diff --git a/tests/phpunit/tests/admin/includes/misc/customizerMobileViewportMeta.php b/tests/phpunit/tests/admin/includes/misc/customizerMobileViewportMeta.php
new file mode 100644
index 0000000000000..c0dcd875627d2
--- /dev/null
+++ b/tests/phpunit/tests/admin/includes/misc/customizerMobileViewportMeta.php
@@ -0,0 +1,55 @@
+<?php
+
+/**
+ * Tests for _customizer_mobile_viewport_meta().
+ *
+ * @group admin
+ * @group includes
+ *
+ * @covers ::_customizer_mobile_viewport_meta
+ */
+class Tests_Customizer_Mobile_Viewport_Meta extends WP_UnitTestCase {
+
+	/**
+	 * Tests _customizer_mobile_viewport_meta().
+	 *
+	 * @dataProvider data_customizer_mobile_viewport_meta
+	 *
+	 * @ticket 65186
+	 *
+	 * @param string $viewport_meta Original viewport meta.
+	 * @param string $expected      Expected viewport meta.
+	 */
+	public function test_customizer_mobile_viewport_meta( $viewport_meta, $expected ) {
+		$this->assertSame( $expected, _customizer_mobile_viewport_meta( $viewport_meta ) );
+	}
+
+	/**
+	 * Data provider for test_customizer_mobile_viewport_meta.
+	 *
+	 * @return array<string, array{
+	 *     viewport_meta: string,
+	 *     expected:      string,
+	 * }>
+	 */
+	public function data_customizer_mobile_viewport_meta(): array {
+		return array(
+			'default'                       => array(
+				'viewport_meta' => 'width=device-width,initial-scale=1.0',
+				'expected'      => 'width=device-width,initial-scale=1.0,minimum-scale=0.5,maximum-scale=1.2',
+			),
+			'empty'                         => array(
+				'viewport_meta' => '',
+				'expected'      => ',minimum-scale=0.5,maximum-scale=1.2',
+			),
+			'with trailing comma'           => array(
+				'viewport_meta' => 'width=device-width,initial-scale=1.0,',
+				'expected'      => 'width=device-width,initial-scale=1.0,minimum-scale=0.5,maximum-scale=1.2',
+			),
+			'with multiple trailing commas' => array(
+				'viewport_meta' => 'width=device-width,initial-scale=1.0,,',
+				'expected'      => 'width=device-width,initial-scale=1.0,minimum-scale=0.5,maximum-scale=1.2',
+			),
+		);
+	}
+}