Add test codifying set_attribute bug modifying breadcrumbs.

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

@@ -290,6 +290,49 @@ public function test_finds_correct_tag_given_breadcrumbs( $html, $breadcrumbs, $
 		$this->assertTrue( $p->get_attribute( 'target' ), "Found {$p->get_tag()} element didn't contain the necessary 'target' attribute." );
 	}
 
+	/**
+	 * Ensures that updates to a tag's attributes doesn't shift
+	 * the current position into the input HTML document.
+	 *
+	 * @ticket 59607
+	 */
+	public function test_remains_stable_when_editing_attributes() {
+		$p = WP_HTML_Processor::create_fragment( '<div><button>First<button><b here>Second' );
+		$p->next_tag( array( 'breadcrumbs' => array( 'BUTTON', 'B' ) ) );
+
+		$this->assertSame(
+			array( 'HTML', 'BODY', 'DIV', 'BUTTON', 'B' ),
+			$p->get_breadcrumbs(),
+			'Found the wrong nested structure at the matched tag.'
+		);
+
+		$p->set_attribute( 'a-name', 'a-value' );
+
+		$this->assertTrue(
+			$p->get_attribute( 'here' ),
+			'Should have found the B tag but could not find expected "here" attribute.'
+		);
+
+		$this->assertSame(
+			array( 'HTML', 'BODY', 'DIV', 'BUTTON', 'B' ),
+			$p->get_breadcrumbs(),
+			'Found the wrong nested structure at the matched tag.'
+		);
+
+		$p->get_updated_html();
+
+		$this->assertTrue(
+			$p->get_attribute( 'here' ),
+			'Should have stayed at the B tag but could not find expected "here" attribute.'
+		);
+
+		$this->assertSame(
+			array( 'HTML', 'BODY', 'DIV', 'BUTTON', 'B' ),
+			$p->get_breadcrumbs(),
+			'Found the wrong nested structure at the matched tag after updating attributes.'
+		);
+	}
+
 	/**
 	 * @ticket 58517
 	 *