diff --git a/tests/phpunit/tests/admin/pluginUpgraderSkin.php b/tests/phpunit/tests/admin/pluginUpgraderSkin.php
new file mode 100644
index 0000000000000..3f4e96caf03b4
--- /dev/null
+++ b/tests/phpunit/tests/admin/pluginUpgraderSkin.php
@@ -0,0 +1,386 @@
+<?php
+
+/**
+ * Tests for Plugin_Upgrader_Skin.
+ *
+ * @group admin
+ * @group plugins
+ * @group upgrade
+ *
+ * @covers Plugin_Upgrader_Skin
+ */
+class Tests_Admin_PluginUpgraderSkin extends WP_UnitTestCase {
+
+	/**
+	 * Plugin file of a plugin that is installed in the test data.
+	 *
+	 * @var string
+	 */
+	const PLUGIN = 'custom-internationalized-plugin/custom-internationalized-plugin.php';
+
+	/**
+	 * Administrator user ID.
+	 *
+	 * @var int
+	 */
+	private static $admin_id;
+
+	/**
+	 * Loads the upgrader classes and sets up the users.
+	 *
+	 * @param WP_UnitTest_Factory $factory Test factory.
+	 */
+	public static function wpSetUpBeforeClass( WP_UnitTest_Factory $factory ) {
+		require_once ABSPATH . 'wp-admin/includes/class-wp-upgrader.php';
+
+		self::$admin_id = $factory->user->create( array( 'role' => 'administrator' ) );
+
+		if ( is_multisite() ) {
+			grant_super_admin( self::$admin_id );
+		}
+	}
+
+	/**
+	 * Revokes the super admin fixture.
+	 */
+	public static function wpTearDownAfterClass() {
+		if ( is_multisite() ) {
+			revoke_super_admin( self::$admin_id );
+		}
+	}
+
+	/**
+	 * Clears the plugins cache.
+	 */
+	public function tear_down() {
+		wp_cache_delete( 'plugins', 'plugins' );
+
+		parent::tear_down();
+	}
+
+	/**
+	 * Tests the defaults of the skin.
+	 *
+	 * @ticket 66551
+	 *
+	 * @covers Plugin_Upgrader_Skin::__construct
+	 */
+	public function test_constructor_should_use_defaults() {
+		$skin = new Plugin_Upgrader_Skin();
+
+		$this->assertSame( 'Update Plugin', $skin->options['title'], 'The title should be set.' );
+		$this->assertSame( '', $skin->options['url'], 'The URL should be empty.' );
+		$this->assertSame( '', $skin->options['nonce'], 'The nonce should be empty.' );
+		$this->assertSame( '', $skin->plugin, 'The plugin should be empty.' );
+		$this->assertFalse( $skin->plugin_active, 'The plugin should not be active.' );
+		$this->assertFalse( $skin->plugin_network_active, 'The plugin should not be network active.' );
+	}
+
+	/**
+	 * Tests that the arguments override the defaults.
+	 *
+	 * @ticket 66551
+	 *
+	 * @covers Plugin_Upgrader_Skin::__construct
+	 */
+	public function test_constructor_should_use_arguments() {
+		$skin = new Plugin_Upgrader_Skin(
+			array(
+				'url'    => 'update.php?action=upgrade-plugin',
+				'plugin' => self::PLUGIN,
+				'nonce'  => 'upgrade-plugin_' . self::PLUGIN,
+				'title'  => 'My title',
+			)
+		);
+
+		$this->assertSame( 'update.php?action=upgrade-plugin', $skin->options['url'] );
+		$this->assertSame( 'upgrade-plugin_' . self::PLUGIN, $skin->options['nonce'] );
+		$this->assertSame( 'My title', $skin->options['title'] );
+		$this->assertSame( self::PLUGIN, $skin->plugin );
+	}
+
+	/**
+	 * Tests that the skin knows when the plugin is active.
+	 *
+	 * @ticket 66551
+	 *
+	 * @covers Plugin_Upgrader_Skin::__construct
+	 */
+	public function test_constructor_should_detect_an_active_plugin() {
+		update_option( 'active_plugins', array( self::PLUGIN ) );
+
+		$skin = new Plugin_Upgrader_Skin( array( 'plugin' => self::PLUGIN ) );
+
+		$this->assertTrue( $skin->plugin_active, 'The plugin should be active.' );
+		$this->assertFalse( $skin->plugin_network_active, 'The plugin should not be network active.' );
+	}
+
+	/**
+	 * Tests that the skin knows when the plugin is active for the network.
+	 *
+	 * @ticket 66551
+	 *
+	 * @group ms-required
+	 *
+	 * @covers Plugin_Upgrader_Skin::__construct
+	 */
+	public function test_constructor_should_detect_a_network_active_plugin() {
+		update_site_option( 'active_sitewide_plugins', array( self::PLUGIN => time() ) );
+
+		$skin = new Plugin_Upgrader_Skin( array( 'plugin' => self::PLUGIN ) );
+
+		$this->assertTrue( $skin->plugin_active, 'A network active plugin should be active.' );
+		$this->assertTrue( $skin->plugin_network_active, 'The plugin should be network active.' );
+	}
+
+	/**
+	 * Tests that an active plugin is activated again in a frame after it is updated.
