ScrollPane: improved/fixed calculation of left/right padding for rounded border

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

@@ -41,11 +41,8 @@ public class FlatScrollPaneBorder
 
 		// if view is rounded, increase left and right insets to avoid that the viewport
 		// is painted over the rounded border on the corners
-		int arc = getArc( c );
-		if( arc > 0 ) {
-			// increase insets by radius minus lineWidth because radius is measured
-			// from the outside of the line, but insets from super include lineWidth
-			int padding = UIScale.scale( (arc / 2) - getLineWidth( c ) );
+		int padding = getLeftRightPadding( c );
+		if( padding > 0 ) {
 			insets.left += padding;
 			insets.right += padding;
 		}
@@ -60,4 +57,22 @@ public class FlatScrollPaneBorder
 
 		return arc;
 	}
+
+	/**
+	 * Returns the scaled left/right padding used when arc is larger than zero.
+	 * <p>
+	 * This is the distance from the inside of the left border to the left side of the view component.
+	 * On the right side, this is the distance between the right side of the view component and
+	 * the vertical scrollbar. Or the inside of the right border if the scrollbar is hidden.
+	 */
+	public int getLeftRightPadding( Component c ) {
+		// Subtract lineWidth from radius because radius is given for the outside
+		// of the painted line, but insets from super already include lineWidth.
+		// Reduce padding by 10% to make padding slightly smaller because it is not recognizable
+		// when the view is minimally painted over the beginning of the border curve.
+		int arc = getArc( c );
+		return (arc > 0)
+			? Math.max( Math.round( UIScale.scale( ((arc / 2f) - getLineWidth( c )) * 0.9f ) ), 0 )
+			: 0;
+	}
 }

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

@@ -521,17 +521,19 @@ public class FlatScrollPaneUI
 			super.layoutContainer( parent );
 
 			JScrollPane scrollPane = (JScrollPane) parent;
-			float arc = getBorderArc( scrollPane );
-			if( arc > 0 ) {
+			Border border = scrollPane.getBorder();
+			int padding;
+			if( border instanceof FlatScrollPaneBorder &&
+				(padding = ((FlatScrollPaneBorder)border).getLeftRightPadding( scrollPane )) > 0 )
+			{
 				JScrollBar vsb = getVerticalScrollBar();
 				if( vsb != null && vsb.isVisible() ) {
 					// move vertical scrollbar to trailing edge
-					int padding = Math.round( (arc / 2) - FlatUIUtils.getBorderLineWidth( scrollPane ) );
-					Insets insets = parent.getInsets();
+					Insets insets = scrollPane.getInsets();
 					Rectangle r = vsb.getBounds();
 					int y = Math.max( r.y, insets.top + padding );
-					int y2 = Math.min( r.y + r.height, parent.getHeight() - insets.bottom - padding );
-					boolean ltr = parent.getComponentOrientation().isLeftToRight();
+					int y2 = Math.min( r.y + r.height, scrollPane.getHeight() - insets.bottom - padding );
+					boolean ltr = scrollPane.getComponentOrientation().isLeftToRight();
 
 					vsb.setBounds( r.x + (ltr ? padding : -padding), y, r.width, y2 - y );
 				}

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

@@ -23,9 +23,11 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.stream.Collectors;
 import javax.swing.*;
+import javax.swing.border.Border;
 import javax.swing.table.AbstractTableModel;
 import javax.swing.tree.*;
 import com.formdev.flatlaf.FlatClientProperties;
+import com.formdev.flatlaf.ui.FlatScrollPaneBorder;
 import com.formdev.flatlaf.util.UIScale;
 import net.miginfocom.swing.*;
 
@@ -203,6 +205,11 @@ public class FlatRoundedScrollPaneTest
 		int arc = arcSlider.getValue();
 		for( JScrollPane scrollPane : allJScrollPanes )
 			scrollPane.putClientProperty( FlatClientProperties.STYLE, "arc: " + arc );
+
+		Border border = allJScrollPanes[0].getBorder();
+		paddingField.setText( border instanceof FlatScrollPaneBorder
+			? Integer.toString( ((FlatScrollPaneBorder)border).getLeftRightPadding( allJScrollPanes[0] ) )
+			: "?" );
 	}
 
 	private void initComponents() {
@@ -210,6 +217,8 @@ public class FlatRoundedScrollPaneTest
 		splitPane2 = new JSplitPane();
 		panel1 = new FlatTestPanel();
 		listLabel = new JLabel();
+		paddingLabel = new JLabel();
+		paddingField = new JLabel();
 		treeLabel = new JLabel();
 		tableLabel = new JLabel();
 		autoResizeModeCheckBox = new JCheckBox();
@@ -237,8 +246,8 @@ public class FlatRoundedScrollPaneTest
 		arcSlider = new JSlider();
 		cornersCheckBox = new JCheckBox();
 		columnHeaderCheckBox = new JCheckBox();
-		rowHeaderCheckBox = new JCheckBox();
 		horizontalScrollBarCheckBox = new JCheckBox();
+		rowHeaderCheckBox = new JCheckBox();
 		verticalScrollBarCheckBox = new JCheckBox();
 
