ScrollBar:

- added `ScrollBar.minimumButtonSize` to specify minimum scroll arrow button size
- center and scale arrows in scroll up/down buttons

(issue #493)

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

@@ -37,7 +37,7 @@ public class FlatArrowButton
 	extends BasicArrowButton
 	implements UIResource
 {
-	public static final int DEFAULT_ARROW_WIDTH = 8;
+	public static final int DEFAULT_ARROW_WIDTH = 9;
 
 	protected boolean chevron;
 	protected Color foreground;
@@ -211,6 +211,6 @@ public class FlatArrowButton
 		if( vert && parent instanceof JComponent && FlatUIUtils.hasRoundBorder( (JComponent) parent ) )
 			x -= scale( parent.getComponentOrientation().isLeftToRight() ? 1 : -1 );
 
-		FlatUIUtils.paintArrow( g, x, 0, getWidth(), getHeight(), getDirection(), chevron, arrowWidth, xOffset, yOffset );
+		FlatUIUtils.paintArrow( g, x, 0, getWidth(), getHeight(), getDirection(), chevron, getArrowWidth(), getXOffset(), getYOffset() );
 	}
 }

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

@@ -57,6 +57,7 @@ import com.formdev.flatlaf.util.UIScale;
  *
  * <!-- FlatScrollBarUI -->
  *
+ * @uiDefault ScrollBar.minimumButtonSize			Dimension
  * @uiDefault ScrollBar.trackInsets					Insets
  * @uiDefault ScrollBar.thumbInsets					Insets
  * @uiDefault ScrollBar.trackArc					int
@@ -83,6 +84,7 @@ public class FlatScrollBarUI
 	// overrides BasicScrollBarUI.supportsAbsolutePositioning (which is private)
 	@Styleable protected boolean allowsAbsolutePositioning;
 
+	/** @since 2.1 */ @Styleable protected Dimension minimumButtonSize;
 	@Styleable protected Insets trackInsets;
 	@Styleable protected Insets thumbInsets;
 	@Styleable protected int trackArc;
@@ -142,6 +144,7 @@ public class FlatScrollBarUI
 
 		allowsAbsolutePositioning = super.getSupportsAbsolutePositioning();
 
+		minimumButtonSize = UIManager.getDimension( "ScrollBar.minimumButtonSize" );
 		trackInsets = UIManager.getInsets( "ScrollBar.trackInsets" );
 		thumbInsets = UIManager.getInsets( "ScrollBar.thumbInsets" );
 		trackArc = UIManager.getInt( "ScrollBar.trackArc" );
@@ -171,6 +174,7 @@ public class FlatScrollBarUI
 	protected void uninstallDefaults() {
 		super.uninstallDefaults();
 
+		minimumButtonSize = null;
 		trackInsets = null;
 		thumbInsets = null;
 		hoverTrackColor = null;
@@ -451,7 +455,6 @@ public class FlatScrollBarUI
 			super( direction, type, foreground, disabledForeground,
 				hoverForeground, hoverBackground, pressedForeground, pressedBackground );
 
-			setArrowWidth( FlatArrowButton.DEFAULT_ARROW_WIDTH - 2 );
 			setFocusable( false );
 			setRequestFocusEnabled( false );
 		}
@@ -461,6 +464,18 @@ public class FlatScrollBarUI
 				null, hoverButtonBackground, null, pressedButtonBackground );
 		}
 
+		@Override
+		public int getArrowWidth() {
+			// scale arrow size depending on scroll bar width
+			// (6 is default arrow width; 10 is base scroll bar width)
+			int arrowWidth = Math.round( 6 * (scrollBarWidth / 10f) );
+
+			// compute arrow size that leaves equal space on both sides (arrow is centered)
+			arrowWidth = scrollBarWidth - (((scrollBarWidth - arrowWidth) / 2) * 2);
+
+			return arrowWidth;
+		}
+
 		@Override
 		protected Color deriveBackground( Color background ) {
 			return FlatUIUtils.deriveColor( background, scrollbar.getBackground() );
@@ -469,8 +484,9 @@ public class FlatScrollBarUI
 		@Override
 		public Dimension getPreferredSize() {
 			if( isShowButtons() ) {
-				int w = UIScale.scale( scrollBarWidth );
-				return new Dimension( w, w );
+				int w = UIScale.scale( Math.max( scrollBarWidth, (minimumButtonSize != null) ? minimumButtonSize.width : 0 ) );
+				int h = UIScale.scale( Math.max( scrollBarWidth, (minimumButtonSize != null) ? minimumButtonSize.height : 0 ) );
+				return new Dimension( w, h );
 			} else
 				return new Dimension();
 		}

