ToggleButton: support underline toggle button style

CHANGELOG.md

@@ -11,6 +11,8 @@ FlatLaf Change Log
 - Label and ToolTip: Fixed font sizes for HTML headings.
 - Button and ToggleButton: Support square button style (set client property
   `JButton.buttonType` to `square`).
+- ToggleButton: Support underline toggle button style (set client property
+  `JButton.buttonType` to `underline`).
 
 
 ## 0.23.1

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

@@ -42,6 +42,15 @@ public interface FlatClientProperties
 	 */
 	String BUTTON_TYPE_SQUARE = "square";
 
+	/**
+	 * Paint the toggle button in underline style.
+	 * <p>
+	 * <strong>Components</strong> {@link javax.swing.JToggleButton}
+	 *
+	 * @see #TOGGLE_BUTTON_TYPE
+	 */
+	String BUTTON_TYPE_UNDERLINE = "underline";
+
 	/**
 	 * Paint a help button (circle with question mark).
 	 * <p>

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

@@ -65,7 +65,7 @@ public class FlatButtonBorder
 
 	@Override
 	public void paintBorder( Component c, Graphics g, int x, int y, int width, int height ) {
-		if( FlatButtonUI.isContentAreaFilled( c ) && !FlatButtonUI.isHelpButton( c ) )
+		if( FlatButtonUI.isContentAreaFilled( c ) && !FlatButtonUI.isHelpButton( c ) && !FlatToggleButtonUI.isUnderlineButton( c ) )
 			super.paintBorder( c, g, x, y, width, height );
 	}
 

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

@@ -232,44 +232,47 @@ public class FlatButtonUI
 			return;
 		}
 
-		if( isContentAreaFilled( c ) ) {
-			Color background = getBackground( c );
-			if( background != null ) {
-				Graphics2D g2 = (Graphics2D) g.create();
-				try {
-					FlatUIUtils.setRenderingHints( g2 );
-
-					Border border = c.getBorder();
-					float focusWidth = (border instanceof FlatBorder) ? scale( (float) this.focusWidth ) : 0;
-					float arc = ((border instanceof FlatButtonBorder && !isSquareButton( c )) || isToolBarButton( c ))
-						? scale( (float) this.arc ) : 0;
-					boolean def = isDefaultButton( c );
-
-					// paint shadow
-					Color shadowColor = def ? defaultShadowColor : this.shadowColor;
-					if( shadowColor != null && shadowWidth > 0 && focusWidth > 0 && !c.hasFocus() && c.isEnabled() ) {
-						g2.setColor( shadowColor );
-						g2.fill( new RoundRectangle2D.Float( focusWidth, focusWidth + UIScale.scale( (float) shadowWidth ),
-							c.getWidth() - focusWidth * 2, c.getHeight() - focusWidth * 2, arc, arc ) );
-					}
-
-					// paint background
-					Color startBg = def ? defaultBackground : startBackground;
-					Color endBg = def ? defaultEndBackground : endBackground;
-					if( background == startBg && endBg != null && !startBg.equals( endBg ) )
-						g2.setPaint( new GradientPaint( 0, 0, startBg, 0, c.getHeight(), endBg ) );
-					else
-						FlatUIUtils.setColor( g2, background, def ? defaultBackground : c.getBackground() );
-					FlatUIUtils.paintComponentBackground( g2, 0, 0, c.getWidth(), c.getHeight(), focusWidth, arc );
-				} finally {
-					g2.dispose();
-				}
-			}
-		}
+		if( isContentAreaFilled( c ) )
+			paintBackground( g, c );
 