 		//======== this ========
@@ -273,6 +282,14 @@ public class FlatRoundedScrollPaneTest
 				listLabel.setHorizontalTextPosition(SwingConstants.LEADING);
 				panel1.add(listLabel, "cell 0 0,aligny top,growy 0");
 
+				//---- paddingLabel ----
+				paddingLabel.setText("Padding:");
+				panel1.add(paddingLabel, "cell 0 0,alignx trailing,growx 0");
+
+				//---- paddingField ----
+				paddingField.setText("0");
+				panel1.add(paddingField, "cell 0 0,alignx trailing,growx 0");
+
 				//---- treeLabel ----
 				treeLabel.setText("JTree:");
 				treeLabel.setHorizontalTextPosition(SwingConstants.LEADING);
@@ -382,12 +399,11 @@ public class FlatRoundedScrollPaneTest
 				// columns
 				"[fill]" +
 				"[grow,fill]para" +
-				"[fill]" +
-				"[fill]" +
-				"[fill]" +
-				"[fill]" +
-				"[fill]",
+				"[]" +
+				"[]" +
+				"[]",
 				// rows
+				"[]" +
 				"[]"));
 
 			//---- arcLabel ----
@@ -403,7 +419,7 @@ public class FlatRoundedScrollPaneTest
 			arcSlider.setPaintTicks(true);
 			arcSlider.setPaintLabels(true);
 			arcSlider.addChangeListener(e -> arcSliderChanged());
-			panel3.add(arcSlider, "cell 1 0");
+			panel3.add(arcSlider, "cell 1 0 1 2");
 
 			//---- cornersCheckBox ----
 			cornersCheckBox.setText("Corners");
@@ -415,22 +431,22 @@ public class FlatRoundedScrollPaneTest
 			columnHeaderCheckBox.addActionListener(e -> columnHeaderChanged());
 			panel3.add(columnHeaderCheckBox, "cell 3 0");
 
-			//---- rowHeaderCheckBox ----
-			rowHeaderCheckBox.setText("Row Header");
-			rowHeaderCheckBox.addActionListener(e -> rowHeaderChanged());
-			panel3.add(rowHeaderCheckBox, "cell 4 0");
-
 			//---- horizontalScrollBarCheckBox ----
 			horizontalScrollBarCheckBox.setText("Horizontal ScrollBar");
 			horizontalScrollBarCheckBox.setSelected(true);
 			horizontalScrollBarCheckBox.addActionListener(e -> horizontalScrollBarChanged());
-			panel3.add(horizontalScrollBarCheckBox, "cell 5 0");
+			panel3.add(horizontalScrollBarCheckBox, "cell 4 0");
+
+			//---- rowHeaderCheckBox ----
+			rowHeaderCheckBox.setText("Row Header");
+			rowHeaderCheckBox.addActionListener(e -> rowHeaderChanged());
+			panel3.add(rowHeaderCheckBox, "cell 3 1");
 
 			//---- verticalScrollBarCheckBox ----
 			verticalScrollBarCheckBox.setText("Vertical ScrollBar");
 			verticalScrollBarCheckBox.setSelected(true);
 			verticalScrollBarCheckBox.addActionListener(e -> verticalScrollBarChanged());
-			panel3.add(verticalScrollBarCheckBox, "cell 6 0");
+			panel3.add(verticalScrollBarCheckBox, "cell 4 1");
 		}
 		add(panel3, "cell 0 1");
 		// JFormDesigner - End of component initialization  //GEN-END:initComponents
@@ -440,6 +456,8 @@ public class FlatRoundedScrollPaneTest
 	private JSplitPane splitPane2;
 	private FlatTestPanel panel1;
 	private JLabel listLabel;
+	private JLabel paddingLabel;
+	private JLabel paddingField;
 	private JLabel treeLabel;
 	private JLabel tableLabel;
 	private JCheckBox autoResizeModeCheckBox;
@@ -466,8 +484,8 @@ public class FlatRoundedScrollPaneTest
 	private JSlider arcSlider;
 	private JCheckBox cornersCheckBox;
 	private JCheckBox columnHeaderCheckBox;
-	private JCheckBox rowHeaderCheckBox;
 	private JCheckBox horizontalScrollBarCheckBox;
+	private JCheckBox rowHeaderCheckBox;
 	private JCheckBox verticalScrollBarCheckBox;
 	// JFormDesigner - End of variables declaration  //GEN-END:variables
 