+	 *
+	 * @ticket 66551
+	 *
+	 * @covers Plugin_Upgrader_Skin::after
+	 */
+	public function test_after_should_reactivate_an_active_plugin_in_a_frame() {
+		wp_set_current_user( self::$admin_id );
+		update_option( 'active_plugins', array( self::PLUGIN ) );
+
+		$output = $this->run_after( $this->get_skin( true ) );
+
+		$this->assertStringContainsString( '<iframe title="Update progress"', $output, 'The frame should be output.' );
+		$this->assertStringContainsString( 'src="update.php?action=activate-plugin&amp;', $output, 'The frame should activate the plugin.' );
+		$this->assertStringContainsString( '&amp;plugin=custom-internationalized-plugin%2Fcustom-internationalized-plugin.php&amp;_wpnonce=', $output, 'The frame should activate the updated plugin.' );
+		$this->assertStringNotContainsString( 'Activate Plugin', $output, 'An active plugin should not get a link to activate it.' );
+		$this->assertStringContainsString( 'Go to Plugins page', $output, 'The Plugins page should be linked.' );
+	}
+
+	/**
+	 * Tests that a plugin that was not active is not activated in a frame.
+	 *
+	 * @ticket 66551
+	 *
+	 * @covers Plugin_Upgrader_Skin::after
+	 */
+	public function test_after_should_not_output_a_frame_for_an_inactive_plugin() {
+		wp_set_current_user( self::$admin_id );
+
+		$output = $this->run_after( $this->get_skin( true ) );
+
+		$this->assertStringNotContainsString( '<iframe', $output );
+	}
+
+	/**
+	 * Tests that no frame is output without a plugin, or when the update failed.
+	 *
+	 * @ticket 66551
+	 *
+	 * @covers Plugin_Upgrader_Skin::after
+	 *
+	 * @dataProvider data_results_without_a_frame
+	 *
+	 * @param mixed $result      The result of the update.
+	 * @param bool  $has_plugin  Whether the update result has an installed plugin.
+	 */
+	public function test_after_should_not_output_a_frame_without_a_plugin_or_after_an_error( $result, $has_plugin ) {
+		wp_set_current_user( self::$admin_id );
+		update_option( 'active_plugins', array( self::PLUGIN ) );
+
+		$output = $this->run_after( $this->get_skin( $result, $has_plugin ) );
+
+		$this->assertStringNotContainsString( '<iframe', $output );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public static function data_results_without_a_frame() {
+		return array(
+			'no plugin was installed' => array( true, false ),
+			'an error'                => array( new WP_Error( 'failed', 'Failed.' ), true ),
+		);
+	}
+
+	/**
+	 * Tests the links after a successful update of a plugin that is not active.
+	 *
+	 * @ticket 66551
+	 *
+	 * @covers Plugin_Upgrader_Skin::after
+	 */
+	public function test_after_should_link_to_activate_the_plugin_and_to_the_plugins_page() {
+		wp_set_current_user( self::$admin_id );
+
+		$output = $this->run_after( $this->get_skin( true ) );
+
+		$this->assertStringContainsString( '<a href="', $output );
+		$this->assertStringContainsString( 'plugins.php?action=activate&amp;plugin=custom-internationalized-plugin%2Fcustom-internationalized-plugin.php&amp;_wpnonce=', $output, 'The link to activate the plugin should be output.' );
+		$this->assertStringContainsString( 'target="_parent">Activate Plugin</a> | <a href="', $output, 'The links should be separated.' );
+		$this->assertStringContainsString( '<a href="' . self_admin_url( 'plugins.php' ) . '" target="_parent">Go to Plugins page</a>', $output, 'The link to the Plugins page should be output.' );
+	}
+
+	/**
+	 * Tests that the link to activate the plugin is not output.
+	 *
+	 * @ticket 66551
+	 *
+	 * @covers Plugin_Upgrader_Skin::after
+	 *
+	 * @dataProvider data_results_without_an_activate_link
+	 *
+	 * @param mixed $result The result of the update.
+	 */
+	public function test_after_should_not_link_to_activate_the_plugin_after_a_failure( $result ) {
+		wp_set_current_user( self::$admin_id );
+
+		$output = $this->run_after( $this->get_skin( $result ) );
+
+		$this->assertStringNotContainsString( 'Activate Plugin', $output );
+		$this->assertStringContainsString( 'Go to Plugins page', $output );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public static function data_results_without_an_activate_link() {
+		return array(
+			'no result' => array( false ),
+			'an error'  => array( new WP_Error( 'failed', 'Failed.' ) ),
+		);
+	}
+
+	/**
+	 * Tests that the plugin is not offered for activation to a user who cannot activate it.
