8364146: JList getScrollableUnitIncrement return 0

Reviewed-by: prr, tr

src/java.desktop/share/classes/javax/swing/JList.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2025, 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
@@ -2497,6 +2497,9 @@ public class JList<E> extends JComponent implements Scrollable, Accessible
     /**
      * Returns the distance to scroll to expose the next or previous
      * row (for vertical scrolling) or column (for horizontal scrolling).
+     * The scrolling distance returned will be positive,
+     * unless scrolling for the specified parameters is already
+     * at its furthest extent, in which case it will return zero.
      * <p>
      * For horizontal scrolling, if the layout orientation is {@code VERTICAL},
      * then the list's font size is returned (or {@code 1} if the font is
@@ -2507,8 +2510,9 @@ public class JList<E> extends JComponent implements Scrollable, Accessible
      *                    {@code SwingConstants.VERTICAL}
      * @param direction less or equal to zero to scroll up/back,
      *                  greater than zero for down/forward
-     * @return the "unit" increment for scrolling in the specified direction;
-     *         always positive
+     * @return the non-negative "unit" increment
+     *         for scrolling in the specified direction
+     *
      * @see #getScrollableBlockIncrement
      * @see Scrollable#getScrollableUnitIncrement
      * @throws IllegalArgumentException if {@code visibleRect} is {@code null}, or
@@ -2529,7 +2533,8 @@ public class JList<E> extends JComponent implements Scrollable, Accessible
                 /* Scroll Down */
                 if (direction > 0) {
                     Rectangle r = getCellBounds(row, row);
-                    return (r == null) ? 0 : r.height - (visibleRect.y - r.y);
+                    return (r == null) ? 0 :
+                            ((r.height - (visibleRect.y - r.y)) < 0) ? 0 : r.height - (visibleRect.y - r.y);
                 }
                 /* Scroll Up */
                 else {

test/jdk/javax/swing/JList/JListTest.java

@@ -0,0 +1,86 @@
+/*
+ * Copyright (c) 2025, 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.
+ */
+
+/*
+ * @test
+ * @bug 8364146
+ * @key headful
+ * @summary Verifies JList getScrollableUnitIncrement return non-negative number
+ * @run main JListTest
+ */
+
+import java.awt.Dimension;
+import java.awt.Rectangle;
+import javax.swing.JFrame;
+import javax.swing.JList;
+import javax.swing.JScrollPane;
+import javax.swing.SwingConstants;
+import javax.swing.SwingUtilities;
+public class JListTest {
+
+    private static JFrame f;
+
+    public static void main(String[] argv) throws Exception {
+
+        SwingUtilities.invokeAndWait(() -> {
+            try {
+                f = new JFrame();
+                String[] data = {"One", "Two", "Three", "Four", "Five", "Six ",
+                                 "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelv"};
+                JList<String> list = new JList<>(data);
+                list.setLayoutOrientation(JList.HORIZONTAL_WRAP);
+
+                JScrollPane sp = new JScrollPane(list);
+                sp.setPreferredSize(new Dimension(200, 200));
+                f.add(sp);
+                f.pack();
+                f.setVisible(true);
+
+                Rectangle cell = list.getCellBounds(1, data.length);
+                System.out.println(cell);
+                cell.y = list.getHeight() + 10;
+                int unit = list.getScrollableUnitIncrement(
+                                cell,
+                                SwingConstants.VERTICAL,
+                                -1);
+                System.out.println("Scrollable unit increment: " + unit);
+
+                if (unit < 0) {
+                    throw new RuntimeException("JList scrollable unit increment should be greater than 0.");
+                }
+                unit = list.getScrollableUnitIncrement(
+                                cell,
+                                SwingConstants.VERTICAL,
+                                1);
+                System.out.println("Scrollable unit increment: " + unit);
+                if (unit < 0) {
+                    throw new RuntimeException("JList scrollable unit increment should be greater than 0.");
+                }
+            } finally {
+                if (f != null) {
+                    f.dispose();
+                }
+            }
+        });
+    }
+}