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

@@ -26,6 +26,18 @@ new FormModel {
 					}, new FormLayoutConstraints( class net.miginfocom.layout.CC ) {
 						"value": "cell 0 0,aligny top,growy 0"
 					} )
+					add( new FormComponent( "javax.swing.JLabel" ) {
+						name: "paddingLabel"
+						"text": "Padding:"
+					}, new FormLayoutConstraints( class net.miginfocom.layout.CC ) {
+						"value": "cell 0 0,alignx trailing,growx 0"
+					} )
+					add( new FormComponent( "javax.swing.JLabel" ) {
+						name: "paddingField"
+						"text": "0"
+					}, new FormLayoutConstraints( class net.miginfocom.layout.CC ) {
+						"value": "cell 0 0,alignx trailing,growx 0"
+					} )
 					add( new FormComponent( "javax.swing.JLabel" ) {
 						name: "treeLabel"
 						"text": "JTree:"
@@ -156,8 +168,8 @@ new FormModel {
 			} )
 			add( new FormContainer( "javax.swing.JPanel", new FormLayoutManager( class net.miginfocom.swing.MigLayout ) {
 				"$layoutConstraints": "hidemode 3"
-				"$columnConstraints": "[fill][grow,fill]para[fill][fill][fill][fill][fill]"
-				"$rowConstraints": "[]"
+				"$columnConstraints": "[fill][grow,fill]para[][][]"
+				"$rowConstraints": "[][]"
 			} ) {
 				name: "panel3"
 				add( new FormComponent( "javax.swing.JLabel" ) {
@@ -180,7 +192,7 @@ new FormModel {
 					"paintLabels": true
 					addEvent( new FormEvent( "javax.swing.event.ChangeListener", "stateChanged", "arcSliderChanged", false ) )
 				}, new FormLayoutConstraints( class net.miginfocom.layout.CC ) {
-					"value": "cell 1 0"
+					"value": "cell 1 0 1 2"
 				} )
 				add( new FormComponent( "javax.swing.JCheckBox" ) {
 					name: "cornersCheckBox"
@@ -196,20 +208,20 @@ new FormModel {
 				}, new FormLayoutConstraints( class net.miginfocom.layout.CC ) {
 					"value": "cell 3 0"
 				} )
-				add( new FormComponent( "javax.swing.JCheckBox" ) {
-					name: "rowHeaderCheckBox"
-					"text": "Row Header"
-					addEvent( new FormEvent( "java.awt.event.ActionListener", "actionPerformed", "rowHeaderChanged", false ) )
-				}, new FormLayoutConstraints( class net.miginfocom.layout.CC ) {
-					"value": "cell 4 0"
-				} )
 				add( new FormComponent( "javax.swing.JCheckBox" ) {
 					name: "horizontalScrollBarCheckBox"
 					"text": "Horizontal ScrollBar"
 					"selected": true
 					addEvent( new FormEvent( "java.awt.event.ActionListener", "actionPerformed", "horizontalScrollBarChanged", false ) )
 				}, new FormLayoutConstraints( class net.miginfocom.layout.CC ) {
-					"value": "cell 5 0"
+					"value": "cell 4 0"
+				} )
+				add( new FormComponent( "javax.swing.JCheckBox" ) {
+					name: "rowHeaderCheckBox"
+					"text": "Row Header"
+					addEvent( new FormEvent( "java.awt.event.ActionListener", "actionPerformed", "rowHeaderChanged", false ) )
+				}, new FormLayoutConstraints( class net.miginfocom.layout.CC ) {
+					"value": "cell 3 1"
 				} )
 				add( new FormComponent( "javax.swing.JCheckBox" ) {
 					name: "verticalScrollBarCheckBox"
@@ -217,7 +229,7 @@ new FormModel {
 					"selected": true
 					addEvent( new FormEvent( "java.awt.event.ActionListener", "actionPerformed", "verticalScrollBarChanged", false ) )
 				}, new FormLayoutConstraints( class net.miginfocom.layout.CC ) {
-					"value": "cell 6 0"
+					"value": "cell 4 1"
 				} )
 			}, new FormLayoutConstraints( class net.miginfocom.layout.CC ) {
 				"value": "cell 0 1"