 		paint( g, c );
 	}
 
+	protected void paintBackground( Graphics g, JComponent c ) {
+		Color background = getBackground( c );
+		if( background != null ) {
+			Graphics2D g2 = (Graphics2D) g.create();
+			try {
+				FlatUIUtils.setRenderingHints( g2 );
+
+				Border border = c.getBorder();
+				float focusWidth = (border instanceof FlatBorder) ? scale( (float) this.focusWidth ) : 0;
+				float arc = ((border instanceof FlatButtonBorder && !isSquareButton( c )) || isToolBarButton( c ))
+					? scale( (float) this.arc ) : 0;
+				boolean def = isDefaultButton( c );
+
+				// paint shadow
+				Color shadowColor = def ? defaultShadowColor : this.shadowColor;
+				if( shadowColor != null && shadowWidth > 0 && focusWidth > 0 && !c.hasFocus() && c.isEnabled() ) {
+					g2.setColor( shadowColor );
+					g2.fill( new RoundRectangle2D.Float( focusWidth, focusWidth + UIScale.scale( (float) shadowWidth ),
+						c.getWidth() - focusWidth * 2, c.getHeight() - focusWidth * 2, arc, arc ) );
+				}
+
+				// paint background
+				Color startBg = def ? defaultBackground : startBackground;
+				Color endBg = def ? defaultEndBackground : endBackground;
+				if( background == startBg && endBg != null && !startBg.equals( endBg ) )
+					g2.setPaint( new GradientPaint( 0, 0, startBg, 0, c.getHeight(), endBg ) );
+				else
+					FlatUIUtils.setColor( g2, background, def ? defaultBackground : c.getBackground() );
+				FlatUIUtils.paintComponentBackground( g2, 0, 0, c.getWidth(), c.getHeight(), focusWidth, arc );
+			} finally {
+				g2.dispose();
+			}
+		}
+	}
+
 	@Override
 	protected void paintText( Graphics g, AbstractButton b, Rectangle textRect, String text ) {
 		if( isHelpButton( b ) )

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

@@ -16,12 +16,17 @@
 
 package com.formdev.flatlaf.ui;
 
+import static com.formdev.flatlaf.FlatClientProperties.*;
 import java.awt.Color;
+import java.awt.Component;
+import java.awt.Graphics;
 import javax.swing.AbstractButton;
 import javax.swing.ButtonModel;
 import javax.swing.JComponent;
+import javax.swing.JToggleButton;
 import javax.swing.UIManager;
 import javax.swing.plaf.ComponentUI;
+import com.formdev.flatlaf.util.UIScale;
 
 /**
  * Provides the Flat LaF UI delegate for {@link javax.swing.JToggleButton}.
@@ -55,6 +60,13 @@ import javax.swing.plaf.ComponentUI;
  * @uiDefault ToggleButton.disabledSelectedBackground	Color
  * @uiDefault ToggleButton.toolbar.selectedBackground	Color
  *
+ * @uiDefault ToggleButton.underline.underlineHeight		int
+ * @uiDefault ToggleButton.underline.underlineColor			Color
+ * @uiDefault ToggleButton.underline.disabledUnderlineColor	Color
+ * @uiDefault ToggleButton.underline.selectedBackground		Color	optional
+ * @uiDefault ToggleButton.underline.hoverBackground		Color
+ * @uiDefault ToggleButton.underline.focusBackground		Color
+ *
  *
  * @author Karl Tauber
  */
@@ -67,6 +79,13 @@ public class FlatToggleButtonUI
 
 	protected Color toolbarSelectedBackground;
 
+	protected int underlineHeight;
+	protected Color underlineColor;
+	protected Color disabledUnderlineColor;
+	protected Color underlineSelectedBackground;
+	protected Color underlineHoverBackground;
+	protected Color underlineFocusBackground;
+
 	private boolean defaults_initialized = false;
 
 	private static ComponentUI instance;
@@ -93,6 +112,13 @@ public class FlatToggleButtonUI
 
 			toolbarSelectedBackground = UIManager.getColor( "ToggleButton.toolbar.selectedBackground" );
 
+			underlineHeight = UIManager.getInt( "ToggleButton.underline.underlineHeight" );
+			underlineColor = UIManager.getColor( "ToggleButton.underline.underlineColor" );
+			disabledUnderlineColor = UIManager.getColor( "ToggleButton.underline.disabledUnderlineColor" );
+			underlineSelectedBackground = UIManager.getColor( "ToggleButton.underline.selectedBackground" );
+			underlineHoverBackground = UIManager.getColor( "ToggleButton.underline.hoverBackground" );
+			underlineFocusBackground = UIManager.getColor( "ToggleButton.underline.focusBackground" );
+
 			defaults_initialized = true;
 		}
 	}
@@ -103,6 +129,36 @@ public class FlatToggleButtonUI
 		defaults_initialized = false;
 	}
 
