SplitPane: allow limiting one-touch expanding to a single side (issue #355)

CHANGELOG.md

@@ -5,6 +5,8 @@ FlatLaf Change Log
 
 #### New features and improvements
 
+- SplitPane: Allow limiting one-touch expanding to a single side (set client
+  property `JSplitPane.expandableSide` to `"left"` or `"right"`). (issue #355)
 - TabbedPane: Selected tab underline color now changes depending on whether the
   focus is within the tab content. (issue #398)
 - IntelliJ Themes:

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

@@ -391,6 +391,40 @@ public interface FlatClientProperties
 	 */
 	String SCROLL_PANE_SMOOTH_SCROLLING = "JScrollPane.smoothScrolling";
 
+	//---- JSplitPane ---------------------------------------------------------
+
+	/**
+	 * Specifies what side of the spilt pane is allowed to expand
+	 * via one-touch expanding arrow buttons.
+	 * Requires that one-touch expanding is enabled with
+	 * {@link javax.swing.JSplitPane#setOneTouchExpandable(boolean)}.
+	 * <p>
+	 * <strong>Component</strong> {@link javax.swing.JSplitPane}<br>
+	 * <strong>Value type</strong> {@link java.lang.String}<br>
+	 * <strong>Allowed Values</strong>
+	 *     {@link #SPLIT_PANE_EXPANDABLE_SIDE_LEFT} or
+	 *     {@link #SPLIT_PANE_EXPANDABLE_SIDE_RIGHT}
+	 *
+	 * @since 2.2
+	 */
+	String SPLIT_PANE_EXPANDABLE_SIDE = "JSplitPane.expandableSide";
+
+	/**
+	 * Allow expanding only left/top side of the split pane.
+	 *
+	 * @see #SPLIT_PANE_EXPANDABLE_SIDE
+	 * @since 2.2
+	 */
+	String SPLIT_PANE_EXPANDABLE_SIDE_LEFT = "left";
+
+	/**
+	 * Allow expanding only right/bottom side of the split pane.
+	 *
+	 * @see #SPLIT_PANE_EXPANDABLE_SIDE
+	 * @since 2.2
+	 */
+	String SPLIT_PANE_EXPANDABLE_SIDE_RIGHT = "right";
+
 	//---- JTabbedPane --------------------------------------------------------
 
 	/**

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

@@ -34,6 +34,7 @@ import javax.swing.UIManager;
 import javax.swing.plaf.ComponentUI;
 import javax.swing.plaf.basic.BasicSplitPaneDivider;
 import javax.swing.plaf.basic.BasicSplitPaneUI;
+import com.formdev.flatlaf.FlatClientProperties;
 import com.formdev.flatlaf.ui.FlatStylingSupport.Styleable;
 import com.formdev.flatlaf.ui.FlatStylingSupport.StyleableUI;
 import com.formdev.flatlaf.ui.FlatStylingSupport.UnknownStyleException;
@@ -352,7 +353,7 @@ public class FlatSplitPaneUI
 				if( leftButton == null || rightButton == null || !splitPane.isOneTouchExpandable() )
 					return;
 
-				// increase side of buttons, which makes them easier to hit by the user
+				// increase size of buttons, which makes them easier to hit by the user
 				// and avoids cut arrows at small divider sizes
 				int extraSize = UIScale.scale( 4 );
 				if( orientation == JSplitPane.VERTICAL_SPLIT ) {
@@ -367,10 +368,19 @@ public class FlatSplitPaneUI
 
 				// hide buttons if not applicable
 				boolean leftCollapsed = isLeftCollapsed();
-				if( leftCollapsed )
+				boolean rightCollapsed = isRightCollapsed();
+				if( leftCollapsed || rightCollapsed ) {
+					leftButton.setVisible( !leftCollapsed );
+					rightButton.setVisible( !rightCollapsed );
+				} else {
+					Object expandableSide = splitPane.getClientProperty( FlatClientProperties.SPLIT_PANE_EXPANDABLE_SIDE );
+					leftButton.setVisible( expandableSide == null || !FlatClientProperties.SPLIT_PANE_EXPANDABLE_SIDE_LEFT.equals( expandableSide ) );
+					rightButton.setVisible( expandableSide == null || !FlatClientProperties.SPLIT_PANE_EXPANDABLE_SIDE_RIGHT.equals( expandableSide ) );
+				}
+
+				// move right button if left button is hidden
+				if( !leftButton.isVisible() )
 					rightButton.setLocation( leftButton.getLocation() );
-				leftButton.setVisible( !leftCollapsed );
-				rightButton.setVisible( !isRightCollapsed() );
 			}
 		}
 	}