Styling:

- check for duplicate keys in StyleableInfos to find "overlapping" fields/properties (e.g. `borderColor` in `FlatBorder` and in `FlatComboBoxUI`)
- Button and ToggleButton: fixed styling `toolbar.spacingInsets`

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

@@ -133,7 +133,6 @@ public class FlatButtonUI
 	@Styleable protected Color shadowColor;
 	@Styleable(dot=true) protected Color defaultShadowColor;
 
-	@Styleable(dot=true) protected Insets toolbarSpacingInsets;
 	@Styleable(dot=true) protected Color toolbarHoverBackground;
 	@Styleable(dot=true) protected Color toolbarPressedBackground;
 	@Styleable(dot=true) protected Color toolbarSelectedBackground;
@@ -202,7 +201,6 @@ public class FlatButtonUI
 			shadowColor = UIManager.getColor( "Button.shadowColor" );
 			defaultShadowColor = UIManager.getColor( "Button.default.shadowColor" );
 
-			toolbarSpacingInsets = UIManager.getInsets( "Button.toolbar.spacingInsets" );
 			toolbarHoverBackground = UIManager.getColor( prefix + "toolbar.hoverBackground" );
 			toolbarPressedBackground = UIManager.getColor( prefix + "toolbar.pressedBackground" );
 			toolbarSelectedBackground = UIManager.getColor( prefix + "toolbar.selectedBackground" );
@@ -416,8 +414,8 @@ public class FlatButtonUI
 			int width = c.getWidth();
 			int height = c.getHeight();
 
-			if( isToolBarButton ) {
+			if( isToolBarButton && c.getBorder() instanceof FlatButtonBorder ) {
-				Insets spacing = UIScale.scale( toolbarSpacingInsets );
+				Insets spacing = UIScale.scale( ((FlatButtonBorder)c.getBorder()).toolbarSpacingInsets );
 				x += spacing.left;
 				y += spacing.top;
 				width -= spacing.left + spacing.right;

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

@@ -42,7 +42,6 @@ import java.awt.event.MouseEvent;
 import java.awt.event.MouseListener;
 import java.awt.geom.Rectangle2D;
 import java.beans.PropertyChangeListener;
-import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicBoolean;
 import javax.swing.AbstractAction;
@@ -495,7 +494,7 @@ public class FlatComboBoxUI
 	/** @since 2 */
 	@Override
 	public Map<String, Class<?>> getStyleableInfos( JComponent c ) {
-		Map<String, Class<?>> infos = new LinkedHashMap<>();
+		Map<String, Class<?>> infos = new FlatStylingSupport.StyleableInfosMap<>();
 		infos.put( "padding", Insets.class );
 		FlatStylingSupport.collectAnnotatedStyleableInfos( this, infos );
 		FlatStylingSupport.collectStyleableInfos( comboBox.getBorder(), infos );

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

@@ -25,7 +25,6 @@ import java.awt.Insets;
 import java.awt.Rectangle;
 import java.awt.event.MouseEvent;
 import java.beans.PropertyChangeListener;
-import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicBoolean;
 import javax.swing.JComponent;
@@ -222,7 +221,7 @@ public class FlatInternalFrameUI
 
 		@Override
 		public Map<String, Class<?>> getStyleableInfos() {
-			Map<String, Class<?>> infos = new LinkedHashMap<>();
+			Map<String, Class<?>> infos = new FlatStylingSupport.StyleableInfosMap<>();
 			FlatStylingSupport.collectAnnotatedStyleableInfos( this, infos );
 			infos.put( "borderMargins", Insets.class );
 			infos.put( "activeDropShadowColor", Color.class );

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

@@ -153,6 +153,7 @@ public class FlatMenuItemRenderer
 		Map<String, Class<?>> infos = FlatStylingSupport.getAnnotatedStyleableInfos( this );
 		if( checkIcon instanceof FlatCheckBoxMenuItemIcon )
 			FlatStylingSupport.putAllPrefixKey( infos, "icon.", ((FlatCheckBoxMenuItemIcon)checkIcon).getStyleableInfos() );
+		infos.remove( "icon.selectionForeground" );
 		if( arrowIcon instanceof FlatMenuArrowIcon )
 			FlatStylingSupport.putAllPrefixKey( infos, "icon.", ((FlatMenuArrowIcon)arrowIcon).getStyleableInfos() );
 		infos.remove( "icon.selectionForeground" );

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

@@ -20,7 +20,6 @@ import java.awt.Color;
 import java.awt.Dimension;
 import java.awt.Graphics;
 import java.beans.PropertyChangeListener;
-import java.util.LinkedHashMap;
 import java.util.Map;
 import javax.swing.Icon;
 import javax.swing.JComponent;
@@ -145,7 +144,7 @@ public class FlatMenuItemUI
 	}
 
 	static Map<String, Class<?>> getStyleableInfos( FlatMenuItemRenderer renderer ) {
-		Map<String, Class<?>> infos = new LinkedHashMap<>();
+		Map<String, Class<?>> infos = new FlatStylingSupport.StyleableInfosMap<>();
 		infos.put( "selectionBackground", Color.class );
 		infos.put( "selectionForeground", Color.class );
 		infos.put( "disabledForeground", Color.class );

