support changing default font used for all components with automatic scaling UI if using larger font

flatlaf-core/src/main/java/com/formdev/flatlaf/FlatLaf.java

@@ -49,8 +49,10 @@ import javax.swing.SwingUtilities;
 import javax.swing.UIDefaults;
 import javax.swing.UIManager;
 import javax.swing.UnsupportedLookAndFeelException;
+import javax.swing.UIDefaults.ActiveValue;
 import javax.swing.plaf.ColorUIResource;
 import javax.swing.plaf.FontUIResource;
+import javax.swing.plaf.UIResource;
 import javax.swing.plaf.basic.BasicLookAndFeel;
 import javax.swing.text.html.HTMLEditorKit;
 import com.formdev.flatlaf.util.SystemInfo;
@@ -345,14 +347,22 @@ public abstract class FlatLaf
 
 		uiFont = UIScale.applyCustomScaleFactor( uiFont );
 
+		// use active value for all fonts to allow changing fonts in all components
+		// (similar as in Nimbus L&F) with:
+		//     UIManager.put( "defaultFont", myFont );
+		Object activeFont =  new ActiveFont( 1 );
+
 		// override fonts
 		for( Object key : defaults.keySet() ) {
 			if( key instanceof String && (((String)key).endsWith( ".font" ) || ((String)key).endsWith( "Font" )) )
-				defaults.put( key, uiFont );
+				defaults.put( key, activeFont );
 		}
 
 		// use smaller font for progress bar
-		defaults.put( "ProgressBar.font", UIScale.scaleFont( uiFont, 0.85f ) );
+		defaults.put( "ProgressBar.font", new ActiveFont( 0.85f ) );
+
+		// set default font
+		defaults.put( "defaultFont", uiFont );
 	}
 
 	/**
@@ -562,4 +572,42 @@ public abstract class FlatLaf
 
 		return false;
 	}
+
+	//---- class ActiveFont ---------------------------------------------------
+
+	private static class ActiveFont
+		implements ActiveValue
+	{
+		private final float scaleFactor;
+
+		// cache (scaled) font
+		private Font font;
+		private Font lastDefaultFont;
+
+		ActiveFont( float scaleFactor ) {
+			this.scaleFactor = scaleFactor;
+		}
+
+		@Override
+		public Object createValue( UIDefaults table ) {
+			Font defaultFont = UIManager.getFont( "defaultFont" );
+
+			if( lastDefaultFont != defaultFont ) {
+				lastDefaultFont = defaultFont;
+
+				if( scaleFactor != 1 ) {
+					// scale font
+					int newFontSize = Math.round( defaultFont.getSize() * scaleFactor );
+					font = new FontUIResource( defaultFont.deriveFont( (float) newFontSize ) );
+				} else {
+					// make sure that font is a UIResource for LaF switching
+					font = (defaultFont instanceof UIResource)
+						? defaultFont
+						: new FontUIResource( defaultFont );
+				}
+			}
+
+			return font;
+		}
+	}
 }

flatlaf-core/src/main/java/com/formdev/flatlaf/util/UIScale.java

@@ -24,6 +24,7 @@ import java.awt.GraphicsEnvironment;
 import java.awt.Insets;
 import java.beans.PropertyChangeEvent;
 import java.beans.PropertyChangeListener;
+import java.beans.PropertyChangeSupport;
 import java.lang.reflect.Method;
 import javax.swing.LookAndFeel;
 import javax.swing.UIManager;
@@ -48,12 +49,15 @@ import javax.swing.plaf.UIResource;
  *
  * 2) user scaling mode
  *
- * This mode is mainly for Java 8 compatibility, but is also used on Linux.
+ * This mode is mainly for Java 8 compatibility, but is also used on Linux
+ * or if the default font is changed.
  * The user scale factor is computed based on the used font.
  * The JRE does not scale anything.
  * So we have to invoke {@link #scale(float)} where necessary.
  * There is only one user scale factor for all displays.
- * The user scale factor may change if the active LaF or "Label.font" has changed.
+ * The user scale factor may change if the active LaF, "defaultFont" or "Label.font" has changed.
+ * If system scaling mode is available the user scale factor is usually 1,
+ * but may be larger on Linux or if the default font is changed.
  *
  * @author Karl Tauber
  */
@@ -61,6 +65,20 @@ public class UIScale
 {
 	private static final boolean DEBUG = false;
 
