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

CHANGELOG.md

@@ -16,6 +16,8 @@ FlatLaf Change Log
 
 #### Fixed bugs
 
+- macOS: Setting window background (of undecorated window) to translucent color
+  (alpha < 255) did not show the window translucent. (issue #705)
 - JIDE CommandMenuBar: Fixed `ClassCastException` when JIDE command bar displays
   `JideMenu` in popup. (PR #794)
 

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatRootPaneUI.java

@@ -30,6 +30,7 @@ import java.awt.LayoutManager2;
 import java.awt.Window;
 import java.awt.event.ComponentListener;
 import java.beans.PropertyChangeEvent;
+import java.beans.PropertyChangeListener;
 import java.util.function.Function;
 import javax.swing.JComponent;
 import javax.swing.JDialog;
@@ -85,6 +86,7 @@ public class FlatRootPaneUI
 	private Object nativeWindowBorderData;
 	private LayoutManager oldLayout;
 	private ComponentListener macFullWindowContentListener;
+	private PropertyChangeListener macWindowBackgroundListener;
 
 	public static ComponentUI createUI( JComponent c ) {
 		return new FlatRootPaneUI();
@@ -153,6 +155,8 @@ public class FlatRootPaneUI
 			if( background == null || background instanceof UIResource )
 				parent.setBackground( UIManager.getColor( "control" ) );
 		}
+
+		macClearBackgroundForTranslucentWindow( c );
 	}
 
 	@Override
@@ -174,6 +178,7 @@ public class FlatRootPaneUI
 
 		if( SystemInfo.isMacFullWindowContentSupported )
 			macFullWindowContentListener = FullWindowContentSupport.macInstallListeners( root );
+		macInstallWindowBackgroundListener( root );
 	}
 
 	@Override
@@ -184,6 +189,7 @@ public class FlatRootPaneUI
 			FullWindowContentSupport.macUninstallListeners( root, macFullWindowContentListener );
 			macFullWindowContentListener = null;
 		}
+		macUninstallWindowBackgroundListener( root );
 	}
 
 	/** @since 1.1.2 */
@@ -296,7 +302,7 @@ public class FlatRootPaneUI
 				}
 			}
 
-			FlatNativeMacLibrary.setWindowButtonsSpacing( getParentWindow(), buttonsSpacing );
+			FlatNativeMacLibrary.setWindowButtonsSpacing( getParentWindow( rootPane ), buttonsSpacing );
 		}
 
 		// update buttons bounds client property
@@ -313,7 +319,7 @@ public class FlatRootPaneUI
 
 		// reset window buttons spacing
 		if( isMacButtonsSpacingSupported() )
-			FlatNativeMacLibrary.setWindowButtonsSpacing( getParentWindow(), FlatNativeMacLibrary.BUTTONS_SPACING_DEFAULT );
+			FlatNativeMacLibrary.setWindowButtonsSpacing( getParentWindow( rootPane ), FlatNativeMacLibrary.BUTTONS_SPACING_DEFAULT );
 
 		// remove buttons bounds client property
 		FullWindowContentSupport.macUninstallFullWindowContentButtonsBoundsProperty( rootPane );
@@ -323,11 +329,60 @@ public class FlatRootPaneUI
 		return SystemInfo.isMacOS && SystemInfo.isJava_17_orLater && FlatNativeMacLibrary.isLoaded();
 	}
 
-	private Window getParentWindow() {
+	private void macInstallWindowBackgroundListener( JRootPane c ) {
+		if( !SystemInfo.isMacOS )
+			return;
+
+		Window window = getParentWindow( c );
+		if( window != null && macWindowBackgroundListener == null ) {
+			macWindowBackgroundListener = e -> macClearBackgroundForTranslucentWindow( c );
+			window.addPropertyChangeListener( "background", macWindowBackgroundListener );
+		}
+	}
+
+	private void macUninstallWindowBackgroundListener( JRootPane c ) {
+		if( !SystemInfo.isMacOS )
+			return;
+
+		Window window = getParentWindow( c );
+		if( window != null && macWindowBackgroundListener != null ) {
+			window.removePropertyChangeListener( "background", macWindowBackgroundListener );
+			macWindowBackgroundListener = null;
+		}
+	}
+
+	/**
+	 * When setting window background to translucent color (alpha < 255),
+	 * Swing paints that window translucent on Windows and Linux, but not on macOS.
+	 * The reason for this is that FlatLaf sets the background color of the root pane,
+	 * and Swing behaves a bit differently on macOS than on other platforms in that case.
+	 * Other L&Fs do not set root pane background, which is {@code null} by default.
+	 * <p>
+	 * To fix this problem, set the root pane background to {@code null}
+	 * if windows uses a translucent background.
+	 */
+	private void macClearBackgroundForTranslucentWindow( JRootPane c ) {
+		if( !SystemInfo.isMacOS )
+			return;
+
+		Window window = getParentWindow( c );
+		if( window != null ) {
+			Color windowBackground = window.getBackground();
+			if( windowBackground != null &&
+				windowBackground.getAlpha() < 255 &&
+				c.getBackground() instanceof UIResource )
+			{
+				// clear root pane background
+				c.setBackground( null );
+			}
+		}
+	}
+
+	private Window getParentWindow( JRootPane c ) {
 		// not using SwingUtilities.windowForComponent() or SwingUtilities.getWindowAncestor()
 		// here because root panes may be nested and used anywhere (e.g. in JInternalFrame)
 		// but we're only interested in the "root" root pane, which is a direct child of the window
-		Container parent = rootPane.getParent();
+		Container parent = c.getParent();
 		return (parent instanceof Window) ? (Window) parent : null;
 	}
 
@@ -389,6 +444,12 @@ public class FlatRootPaneUI
 				break;
 
 			case "ancestor":
+				if( e.getNewValue() instanceof Window )
+					macClearBackgroundForTranslucentWindow( rootPane );
+
+				macUninstallWindowBackgroundListener( rootPane );
+				macInstallWindowBackgroundListener( rootPane );
+
 				// FlatNativeMacLibrary.setWindowButtonsSpacing() and
 				// FullWindowContentSupport.macUpdateFullWindowContentButtonsBoundsProperty()
 				// require a native window, but setting the client properties

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() );
+		if( isOpaque() ) {
-		g.fillRect( 0, 0, width, height );
+			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"
 			} )