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

@@ -20,7 +20,6 @@ import java.awt.Color;
 import java.awt.Component;
 import java.awt.Container;
 import java.awt.Insets;
-import java.util.LinkedHashMap;
 import java.util.Map;
 import javax.swing.JScrollPane;
 import javax.swing.UIManager;
@@ -61,7 +60,7 @@ public class FlatPopupMenuBorder
 	/** @since 2 */
 	@Override
 	public Map<String, Class<?>> getStyleableInfos() {
-		Map<String, Class<?>> infos = new LinkedHashMap<>();
+		Map<String, Class<?>> infos = new FlatStylingSupport.StyleableInfosMap<>();
 		infos.put( "borderInsets", Insets.class );
 		infos.put( "borderColor", Color.class );
 		return infos;

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

@@ -24,7 +24,6 @@ import java.awt.Rectangle;
 import java.awt.event.MouseAdapter;
 import java.awt.event.MouseEvent;
 import java.beans.PropertyChangeListener;
-import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.Objects;
 import javax.swing.InputMap;
@@ -259,7 +258,7 @@ public class FlatScrollBarUI
 	/** @since 2 */
 	@Override
 	public Map<String, Class<?>> getStyleableInfos( JComponent c ) {
-		Map<String, Class<?>> infos = new LinkedHashMap<>();
+		Map<String, Class<?>> infos = new FlatStylingSupport.StyleableInfosMap<>();
 		infos.put( "track", Color.class );
 		infos.put( "thumb", Color.class );
 		infos.put( "width", int.class );

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

@@ -25,6 +25,7 @@ import java.lang.reflect.Field;
 import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
@@ -612,7 +613,7 @@ public class FlatStylingSupport
 	}
 
 	public static Map<String, Class<?>> getAnnotatedStyleableInfos( Object obj, Border border ) {
-		Map<String, Class<?>> infos = new LinkedHashMap<>();
+		Map<String, Class<?>> infos = new StyleableInfosMap<>();
 		collectAnnotatedStyleableInfos( obj, infos );
 		collectStyleableInfos( border, infos );
 		return infos;
@@ -623,6 +624,7 @@ public class FlatStylingSupport
 	 * The key is the name of the field and the value the type of the field.
 	 */
 	public static void collectAnnotatedStyleableInfos( Object obj, Map<String, Class<?>> infos ) {
+		HashSet<String> processedFields = new HashSet<>();
 		Class<?> cls = obj.getClass();
 
 		for(;;) {
@@ -637,6 +639,13 @@ public class FlatStylingSupport
 				String name = f.getName();
 				Class<?> type = f.getType();
 
+				// for the case that the same field name is used in a class and in
+				// one of its superclasses (e.g. field 'borderColor' in FlatButtonBorder
+				// and in FlatBorder), do not process field in superclass
+				if( processedFields.contains( name ) )
+					continue;
+				processedFields.add( name );
+
 				// handle "dot" keys (e.g. change field name "iconArrowType" to style key "icon.arrowType")
 				if( styleable.dot() ) {
 					int len = name.length();
@@ -691,4 +700,24 @@ public class FlatStylingSupport
 			return "unknown style '" + super.getMessage() + "'";
 		}
 	}
+
+	//---- class StyleableInfosMap --------------------------------------------
+
+	static class StyleableInfosMap<K,V>
+		extends LinkedHashMap<K,V>
+	{
+		@Override
+		public V put( K key, V value ) {
+			V oldValue = super.put( key, value );
+			if( oldValue != null )
+				throw new IllegalArgumentException( "duplicate key '" + key + "'" );
+			return oldValue;
+		}
+
+		@Override
+		public void putAll( Map<? extends K, ? extends V> m ) {
+			for( Map.Entry<? extends K, ? extends V> e : m.entrySet() )
+				put( e.getKey(), e.getValue() );
+		}
+	}
 }

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

@@ -52,7 +52,6 @@ import java.awt.geom.Rectangle2D;
 import java.beans.PropertyChangeEvent;
 import java.beans.PropertyChangeListener;
 import java.util.Collections;
-import java.util.LinkedHashMap;
 import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
@@ -638,7 +637,7 @@ public class FlatTabbedPaneUI
 	/** @since 2 */
 	@Override
 	public Map<String, Class<?>> getStyleableInfos( JComponent c ) {
-		Map<String, Class<?>> infos = new LinkedHashMap<>();
+		Map<String, Class<?>> infos = new FlatStylingSupport.StyleableInfosMap<>();
 		infos.put( "tabInsets", Insets.class );
 		infos.put( "tabAreaInsets", Insets.class );
 		infos.put( "textIconGap", int.class );