macOS: setting window background (of undecorated window) to translucent color (alpha < 255) did not show the window translucent (issue #705)

flatlaf-testing/src/main/java/com/formdev/flatlaf/testing/FlatTestPanel.java

@@ -32,8 +32,10 @@ public class FlatTestPanel
 		int width = getWidth();
 		int height = getHeight();
 
-		g.setColor( super.getBackground() );
-		g.fillRect( 0, 0, width, height );
+		if( isOpaque() ) {
+			g.setColor( super.getBackground() );
+			g.fillRect( 0, 0, width, height );
+		}
 
 		if( isPaintBackgroundPattern() ) {
 			g.setColor( Color.magenta );

flatlaf-testing/src/main/java/com/formdev/flatlaf/testing/FlatWindowDecorationsTest.java

@@ -76,6 +76,14 @@ public class FlatWindowDecorationsTest
 		initComponents();
 	}
 
+	@Override
+	public void updateUI() {
+		super.updateUI();
+
+		if( translucentWindowBackgroundCheckBox != null )
+			translucentWindowBackgroundChanged();
+	}
+
 	@Override
 	public void addNotify() {
 		super.addNotify();
@@ -237,6 +245,27 @@ public class FlatWindowDecorationsTest
 		}
 	}
 
+	private void translucentWindowBackgroundChanged() {
+		boolean selected = translucentWindowBackgroundCheckBox.isSelected();
+		if( selected && !undecoratedCheckBox.isSelected() ) {
+			undecoratedCheckBox.setSelected( true );
+			undecoratedChanged();
+		}
+		undecoratedCheckBox.setEnabled( !selected );
+
+		Color background = selected
+			? new Color( 100, 0, 0, 100 )
+			: UIManager.getColor( "control" );
+
+		Window window = SwingUtilities.windowForComponent( this );
+		window.setBackground( background );
+
+		for( Component c = this; c != null; c = c.getParent() ) {
+			if( c instanceof JComponent )
+				LookAndFeel.installProperty( (JComponent) c, "opaque", !selected );
+		}
+	}
+
 	private void addMenu() {
 		JMenu menu = new JMenu( "Hello" );
 		menu.add( new JMenuItem( "world" ) );
@@ -543,6 +572,7 @@ debug*/
 		colorizeTitleBarCheckBox = new JCheckBox();
 		colorizeMenuBarCheckBox = new JCheckBox();
 		colorizeMenusCheckBox = new JCheckBox();
+		translucentWindowBackgroundCheckBox = new JCheckBox();
 		JButton openDialogButton = new JButton();
 		JButton openFrameButton = new JButton();
 		typeNormalRadioButton = new JRadioButton();
@@ -885,6 +915,7 @@ debug*/
 				"[]" +
 				"[]" +
 				"[]" +
+				"[]para" +
 				"[]"));
 
 			//---- unifiedBackgroundCheckBox ----
@@ -906,6 +937,11 @@ debug*/
 			colorizeMenusCheckBox.setText("colorize menus");
 			colorizeMenusCheckBox.addActionListener(e -> colorizeMenus());
 			panel5.add(colorizeMenusCheckBox, "cell 0 3");
+
+			//---- translucentWindowBackgroundCheckBox ----
+			translucentWindowBackgroundCheckBox.setText("translucent window background");
+			translucentWindowBackgroundCheckBox.addActionListener(e -> translucentWindowBackgroundChanged());
+			panel5.add(translucentWindowBackgroundCheckBox, "cell 0 4");
 		}
 		add(panel5, "cell 2 1");
 
@@ -1169,6 +1205,7 @@ debug*/
 	private JCheckBox colorizeTitleBarCheckBox;
 	private JCheckBox colorizeMenuBarCheckBox;
 	private JCheckBox colorizeMenusCheckBox;
+	private JCheckBox translucentWindowBackgroundCheckBox;
 	private JRadioButton typeNormalRadioButton;
 	private JRadioButton typeUtilityRadioButton;
 	private JRadioButton typeSmallRadioButton;

flatlaf-testing/src/main/java/com/formdev/flatlaf/testing/FlatWindowDecorationsTest.jfd

@@ -1,4 +1,4 @@
-JFDML JFormDesigner: "8.1.0.0.283" Java: "19.0.2" encoding: "UTF-8"
+JFDML JFormDesigner: "8.2.1.0.348" Java: "21.0.1" encoding: "UTF-8"
 
 new FormModel {
 	contentType: "form/swing"
@@ -449,7 +449,7 @@ new FormModel {
 			add( new FormContainer( "javax.swing.JPanel", new FormLayoutManager( class net.miginfocom.swing.MigLayout ) {
 				"$layoutConstraints": "ltr,hidemode 3,gap 0 0"
 				"$columnConstraints": "[left]"
-				"$rowConstraints": "[][][][]"
+				"$rowConstraints": "[][][][]para[]"
 			} ) {
 				name: "panel5"
 				"border": new javax.swing.border.TitledBorder( "Color" )
@@ -493,6 +493,16 @@ new FormModel {
 				}, new FormLayoutConstraints( class net.miginfocom.layout.CC ) {
 					"value": "cell 0 3"
 				} )
+				add( new FormComponent( "javax.swing.JCheckBox" ) {
+					name: "translucentWindowBackgroundCheckBox"
+					"text": "translucent window background"
+					auxiliary() {
+						"JavaCodeGenerator.variableLocal": false
+					}
+					addEvent( new FormEvent( "java.awt.event.ActionListener", "actionPerformed", "translucentWindowBackgroundChanged", false ) )
+				}, new FormLayoutConstraints( class net.miginfocom.layout.CC ) {
+					"value": "cell 0 4"
+				} )
 			}, new FormLayoutConstraints( class net.miginfocom.layout.CC ) {
 				"value": "cell 2 1"
 			} )