text.'
+ );
+
+ $updated_html = $processor->get_updated_html();
+ $this->assertEqualHTML(
+ $expected_html,
+ $updated_html,
+ '',
+ 'Should have preserved the leading newline in the serialized HTML.'
+ );
+
+ $processor = WP_HTML_Processor::create_fragment( $updated_html );
+ $this->assertNotNull( $processor, 'Failed to reprocess updated HTML.' );
+ $this->assertTrue( $processor->next_tag( $tag_name ), 'Failed to find target tag after update.' );
+ $roundtrip_text = '';
+ while ( $processor->next_token() && '#text' === $processor->get_token_name() ) {
+ $roundtrip_text .= $processor->get_modifiable_text();
+ }
+ $this->assertSame(
+ $expected_text,
+ $roundtrip_text,
+ 'Should have preserved the leading newline after reprocessing the updated HTML.'
+ );
+ }
+
+ /**
+ * Data provider.
+ *
+ * @return array
+ */
+ public static function data_modifiable_text_ignores_leading_newline_text_nodes(): array {
+ return array(
+ 'PRE leading newline' => array(
+ 'PRE',
+ "\nAFTER NEWLINE",
+ "\n\nAFTER NEWLINE
",
+ ),
+ 'PRE leading carriage return' => array(
+ 'PRE',
+ "\rAFTER CR",
+ "\n\nAFTER CR
",
+ ),
+ 'PRE leading carriage return + newline' => array(
+ 'PRE',
+ "\r\nAFTER CRLF",
+ "\n\nAFTER CRLF
",
+ ),
+ 'LISTING leading newline' => array(
+ 'LISTING',
+ "\nAFTER NEWLINE",
+ "\n\nAFTER NEWLINE",
+ ),
+ 'LISTING leading carriage return' => array(
+ 'LISTING',
+ "\rAFTER CR",
+ "\n\nAFTER CR",
+ ),
+ 'LISTING leading carriage return + newline' => array(
+ 'LISTING',
+ "\r\nAFTER CRLF",
+ "\n\nAFTER CRLF",
+ ),
+ );
+ }
+
/**
* Ensures that `set_modifiable_text()` returns false for elements that are not special "atomic" elements.
*
diff --git a/tests/phpunit/tests/html-api/wpHtmlTagProcessorModifiableText.php b/tests/phpunit/tests/html-api/wpHtmlTagProcessorModifiableText.php
index 4a09403b7b23e..a88956bf6d3a4 100644
--- a/tests/phpunit/tests/html-api/wpHtmlTagProcessorModifiableText.php
+++ b/tests/phpunit/tests/html-api/wpHtmlTagProcessorModifiableText.php
@@ -665,6 +665,100 @@ public function test_modifiable_text_special_textarea(): void {
);
}
+ /**
+ * Ensures that text node updates preserve a leading newline where the HTML parser
+ * ignores one newline after the opening tag.
+ *
+ * @ticket 64609
+ *
+ * @dataProvider data_modifiable_text_ignores_leading_newline_text_nodes
+ *
+ * @param string $tag_name Expected tag name.
+ * @param string $set_text Text to set.
+ * @param string $expected_html Expected HTML output.
+ */
+ public function test_modifiable_text_preserves_ignored_leading_newline_in_text_nodes( string $tag_name, string $set_text, string $expected_html ): void {
+ $html = "<{$tag_name}>initial";
+ $processor = new WP_HTML_Tag_Processor( $html );
+ $this->assertTrue( $processor->next_tag( $tag_name ), 'Failed to find target tag.' );
+ $this->assertTrue( $processor->next_token(), 'Failed to find text node.' );
+ $this->assertSame( '#text', $processor->get_token_name(), 'Should have found a text node.' );
+
+ $this->assertTrue( $processor->set_modifiable_text( $set_text ) );
+ $expected_text = strtr(
+ $set_text,
+ array(
+ "\r\n" => "\n",
+ "\r" => "\n",
+ )
+ );
+ $this->assertSame(
+ $expected_text,
+ $processor->get_modifiable_text(),
+ 'Should have preserved the leading newline when reading the updated text.'
+ );
+
+ $updated_html = $processor->get_updated_html();
+ $this->assertEqualHTML(
+ $expected_html,
+ $updated_html,
+ '',
+ 'Should have preserved the leading newline in the serialized HTML.'
+ );
+
+ $processor = new WP_HTML_Tag_Processor( $updated_html );
+ $this->assertTrue( $processor->next_tag( $tag_name ), 'Failed to find target tag after update.' );
+ $roundtrip_text = '';
+ while ( $processor->next_token() && '#text' === $processor->get_token_name() ) {
+ $roundtrip_text .= $processor->get_modifiable_text();
+ }
+ $this->assertSame(
+ $expected_text,
+ $roundtrip_text,
+ 'Should have preserved the leading newline after reprocessing the updated HTML.'
+ );
+ }
+
+ /**
+ * Data provider.
+ *
+ * @return array
+ */
+ public static function data_modifiable_text_ignores_leading_newline_text_nodes(): array {
+ return array(
+ 'PRE leading newline' => array(
+ 'PRE',
+ "\nAFTER NEWLINE",
+ "\n\nAFTER NEWLINE
",
+ ),
+ 'PRE leading carriage return' => array(
+ 'PRE',
+ "\rAFTER CR",
+ "\n\nAFTER CR
",
+ ),
+ 'PRE leading carriage return + newline' => array(
+ 'PRE',
+ "\r\nAFTER CRLF",
+ "\n\nAFTER CRLF
",
+ ),
+ 'LISTING leading newline' => array(
+ 'LISTING',
+ "\nAFTER NEWLINE",
+ "\n\nAFTER NEWLINE",
+ ),
+ 'LISTING leading carriage return' => array(
+ 'LISTING',
+ "\rAFTER CR",
+ "\n\nAFTER CR",
+ ),
+ 'LISTING leading carriage return + newline' => array(
+ 'LISTING',
+ "\r\nAFTER CRLF",
+ "\n\nAFTER CRLF",
+ ),
+ );
+ }
+
/**
* Ensures that `set_modifiable_text()` returns false for elements that are
* not special "atomic" elements.