Test idempotency of get_modifiable_text().

tests/phpunit/tests/html-api/wpHtmlTagProcessorModifiableText.php

@@ -0,0 +1,41 @@
+<?php
+/**
+ * Unit tests covering WP_HTML_Tag_Processor modifiable text functionality.
+ *
+ * @package WordPress
+ * @subpackage HTML-API
+ * @group html-api
+ *
+ * @coversDefaultClass WP_HTML_Tag_Processor
+ */
+class Tests_HtmlApi_WpHtmlTagProcessorModifiableText extends WP_UnitTestCase {
+	/**
+	 * Ensures that calls to `get_modifiable_text()` don't change the
+	 * parser state in a way that would corrupt repeated calls.
+	 *
+	 * @ticket 61576
+	 */
+	public function test_get_modifiable_text_is_idempotent() {
+		$processor = new WP_HTML_Tag_Processor( "<pre>\nFirst newline ignored.</pre>" );
+
+		// Find the text node in the middle.
+		while ( '#text' !== $processor->get_token_name() && $processor->next_token() ) {
+			continue;
+		}
+
+		$this->assertSame(
+			'#text',
+			$processor->get_token_name(),
+			'Failed to find text node under test: check test setup.'
+		);
+
+		// The count of 5 isn't important; but calling this multiple times is.
+		for ( $i = 0; $i < 5; $i++ ) {
+			$this->assertSame(
+				'First newline ignored',
+				$processor->get_modifiable_text(),
+				'Should have returned the same modifiable text regardless of how many times it was called.'
+			);
+		}
+	}
+}