8273969: Memory Leak on the Runnable provided to Platform.startup

modules/javafx.graphics/src/main/java/com/sun/javafx/tk/quantum/QuantumToolkit.java

@@ -376,6 +376,7 @@ void runToolkit() {
         launchLatch.countDown();
         try {
             Application.invokeAndWait(this.userRunnable);
+            this.userRunnable = null;
 
             if (getPrimaryTimer().isFullspeed()) {
                 /*

tests/system/src/test/java/test/com/sun/javafx/application/PlatformStartupMemoryLeakTest.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package test.com.sun.javafx.application;
+
+import javafx.application.Platform;
+import org.junit.Test;
+import org.junit.AfterClass;
+import test.util.memory.JMemoryBuddy;
+
+public class PlatformStartupMemoryLeakTest {
+
+    @Test
+    public void testStartupLeak() {
+        JMemoryBuddy.memoryTest((checker) -> {
+            // This Runnable must not turn into a lambda, because then the test wouldn't work anymore.
+            Runnable r = new Runnable() {
+                @Override
+                public void run() {
+                    System.out.println("Startup called!");
+                }
+            };
+            Platform.startup(r);
+            checker.assertCollectable(r);
+        });
+    }
+
+    @AfterClass
+    public static void tearDown() {
+        Platform.exit();
+    }
+
+}