List and Tree: paint cell focus indicator (black rectangle) only if more than one item is selected

CHANGELOG.md

@@ -6,6 +6,8 @@ FlatLaf Change Log
 - Updated colors in "Flat Light" and "Flat IntelliJ" themes with colors from
   "IntelliJ Light Theme", which provides blue coloring that better match
   platform colors.
+- List and Tree: Paint cell focus indicator (black rectangle) only if more than
+  one item is selected.
 - Fixed link color (in HTML text) and separator color in IntelliJ platform
   themes.
 

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

@@ -0,0 +1,72 @@
+/*
+ * Copyright 2019 FormDev Software GmbH
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.formdev.flatlaf.ui;
+
+import java.awt.Component;
+import java.awt.Graphics;
+import javax.swing.JList;
+import javax.swing.SwingUtilities;
+import javax.swing.UIManager;
+
+/**
+ * Cell border for {@link javax.swing.DefaultListCellRenderer}.
+ *
+ * Uses separate cell margins from UI defaults to allow easy customizing.
+ *
+ * @author Karl Tauber
+ */
+public class FlatListCellBorder
+	extends FlatLineBorder
+{
+	protected FlatListCellBorder() {
+		super( UIManager.getInsets( "List.cellMargins" ), UIManager.getColor( "List.cellFocusColor" ) );
+	}
+
+	//---- class Default ------------------------------------------------------
+
+	public static class Default
+		extends FlatListCellBorder
+	{
+		@Override
+		public void paintBorder( Component c, Graphics g, int x, int y, int width, int height ) {
+			// do not paint border
+		}
+	}
+
+	//---- class Focused ------------------------------------------------------
+
+	public static class Focused
+		extends FlatListCellBorder
+	{
+	}
+
+	//---- class Selected -----------------------------------------------------
+
+	public static class Selected
+		extends FlatListCellBorder
+	{
+		@Override
+		public void paintBorder( Component c, Graphics g, int x, int y, int width, int height ) {
+			// paint border only if exactly one item is selected
+			JList<?> list = (JList<?>) SwingUtilities.getAncestorOfClass( JList.class, c );
+			if( list != null && list.getMinSelectionIndex() == list.getMaxSelectionIndex() )
+				return;
+
+			super.paintBorder( c, g, x, y, width, height );
+		}
+	}
+}

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

@@ -52,6 +52,11 @@ import javax.swing.plaf.basic.BasicListUI;
  * @uiDefault List.selectionInactiveBackground		Color
  * @uiDefault List.selectionInactiveForeground		Color
  *
+ * <!-- FlatListCellBorder -->
+ *
+ * @uiDefault List.cellMargins						Insets
+ * @uiDefault List.cellFocusColor					Color
+ *
  * @author Karl Tauber
  */
 public class FlatListUI

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

@@ -85,6 +85,7 @@ public class FlatTreeUI
 	protected Color selectionForeground;
 	protected Color selectionInactiveBackground;
 	protected Color selectionInactiveForeground;
+	protected Color selectionBorderColor;
 
 	public static ComponentUI createUI( JComponent c ) {
 		return new FlatTreeUI();
@@ -100,6 +101,7 @@ public class FlatTreeUI
 		selectionForeground = UIManager.getColor( "Tree.selectionForeground" );
 		selectionInactiveBackground = UIManager.getColor( "Tree.selectionInactiveBackground" );
 		selectionInactiveForeground = UIManager.getColor( "Tree.selectionInactiveForeground" );
+		selectionBorderColor = UIManager.getColor( "Tree.selectionBorderColor" );
 
 		// scale
 		int rowHeight = FlatUIUtils.getUIInt( "Tree.rowHeight", 16 );
@@ -119,6 +121,7 @@ public class FlatTreeUI
 		selectionForeground = null;
 		selectionInactiveBackground = null;
 		selectionInactiveForeground = null;
+		selectionBorderColor = null;
 	}
 
 	/**
@@ -157,11 +160,26 @@ public class FlatTreeUI
 				rendererComponent.setForeground( selectionInactiveForeground );
 		}
 
+		// remove selection border if exactly one item is selected
+		Color oldBorderSelectionColor = null;
+		if( isSelected && hasFocus &&
+			tree.getMinSelectionRow() == tree.getMaxSelectionRow() &&
+			rendererComponent instanceof DefaultTreeCellRenderer )
+		{
+			DefaultTreeCellRenderer renderer = (DefaultTreeCellRenderer) rendererComponent;
+			if( renderer.getBorderSelectionColor() == selectionBorderColor ) {
+				oldBorderSelectionColor = renderer.getBorderSelectionColor();
+				renderer.setBorderSelectionColor( null );
+			}
+		}
+
 		// paint renderer
 		rendererPane.paintComponent( g, rendererComponent, tree, bounds.x, bounds.y, bounds.width, bounds.height, true );
 
-		// restore background selection color
+		// restore background selection color and border selection color
 		if( oldBackgroundSelectionColor != null )
 			((DefaultTreeCellRenderer)rendererComponent).setBackgroundSelectionColor( oldBackgroundSelectionColor );
+		if( oldBorderSelectionColor != null )
+			((DefaultTreeCellRenderer)rendererComponent).setBorderSelectionColor( oldBorderSelectionColor );
 	}
 }

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

@@ -196,9 +196,11 @@ HelpButton.disabledQuestionMarkColor=@@CheckBox.icon.disabledCheckmarkColor
 #---- List ----
 
 List.border=1,0,1,0
-List.cellNoFocusBorder=1,6,1,6
+List.cellMargins=1,6,1,6
-List.focusCellHighlightBorder=1,6,1,6,@cellFocusColor
+List.cellFocusColor=@cellFocusColor
-List.focusSelectedCellHighlightBorder=1,6,1,6,@cellFocusColor
+List.cellNoFocusBorder=com.formdev.flatlaf.ui.FlatListCellBorder$Default
+List.focusCellHighlightBorder=com.formdev.flatlaf.ui.FlatListCellBorder$Focused
+List.focusSelectedCellHighlightBorder=com.formdev.flatlaf.ui.FlatListCellBorder$Selected
 List.selectionInactiveBackground=@selectionInactiveBackground
 List.selectionInactiveForeground=@selectionInactiveForeground