+	private static PropertyChangeSupport changeSupport;
+
+	public static void addPropertyChangeListener( PropertyChangeListener listener ) {
+		if( changeSupport == null )
+			changeSupport = new PropertyChangeSupport( UIScale.class );
+		changeSupport.addPropertyChangeListener( listener );
+	}
+
+	public static void removePropertyChangeListener( PropertyChangeListener listener ) {
+		if( changeSupport == null )
+			return;
+		changeSupport.removePropertyChangeListener( listener );
+	}
+
 	//---- system scaling (Java 9) --------------------------------------------
 
 	private static Boolean jreHiDPI;
@@ -110,27 +128,33 @@ public class UIScale
 			return;
 		initialized = true;
 
-		if( isUserScalingEnabled() ) {
-			// listener to update scale factor if LaF changed or if Label.font changed
-			// (e.g. option "Override default fonts" in IntelliJ IDEA)
-			PropertyChangeListener listener = new PropertyChangeListener() {
-				@Override
-				public void propertyChange( PropertyChangeEvent e ) {
-					String propName = e.getPropertyName();
-					if( "lookAndFeel".equals( propName ) ) {
+		if( !isUserScalingEnabled() )
+			return;
+
+		// listener to update scale factor if LaF changed, "defaultFont" or "Label.font" changed
+		PropertyChangeListener listener = new PropertyChangeListener() {
+			@Override
+			public void propertyChange( PropertyChangeEvent e ) {
+				switch( e.getPropertyName() ) {
+					case "lookAndFeel":
 						// it is not necessary (and possible) to remove listener of old LaF defaults
 						if( e.getNewValue() instanceof LookAndFeel )
 							UIManager.getLookAndFeelDefaults().addPropertyChangeListener( this );
 						updateScaleFactor();
-					} else if( "Label.font".equals( propName ) )
-						updateScaleFactor();
-				}
-			};
-			UIManager.addPropertyChangeListener( listener );
-			UIManager.getLookAndFeelDefaults().addPropertyChangeListener( listener );
+						break;
 
-			updateScaleFactor();
-		}
+					case "defaultFont":
+					case "Label.font":
+						updateScaleFactor();
+						break;
+				}
+			}
+		};
+		UIManager.addPropertyChangeListener( listener );
+		UIManager.getDefaults().addPropertyChangeListener( listener );
+		UIManager.getLookAndFeelDefaults().addPropertyChangeListener( listener );
+
+		updateScaleFactor();
 	}
 
 	private static void updateScaleFactor() {
@@ -141,7 +165,9 @@ public class UIScale
 		// because even if we are on a HiDPI display it is not sure
 		// that a larger font size is set by the current LaF
 		// (e.g. can avoid large icons with small text)
-		Font font = UIManager.getFont( "Label.font" );
+		Font font = UIManager.getFont( "defaultFont" );
+		if( font == null )
+			font = UIManager.getFont( "Label.font" );
 
 		setUserScaleFactor( computeScaleFactor( font ) );
 	}
@@ -168,9 +194,6 @@ public class UIScale
 	}
 
 	private static boolean isUserScalingEnabled() {
-		if( isSystemScalingEnabled() && !SystemInfo.IS_LINUX )
-			return false; // disable user scaling if JRE scales
-
 		// same as in IntelliJ IDEA
 		String hidpi = System.getProperty( "hidpi" );
 		return (hidpi != null) ? Boolean.parseBoolean( hidpi ) : true;
@@ -197,14 +220,6 @@ public class UIScale
 		return new FontUIResource( font.deriveFont( (float) newFontSize ) );
 	}
 
-	/**
-	 * Scales the given font.
-	 */
-	public static FontUIResource scaleFont( FontUIResource font, float scaleFactor ) {
-		int newFontSize = Math.round( font.getSize() * scaleFactor );
-		return new FontUIResource( font.deriveFont( (float) newFontSize ) );
-	}
-
 	/**
 	 * Similar to sun.java2d.SunGraphicsEnvironment.getScaleFactor(String)
 	 */
@@ -242,10 +257,14 @@ public class UIScale
 		else // round scale factor to 1/4
 			scaleFactor = Math.round( scaleFactor * 4f ) / 4f;
 
+		float oldScaleFactor = UIScale.scaleFactor;
 		UIScale.scaleFactor = scaleFactor;
 
 		if( DEBUG )
 			System.out.println( "HiDPI scale factor " + scaleFactor );
+
+		if( changeSupport != null )
+			changeSupport.firePropertyChange( "userScaleFactor", oldScaleFactor, scaleFactor );
 	}
 
 	public static float scale( float value ) {