+	static boolean isUnderlineButton( Component c ) {
+		return c instanceof JToggleButton && clientPropertyEquals( (JToggleButton) c, BUTTON_TYPE, BUTTON_TYPE_UNDERLINE );
+	}
+
+	@Override
+	protected void paintBackground( Graphics g, JComponent c ) {
+		if( isUnderlineButton( c ) ) {
+			int height = c.getHeight();
+			int width = c.getWidth();
+			boolean selected = ((AbstractButton)c).isSelected();
+
+			// paint background
+			Color background = buttonStateColor( c,
+				selected ? underlineSelectedBackground : null,
+				null, underlineFocusBackground, underlineHoverBackground, null );
+			if( background != null ) {
+				g.setColor( background );
+				g.fillRect( 0, 0, width, height );
+			}
+
+			// paint underline if selected
+			if( selected ) {
+				int underlineHeight = UIScale.scale( this.underlineHeight );
+				g.setColor( c.isEnabled() ? underlineColor : disabledUnderlineColor );
+				g.fillRect( 0, height - underlineHeight, width, underlineHeight );
+			}
+		} else
+			super.paintBackground( g, c );
+	}
+
 	@Override
 	protected Color getBackground( JComponent c ) {
 		ButtonModel model = ((AbstractButton)c).getModel();

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

@@ -435,6 +435,14 @@ ToggleButton.pressedBackground=$Button.pressedBackground
 ToggleButton.toolbar.hoverBackground=$Button.toolbar.hoverBackground
 ToggleButton.toolbar.pressedBackground=$Button.toolbar.pressedBackground
 
+# button type "underline"
+ToggleButton.underline.underlineHeight=2
+ToggleButton.underline.underlineColor=$TabbedPane.underlineColor
+ToggleButton.underline.disabledUnderlineColor=$TabbedPane.disabledUnderlineColor
+ToggleButton.underline.selectedBackground=$?TabbedPane.selectedBackground
+ToggleButton.underline.hoverBackground=$TabbedPane.hoverColor
+ToggleButton.underline.focusBackground=$TabbedPane.focusColor
+
 
 #---- ToolBar ----
 

flatlaf-testing/src/main/java/com/formdev/flatlaf/testing/FlatComponentsTest.java

@@ -78,6 +78,8 @@ public class FlatComponentsTest
 		JToggleButton toggleButton10 = new JToggleButton();
 		JToggleButton toggleButton3 = new JToggleButton();
 		JToggleButton toggleButton4 = new JToggleButton();
+		JToggleButton toggleButton5 = new JToggleButton();
+		JToggleButton toggleButton8 = new JToggleButton();
 		JLabel checkBoxLabel = new JLabel();
 		JCheckBox checkBox1 = new JCheckBox();
 		JCheckBox checkBox2 = new JCheckBox();
@@ -339,6 +341,19 @@ public class FlatComponentsTest
 		toggleButton4.setSelected(true);
 		add(toggleButton4, "cell 4 2");
 
+		//---- toggleButton5 ----
+		toggleButton5.setText("underline");
+		toggleButton5.putClientProperty("JButton.buttonType", "underline");
+		toggleButton5.setSelected(true);
+		add(toggleButton5, "cell 5 2");
+
+		//---- toggleButton8 ----
+		toggleButton8.setText("underline");
+		toggleButton8.putClientProperty("JButton.buttonType", "underline");
+		toggleButton8.setEnabled(false);
+		toggleButton8.setSelected(true);
+		add(toggleButton8, "cell 5 2");
+
 		//---- checkBoxLabel ----
 		checkBoxLabel.setText("JCheckBox");
 		add(checkBoxLabel, "cell 0 3");

flatlaf-testing/src/main/java/com/formdev/flatlaf/testing/FlatComponentsTest.jfd

@@ -167,6 +167,23 @@ new FormModel {
 			}, new FormLayoutConstraints( class net.miginfocom.layout.CC ) {
 				"value": "cell 4 2"
 			} )
+			add( new FormComponent( "javax.swing.JToggleButton" ) {
+				name: "toggleButton5"
+				"text": "underline"
+				"$client.JButton.buttonType": "underline"
+				"selected": true
+			}, new FormLayoutConstraints( class net.miginfocom.layout.CC ) {
+				"value": "cell 5 2"
+			} )
+			add( new FormComponent( "javax.swing.JToggleButton" ) {
+				name: "toggleButton8"
+				"text": "underline"
+				"$client.JButton.buttonType": "underline"
+				"enabled": false
+				"selected": true
+			}, new FormLayoutConstraints( class net.miginfocom.layout.CC ) {
+				"value": "cell 5 2"
+			} )
 			add( new FormComponent( "javax.swing.JLabel" ) {
 				name: "checkBoxLabel"
 				"text": "JCheckBox"