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

@@ -1841,7 +1841,7 @@ public class FlatTabbedPaneUI
 			super( direction, arrowType,
 				FlatTabbedPaneUI.this.foreground, FlatTabbedPaneUI.this.disabledForeground,
 				null, buttonHoverBackground, null, buttonPressedBackground );
-			setArrowWidth( 10 );
+			setArrowWidth( 11 );
 		}
 
 		protected void updateStyle() {

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

@@ -786,13 +786,15 @@ public class FlatUIUtils
 		int direction, boolean chevron, int arrowSize, float xOffset, float yOffset )
 	{
 		// compute arrow width/height
-		int aw = UIScale.scale( arrowSize + (chevron ? 0 : 1) );
-		int ah = UIScale.scale( (arrowSize / 2) + (chevron ? 0 : 1) );
+		// - make chevron arrows one pixel smaller because coordinates are based on center of pixels (0.5/0.5)
+		// - make triangle arrows one pixel taller (and round height up) to make them look stronger
+		float aw = UIScale.scale( arrowSize + (chevron ? -1 : 0) );
+		float ah = chevron ? (aw / 2) : UIScale.scale( (arrowSize / 2) + 1 );
 
 		// rotate arrow width/height for horizontal directions
 		boolean vert = (direction == SwingConstants.NORTH || direction == SwingConstants.SOUTH);
 		if( !vert ) {
-			int temp = aw;
+			float temp = aw;
 			aw = ah;
 			ah = temp;
 		}
@@ -804,19 +806,19 @@ public class FlatUIUtils
 		// compute arrow location
 		float ox = ((width - (aw + extra)) / 2f) + UIScale.scale( xOffset );
 		float oy = ((height - (ah + extra)) / 2f) + UIScale.scale( yOffset );
-		int ax = x + ((direction == SwingConstants.WEST) ? -Math.round( -ox ) : Math.round( ox ));
-		int ay = y + ((direction == SwingConstants.NORTH) ? -Math.round( -oy ) : Math.round( oy ));
+		float ax = x + ((direction == SwingConstants.WEST) ? -Math.round( -(ox + aw) ) - aw : Math.round( ox ));
+		float ay = y + ((direction == SwingConstants.NORTH) ? -Math.round( -(oy + ah) ) - ah : Math.round( oy ));
 
 		// paint arrow
 		g.translate( ax, ay );
 /*debug
-		debugPaintArrow( g, Color.red, vert, aw + extra, ah + extra );
+		debugPaintArrow( g, Color.red, vert, Math.round( aw + extra ), Math.round( ah + extra ) );
 debug*/
 		Shape arrowShape = createArrowShape( direction, chevron, aw, ah );
 		if( chevron ) {
 			Stroke oldStroke = g.getStroke();
 			g.setStroke( new BasicStroke( UIScale.scale( 1f ) ) );
-			g.draw( arrowShape );
+			drawShapePure( g, arrowShape );
 			g.setStroke( oldStroke );
 		} else {
 			// triangle
@@ -892,6 +894,23 @@ debug*/
 		return path;
 	}
 
+	/**
+	 * Draws the given shape with disabled stroke normalization.
+	 * The x/y coordinates of the shape are translated by a half pixel.
+	 *
+	 * @since 2.1
+	 */
+	public static void drawShapePure( Graphics2D g, Shape shape ) {
+		Object oldStrokeControl = g.getRenderingHint( RenderingHints.KEY_STROKE_CONTROL );
+		g.setRenderingHint( RenderingHints.KEY_STROKE_CONTROL, RenderingHints.VALUE_STROKE_PURE );
+
+		g.translate( 0.5, 0.5 );
+		g.draw( shape );
+		g.translate( -0.5, -0.5 );
+
+		g.setRenderingHint( RenderingHints.KEY_STROKE_CONTROL, oldStrokeControl );
+	}
+
 	/**
 	 * Draws the given string at the specified location.
 	 * The provided component is used to query text properties and anti-aliasing hints.

flatlaf-core/src/main/resources/com/formdev/flatlaf/FlatLaf.properties

@@ -555,6 +555,7 @@ RootPane.honorDialogMinimumSizeOnResize = true
 #---- ScrollBar ----
 
 ScrollBar.width = 10
+ScrollBar.minimumButtonSize = 12,12
 ScrollBar.minimumThumbSize = 10,10
 ScrollBar.maximumThumbSize = 100000,100000
 ScrollBar.trackInsets = 0,0,0,0

flatlaf-core/src/test/java/com/formdev/flatlaf/ui/TestFlatStyleableInfo.java

@@ -539,6 +539,7 @@ public class TestFlatStyleableInfo
 			"maximumThumbSize", Dimension.class,
 			"allowsAbsolutePositioning", boolean.class,
 
+			"minimumButtonSize", Dimension.class,
 			"trackInsets", Insets.class,
 			"thumbInsets", Insets.class,
 			"trackArc", int.class,

flatlaf-core/src/test/java/com/formdev/flatlaf/ui/TestFlatStyling.java

@@ -684,6 +684,7 @@ public class TestFlatStyling
 		ui.applyStyle( "maximumThumbSize: 1,2" );
 		ui.applyStyle( "allowsAbsolutePositioning: true" );
 
+		ui.applyStyle( "minimumButtonSize: 1,2" );
 		ui.applyStyle( "trackInsets: 1,2,3,4" );
 		ui.applyStyle( "thumbInsets: 1,2,3,4" );
 		ui.applyStyle( "trackArc: 5" );