8252446: Screen.getScreens() is empty sometimes

Reviewed-by: arapte, pbansal

modules/javafx.graphics/src/main/java/javafx/stage/Screen.java

@@ -108,15 +108,13 @@ private static void updateConfiguration() {
             if (newScreen != null) {
                 if (canKeepOld) {
                     canKeepOld = false;
-                    newScreens.clear();
-                    newScreens.addAll(Screen.screens.subList(0, i));
+                    newScreens.setAll(Screen.screens.subList(0, i));
                 }
                 newScreens.add(newScreen);
             }
         }
         if (!canKeepOld) {
-            Screen.screens.clear();
-            Screen.screens.addAll(newScreens);
+            Screen.screens.setAll(newScreens);
         }
 
         configurationDirty.set(false);

tests/system/src/test/java/test/javafx/stage/ScreenTest.java

@@ -0,0 +1,103 @@
+/*
+ * Copyright (c) 2020, 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.javafx.stage;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+import javafx.application.Platform;
+import javafx.collections.ListChangeListener.Change;
+import javafx.collections.ObservableList;
+import javafx.stage.Screen;
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import test.util.Util;
+
+import static org.junit.Assert.*;
+import static org.junit.Assume.assumeTrue;
+
+public class ScreenTest {
+    static CountDownLatch startupLatch = new CountDownLatch(1);
+    static ObservableList<Screen> screens;
+    static volatile boolean screensListenerCalled = false;
+    static volatile boolean screensSizeIsZero = false;
+
+    private static void waitForLatch(CountDownLatch latch, int seconds, String msg) throws Exception {
+        assertTrue("Timeout: " + msg, latch.await(seconds, TimeUnit.SECONDS));
+    }
+
+    /* This test for JDK-8252446 adds a listener on the ObservableList of
+     * screens as the first thing in the platform startup runnable. Even
+     * so, it cannot count on getting a call to the listener for the
+     * initial list of screens. We don't get one on Windows or Linux. We
+     * do get one on Mac, but this isn't guaranteed behavior, so this
+     * test might or might not be effective.
+     */
+    @BeforeClass
+    public static void initFX() throws Exception {
+        Platform.setImplicitExit(false);
+        Platform.startup(() -> {
+            screens = Screen.getScreens();
+            screens.addListener((Change<?> change) -> {
+                final int size = screens.size();
+                System.err.println("Screens list changed, size = " + size);
+                if (size == 0) {
+                    screensSizeIsZero = true;
+                }
+                screensListenerCalled = true;
+            });
+            Platform.runLater(startupLatch::countDown);
+        });
+        waitForLatch(startupLatch, 10, "FX runtime failed to start");
+    }
+
+    @AfterClass
+    public static void exitFX() {
+        Platform.exit();
+    }
+
+    @Test
+    public void testScreensNotEmpty() {
+        assertNotNull(screens);
+        assertFalse("Screens list is empty", screens.size() == 0);
+    }
+
+    @Test
+    public void testScreensNotEmptyInListener() {
+        // Sleep for some time to see whether we get an initial call to our
+        // listener. Since we cannot count on the listener being called at
+        // all, we can't use a latch.
+        Util.sleep(2000);
+
+        // Skip the test if it isn't called.
+        if (!screensListenerCalled) {
+            System.err.println("Skipping test: Screens listener not called");
+        }
+        assumeTrue(screensListenerCalled);
+        assertFalse("Screens list is empty in listener", screensSizeIsZero);
+    }
+
+}