+	 *
+	 * @ticket 66551
+	 *
+	 * @covers Plugin_Upgrader_Skin::after
+	 */
+	public function test_after_should_not_link_to_activate_the_plugin_for_a_user_without_the_capability() {
+		wp_set_current_user( self::factory()->user->create( array( 'role' => 'subscriber' ) ) );
+
+		$output = $this->run_after( $this->get_skin( true ) );
+
+		$this->assertStringNotContainsString( 'Activate Plugin', $output );
+	}
+
+	/**
+	 * Tests that the links can be filtered.
+	 *
+	 * @ticket 66551
+	 *
+	 * @covers Plugin_Upgrader_Skin::after
+	 */
+	public function test_after_should_filter_the_links() {
+		wp_set_current_user( self::$admin_id );
+
+		$filter = new MockAction();
+		add_filter( 'update_plugin_complete_actions', array( $filter, 'filter' ), 10, 2 );
+		add_filter(
+			'update_plugin_complete_actions',
+			static function ( $actions ) {
+				$actions['custom'] = '<a href="#custom">Custom link</a>';
+
+				return $actions;
+			},
+			11
+		);
+
+		$output = $this->run_after( $this->get_skin( true ) );
+		$args   = $filter->get_args();
+
+		$this->assertSame( array( 'activate_plugin', 'plugins_page' ), array_keys( $args[0][0] ), 'The filter should receive both links.' );
+		$this->assertSame( self::PLUGIN, $args[0][1], 'The filter should receive the plugin file.' );
+		$this->assertStringContainsString( 'Go to Plugins page</a> | <a href="#custom">Custom link</a>', $output, 'The added link should be output.' );
+	}
+
+	/**
+	 * Tests that nothing is output about the links when they are removed.
+	 *
+	 * @ticket 66551
+	 *
+	 * @covers Plugin_Upgrader_Skin::after
+	 */
+	public function test_after_should_not_output_links_when_they_are_removed() {
+		wp_set_current_user( self::$admin_id );
+
+		add_filter( 'update_plugin_complete_actions', '__return_empty_array' );
+
+		$skin = $this->get_skin( true );
+
+		get_echo( array( $skin, 'after' ) );
+
+		$this->assertSame( array(), $skin->messages, 'No feedback should be given without links.' );
+	}
+
+	/**
+	 * Tests that the update count is decremented after a successful update only.
+	 *
+	 * @ticket 66551
+	 *
+	 * @covers Plugin_Upgrader_Skin::after
+	 */
+	public function test_after_should_decrement_the_update_count_after_a_successful_update() {
+		wp_set_current_user( self::$admin_id );
+
+		$output = $this->run_after( $this->get_skin( true ) );
+
+		$this->assertStringContainsString( '<script', $output, 'A script should be output.' );
+		$this->assertStringContainsString( '"plugin"', $output, 'The script should decrement the count of plugins.' );
+
+		$output = $this->run_after( $this->get_skin( false ) );
+
+		$this->assertStringNotContainsString( '<script', $output, 'No script should be output without a result.' );
+	}
+
+	/**
+	 * Returns a plugin upgrader skin attached to an upgrader.
+	 *
+	 * @param mixed $result     The result of the update, which the skin gets.
+	 * @param bool  $has_plugin Optional. Whether the upgrader result has the folder of an installed plugin.
+	 *                          Default true.
+	 * @return Plugin_Upgrader_Skin The skin, with the feedback it gets in `$messages`.
+	 */
+	private function get_skin( $result, $has_plugin = true ) {
+		// Collects the feedback, because show_message() ends all output buffers.
+		$skin     = new class( array( 'plugin' => self::PLUGIN ) ) extends Plugin_Upgrader_Skin {
+			/**
+			 * Feedback given to the skin.
+			 *
+			 * @var string[]
+			 */
+			public $messages = array();
+
+			/**
+			 * Collects the feedback.
+			 *
+			 * @param string|array|WP_Error $feedback Message data.
+			 * @param mixed                 ...$args  Optional text replacements.
+			 */
+			public function feedback( $feedback, ...$args ) {
+				$this->messages[] = $feedback;
+			}
+		};
+		$upgrader = new Plugin_Upgrader( $skin );
+
+		$upgrader->result = $has_plugin ? array( 'destination_name' => 'custom-internationalized-plugin' ) : array( 'destination_name' => '' );
+
+		// Done by WP_Upgrader::init(), which also schedules a backup cleanup.
+		$skin->set_upgrader( $upgrader );
+		$skin->result = $result;
+
+		return $skin;
+	}
+
+	/**
+	 * Runs the skin's after() method.
+	 *
+	 * @param Plugin_Upgrader_Skin $skin The skin.
+	 * @return string The output, followed by the feedback of the skin.
+	 */
+	private function run_after( $skin ) {
+		return get_echo( array( $skin, 'after' ) ) . implode( '', $skin->messages );
+	}
+}