macOS: fixed window "flashing" when switching from a light to a dark theme (or vice versa), especially when using animated theme changer

CHANGELOG.md

@@ -9,11 +9,12 @@ FlatLaf Change Log
   named Java modules, it is no longer necessary to add `opens com.myapp.themes;`
   to `module-info.java`. (issue #1026)
 
-
 #### Fixed bugs
 
 - Tree and List: Fixed painting of rounded drop backgrounds. (issue #1023)
-
+- macOS: Fixed window "flashing" when switching from a light to a dark theme (or
+  vice versa). Especially when using animated theme changer (see
+  [FlatLaf Extras](flatlaf-extras)).
 
 #### Incompatibilities
 

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

@@ -20,6 +20,7 @@ import java.awt.Color;
 import java.awt.Component;
 import java.awt.Container;
 import java.awt.Dimension;
+import java.awt.EventQueue;
 import java.awt.Frame;
 import java.awt.Graphics;
 import java.awt.Graphics2D;
@@ -153,8 +154,28 @@ public class FlatRootPaneUI
 		Container parent = c.getParent();
 		if( parent instanceof JFrame || parent instanceof JDialog ) {
 			Color background = parent.getBackground();
-			if( background == null || background instanceof UIResource )
+			if( background == null || background instanceof UIResource ) {
-				parent.setBackground( UIManager.getColor( "control" ) );
+				if( SystemInfo.isMacOS ) {
+					// Setting window background on macOS immediately fills the whole window
+					// with that color, and slightly delayed, the Swing repaint manager
+					// repaints the actual window content. This results in some flashing
+					// when switching from a light to a dark theme (or vice versa).
+					// --> delay setting window background and immediately repaint window content
+					Runnable r = () -> {
+						parent.setBackground( UIManager.getColor( "control" ) );
+						c.paintImmediately( 0, 0, c.getWidth(), c.getHeight() );
+					};
+
+					// for class FlatAnimatedLafChange:
+					// if animated Laf change is in progress, set background color when
+					// animation has finished to avoid/reduce flashing
+					if( c.getClientProperty( "FlatLaf.internal.animatedLafChange" ) != null )
+						c.putClientProperty( "FlatLaf.internal.animatedLafChange.runWhenFinished", r );
+					else
+						EventQueue.invokeLater( r );
+				} else
+					parent.setBackground( UIManager.getColor( "control" ) );
+			}
 		}
 
 		macClearBackgroundForTranslucentWindow( c );

flatlaf-extras/src/main/java/com/formdev/flatlaf/extras/FlatAnimatedLafChange.java

@@ -26,6 +26,7 @@ import java.util.Map;
 import java.util.WeakHashMap;
 import javax.swing.JComponent;
 import javax.swing.JLayeredPane;
+import javax.swing.JRootPane;
 import javax.swing.RootPaneContainer;
 import com.formdev.flatlaf.FlatSystemProperties;
 import com.formdev.flatlaf.util.Animator;
@@ -80,7 +81,7 @@ public class FlatAnimatedLafChange
 		showSnapshot( true, oldUIsnapshots );
 	}
 
-	private static void showSnapshot( boolean useAlpha, Map<JLayeredPane, JComponent> map ) {
+	private static void showSnapshot( boolean old, Map<JLayeredPane, JComponent> map ) {
 		inShowSnapshot = true;
 
 		// create snapshots for all shown windows
@@ -107,7 +108,7 @@ public class FlatAnimatedLafChange
 					if( inShowSnapshot || snapshot.contentsLost() )
 						return;
 
-					if( useAlpha )
+					if( old )
 						((Graphics2D)g).setComposite( AlphaComposite.getInstance( AlphaComposite.SRC_OVER, alpha ) );
 					g.drawImage( snapshot, 0, 0, null );
 				}
@@ -120,13 +121,17 @@ public class FlatAnimatedLafChange
 					snapshot.flush();
 				}
 			};
-			if( !useAlpha )
+			if( !old )
 				snapshotLayer.setOpaque( true );
 			snapshotLayer.setSize( layeredPane.getSize() );
 
 			// add image layer to layered pane
-			layeredPane.add( snapshotLayer, Integer.valueOf( JLayeredPane.DRAG_LAYER + (useAlpha ? 2 : 1) ) );
+			layeredPane.add( snapshotLayer, Integer.valueOf( JLayeredPane.DRAG_LAYER + (old ? 2 : 1) ) );
 			map.put( layeredPane, snapshotLayer );
+
+			// let FlatRootPaneUI know that animated Laf change is in progress
+			if( old )
+				layeredPane.getRootPane().putClientProperty( "FlatLaf.internal.animatedLafChange", true );
 		}
 
 		inShowSnapshot = false;
@@ -180,6 +185,15 @@ public class FlatAnimatedLafChange
 		for( Map.Entry<JLayeredPane, JComponent> e : map.entrySet() ) {
 			e.getKey().remove( e.getValue() );
 			e.getKey().repaint();
+
+			// run Runnable that FlatRootPaneUI put into client properties
+			JRootPane rootPane = e.getKey().getRootPane();
+			rootPane.putClientProperty( "FlatLaf.internal.animatedLafChange", null );
+			Runnable r = (Runnable) rootPane.getClientProperty( "FlatLaf.internal.animatedLafChange.runWhenFinished" );
+			if( r != null ) {
+				rootPane.putClientProperty( "FlatLaf.internal.animatedLafChange.runWhenFinished", null );
+				r.run();
+			}
 		}
 
 		map.clear();