diff --git a/tests/phpunit/includes/abstract-testcase.php b/tests/phpunit/includes/abstract-testcase.php
index 98b3456935716..a55b2e48aa2ed 100644
--- a/tests/phpunit/includes/abstract-testcase.php
+++ b/tests/phpunit/includes/abstract-testcase.php
@@ -75,6 +75,20 @@ public static function set_up_before_class() {
 
 		$class = get_called_class();
 
+		/*
+		 * Record the contents of the uploads directory before any class fixtures
+		 * are created, so that `remove_added_uploads()` can tell pre-existing
+		 * files apart from files added by the tests.
+		 *
+		 * This must happen before `wpSetUpBeforeClass()` runs. Capturing it later
+		 * would treat files uploaded by the first class's fixtures as
+		 * pre-existing, permanently exempting them from cleanup and leaving them
+		 * on disk for the rest of the run.
+		 */
+		if ( null === self::$ignore_files ) {
+			self::$ignore_files = self::scan_user_uploads();
+		}
+
 		if ( method_exists( $class, 'wpSetUpBeforeClass' ) ) {
 			call_user_func( array( $class, 'wpSetUpBeforeClass' ), static::factory() );
 		}
@@ -112,16 +126,22 @@ public function set_up() {
 
 		$this->factory = static::factory();
 
-		if ( ! self::$ignore_files ) {
-			self::$ignore_files = $this->scan_user_uploads();
-		}
-
 		if ( ! self::$hooks_saved ) {
 			$this->_backup_hooks();
 		}
 
 		$this->clean_up_global_scope();
 
+		/*
+		 * Reset the metadata lazyload queue before each test.
+		 *
+		 * `tear_down()` resets the queue after every test, but fixtures created
+		 * in `wpSetUpBeforeClass()` run outside of any test's `tear_down()`.
+		 * Without this reset, anything queued while building class fixtures
+		 * leaks into the first test of the class.
+		 */
+		$this->reset_lazyload_queue();
+
 		/*
 		 * When running core tests, ensure that post types and taxonomies
 		 * are reset for each test. We skip this step for non-core tests,
@@ -207,7 +227,7 @@ public function tear_down() {
 		}
 
 		// Reset comment globals.
-		$comment_globals = array( 'comment_alt', 'comment_depth', 'comment_thread_alt' );
+		$comment_globals = array( 'comment', 'comment_alt', 'comment_depth', 'comment_thread_alt' );
 		foreach ( $comment_globals as $global ) {
 			$GLOBALS[ $global ] = null;
 		}
@@ -230,6 +250,18 @@ public function tear_down() {
 		$this->_restore_hooks();
 		wp_set_current_user( 0 );
 
+		/*
+		 * When running core tests, reset `$_SERVER` after each test.
+		 *
+		 * `set_up()` performs the same reset before each test, but class fixtures
+		 * created in `wpSetUpBeforeClass()` run before the first `set_up()` of a
+		 * class. Without this reset, `$_SERVER` values left behind by a previous
+		 * class are still in place while those fixtures are created.
+		 */
+		if ( defined( 'WP_RUN_CORE_TESTS' ) && WP_RUN_CORE_TESTS ) {
+			$this->reset__SERVER();
+		}
+
 		$this->reset_lazyload_queue();
 
 		WP_Style_Engine_CSS_Rules_Store::remove_all_stores();
@@ -1530,9 +1562,10 @@ public function rmdir( $path ) {
 	/**
 	 * Deletes files added to the `uploads` directory during tests.
 	 *
-	 * This method works in tandem with the `set_up()` and `rmdir()` methods:
-	 * - `set_up()` scans the `uploads` directory before every test, and stores
-	 *   its contents inside of the `$ignore_files` property.
+	 * This method works in tandem with the `set_up_before_class()` and `rmdir()` methods:
+	 * - `set_up_before_class()` scans the `uploads` directory once, before any
+	 *   test or class fixture has run, and stores its contents inside of the
+	 *   `$ignore_files` property.
 	 * - `rmdir()` and its helper methods only delete files that are not listed
 	 *   in the `$ignore_files` property. If called during `tear_down()` in tests,
 	 *   this will only delete files added during the previously run test.
@@ -1550,7 +1583,7 @@ public function remove_added_uploads() {
 	 * @param string $dir Path to the directory to scan.
 	 * @return string[] List of file paths.
 	 */
-	public function files_in_dir( $dir ) {
+	public static function files_in_dir( $dir ) {
 		$files = array();
 
 		$iterator = new RecursiveDirectoryIterator( $dir );
@@ -1571,14 +1604,14 @@ public function files_in_dir( $dir ) {
 	 *
 	 * @return string[] List of file paths.
 	 */
-	public function scan_user_uploads() {
+	public static function scan_user_uploads() {
 		static $files = array();
 		if ( ! empty( $files ) ) {
 			return $files;
 		}
 
 		$uploads = wp_upload_dir();
-		$files   = $this->files_in_dir( $uploads['basedir'] );
+		$files   = self::files_in_dir( $uploads['basedir'] );
 		return $files;
 	}