Add regression unit test for ColorSliderUI repaint bug

jhotdraw-gui/pom.xml

@@ -9,6 +9,12 @@
     <artifactId>jhotdraw-gui</artifactId>
     <packaging>jar</packaging>
     <dependencies>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter</artifactId>
+            <version>5.8.1</version>
+            <scope>test</scope>
+        </dependency>
         <dependency>
             <groupId>${project.groupId}</groupId>
             <artifactId>jhotdraw-utils</artifactId>
@@ -30,5 +36,14 @@
             <version>${project.version}</version>
         </dependency>
     </dependencies>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-surefire-plugin</artifactId>
+                <version>2.22.2</version>
+            </plugin>
+        </plugins>
+    </build>
     <name>jhotdraw-gui</name>
 </project>

jhotdraw-gui/src/main/java/org/jhotdraw/color/ColorSliderUI.java

@@ -314,7 +314,7 @@ public void paintColorTrack(Graphics g, int x, int y, int width, int height, int
             }
         }
         if (colorTrackImage != null) {
-            g.drawImage(colorTrackImage, x, y, slider);
+            g.drawImage(colorTrackImage, x, y, null);
         }
     }
 

jhotdraw-gui/src/main/java/org/jhotdraw/gui/plaf/palette/colorchooser/PaletteColorWheelChooser.java

@@ -105,4 +105,4 @@ public void setColorToModel(Color color) {
     // Variables declaration - do not modify//GEN-BEGIN:variables
     private javax.swing.JSlider brightnessSlider;
     // End of variables declaration//GEN-END:variables
-}
+}

jhotdraw-gui/src/test/java/org/jhotdraw/color/ColorSliderUIRegressionTest.java

@@ -0,0 +1,51 @@
+package org.jhotdraw.color;
+
+import org.junit.jupiter.api.Test;
+
+import javax.swing.*;
+import java.awt.*;
+import java.awt.image.BufferedImage;
+
+import static org.junit.jupiter.api.Assertions.fail;
+import org.jhotdraw.color.ColorSliderModel;
+import org.jhotdraw.color.DefaultColorSliderModel;
+import org.jhotdraw.color.HSVColorSpace;
+
+
+public class ColorSliderUIRegressionTest {
+
+    @Test
+    void paintColorTrack_doesNotCallImageUpdate_andDoesNotCrash() throws Exception {
+        SwingUtilities.invokeAndWait(() -> {
+            // Custom slider that fails if Swing calls imageUpdate on it.
+            // After our fix (drawImage observer = null), this should never be called.
+            JSlider slider = new JSlider(JSlider.HORIZONTAL) {
+                @Override
+                public boolean imageUpdate(Image img, int infoflags, int x, int y, int w, int h) {
+                    fail("imageUpdate() was called. The color track should be drawn with a null ImageObserver.");
+                    return super.imageUpdate(img, infoflags, x, y, w, h);
+                }
+            };
+
+            // Install JHotDraw custom slider UI
+            slider.setUI((ColorSliderUI) ColorSliderUI.createUI(slider));
+
+            // Configure like in the real chooser panels (model + client properties)
+            ColorSliderModel model = new DefaultColorSliderModel(HSVColorSpace.getInstance());
+            model.configureSlider(2, slider);
+
+            // Give it a size so painting runs
+            slider.setSize(300, 50);
+            slider.doLayout();
+
+            // Paint into an offscreen image (headless-friendly)
+            BufferedImage img = new BufferedImage(300, 50, BufferedImage.TYPE_INT_ARGB);
+            Graphics2D g2 = img.createGraphics();
+            try {
+                slider.paint(g2);
+            } finally {
+                g2.dispose();
+            }
+        });
+    }
+}

jhotdraw-samples/jhotdraw-samples-mini/src/main/java/org/jhotdraw/samples/color/WheelsAndSlidersMain.java

@@ -132,10 +132,10 @@ private JPanel createColorWheelChooser(ColorSpace sys, int angularIndex, int rad
         w.setFlipY(flipY);
         w.setModel(m);
         JSlider s = new JSlider(JSlider.VERTICAL);
+        m.configureSlider(verticalIndex, s);
         s.setMajorTickSpacing(10);
         s.setPaintLabels(true);
         s.setPaintTicks(true);
-        m.configureSlider(verticalIndex, s);
         p.add(new JLabel("<html>" + ColorUtil.getName(sys) + "<br>α:" + angularIndex + " r:" + radialIndex + " v:" + verticalIndex), BorderLayout.NORTH);
         p.add(w, BorderLayout.CENTER);
         p.add(s, BorderLayout.EAST);
@@ -181,10 +181,10 @@ private JPanel createSliderChooser(ColorSpace sys, boolean vertical) {
         for (int i = 0; i < m.getComponentCount(); i++) {
             final int comp = i;
             JSlider s = new JSlider(JSlider.HORIZONTAL);
+            m.configureSlider(comp, s);
             s.setMajorTickSpacing(50);
             s.setPaintTicks(true);
             s.setOrientation(vertical ? JSlider.VERTICAL : JSlider.HORIZONTAL);
-            m.configureSlider(comp, s);
             if (vertical) {
                 gbc.gridx = i;
                 gbc.gridy = 0;