SplitPane:

- update divider when client property `JSplitPane.expandableSide` changed
- Extras: added support for `JSplitPane.expandableSide` client property to `FlatSplitPane`

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

@@ -301,6 +301,10 @@ public class FlatSplitPaneUI
 					// necessary to show/hide one-touch buttons on expand/collapse
 					doLayout();
 					break;
+
+				case FlatClientProperties.SPLIT_PANE_EXPANDABLE_SIDE:
+					revalidate();
+					break;
 			}
 		}