8172269: When checking the default behaviour for a scroll tab layout and checking the 'opaque' checkbox, the area behind tabs is not red.

src/java.desktop/share/classes/javax/swing/plaf/basic/BasicTabbedPaneUI.java

@@ -3907,11 +3907,13 @@ private class ScrollableTabPanel extends JPanel implements UIResource {
         public ScrollableTabPanel() {
             super(null);
             setOpaque(tabPane.isOpaque());
-            Color bgColor = UIManager.getColor("TabbedPane.tabAreaBackground");
-            if (bgColor == null) {
-                bgColor = tabPane.getBackground();
+            Color background = tabPane.getBackground();
+            Color tabAreaBackground = UIManager.getColor("TabbedPane.tabAreaBackground");
+            if (background instanceof UIResource && tabAreaBackground != null) {
+                setBackground(tabAreaBackground);
+            } else {
+                setBackground(background);
             }
-            setBackground(bgColor);
         }
         public void paintComponent(Graphics g) {
             super.paintComponent(g);

test/jdk/javax/swing/JTabbedPane/TestBackgroundScrollPolicy.java

@@ -0,0 +1,139 @@
+/*
+ * 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.
+ *
+ * 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.
+ */
+
+import java.awt.*;
+import java.util.ArrayList;
+import java.util.concurrent.CountDownLatch;
+import javax.swing.JFrame;
+import javax.swing.JLabel;
+import javax.swing.JTabbedPane;
+
+import static javax.swing.UIManager.*;
+import static javax.swing.SwingUtilities.*;
+
+/*
+ * @test
+ * @key headful
+ * @bug 8172269
+ * @summary Tests JTabbedPane background for SCROLL_TAB_LAYOUT
+ */
+
+public class TestBackgroundScrollPolicy implements Runnable {
+    private static final ArrayList<String> LIST = new ArrayList<>();
+    private static final LookAndFeelInfo[] INFO = getInstalledLookAndFeels();
+    private static final CountDownLatch LATCH = new CountDownLatch(INFO.length);
+    private static Robot ROBOT;
+
+    public static void main(String[] args) throws Exception {
+        ROBOT = new Robot();
+        invokeLater(new TestBackgroundScrollPolicy());
+        LATCH.await();
+        if (!LIST.isEmpty()) {
+            throw new RuntimeException(LIST.toString());
+        }
+    }
+
+    private static void addOpaqueError(boolean opaque) {
+        LIST.add(getLookAndFeel().getName() + " background color wrong for opaque=" + opaque);
+    }
+
+    private static boolean updateLookAndFeel() {
+        int index = (int) LATCH.getCount() - 1;
+        if (index >= 0) {
+            try {
+                LookAndFeelInfo info = INFO[index];
+                System.err.println("L&F: " + info.getName());
+                setLookAndFeel(info.getClassName());
+                return true;
+            } catch (Exception exception) {
+                exception.printStackTrace();
+            }
+        }
+        return false;
+    }
+
+    private JFrame frame;
+    private JTabbedPane pane;
+
+    public void run() {
+        if (this.frame == null) {
+            if (!updateLookAndFeel()) {
+                return;
+            }
+            this.pane = new JTabbedPane();
+            this.pane.setOpaque(false);
+            this.pane.setBackground(Color.RED);
+            for (int i = 0; i < 3; i++) {
+                this.pane.addTab("Tab " + i, new JLabel("Content area " + i));
+            }
+            this.pane.setTabLayoutPolicy(JTabbedPane.SCROLL_TAB_LAYOUT);
+            this.frame = new JFrame(getClass().getSimpleName());
+            this.frame.getContentPane().setBackground(Color.BLUE);
+            this.frame.add(this.pane);
+            this.frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
+            this.frame.setSize(400, 200);
+            this.frame.setLocationRelativeTo(null);
+            this.frame.setVisible(true);
+        } else {
+            Point point = new Point(this.pane.getWidth() - 2, 2);
+            convertPointToScreen(point, this.pane);
+            Color actual = ROBOT.getPixelColor(point.x, point.y);
+
+            boolean opaque = this.pane.isOpaque();
+            Color expected = opaque
+                    ? this.pane.getBackground()
+                    : this.frame.getContentPane().getBackground();
+
+            System.out.println("expected " + expected + " actual " + actual);
+            if (!expected.equals(actual)){
+                addOpaqueError(opaque);
+            }
+            if (!opaque) {
+                this.pane.setOpaque(true);
+                this.pane.repaint();
+            } else {
+                this.frame.dispose();
+                this.frame = null;
+                this.pane = null;
+                LATCH.countDown();
+            }
+
+        }
+        ROBOT.delay(2000);
+        SecondaryLoop secondaryLoop =
+                Toolkit.getDefaultToolkit().getSystemEventQueue()
+                        .createSecondaryLoop();
+        new Thread() {
+            @Override
+            public void run() {
+                try {
+                    Thread.sleep(200);
+                } catch (InterruptedException e) {
+                }
+                secondaryLoop.exit();
+                invokeLater(TestBackgroundScrollPolicy.this);
+            }
+        }.start();
+        secondaryLoop.enter();
+    }
+}