Fixed memory leak in smooth scrolling due to incorrect visibility check.

Author: Chrriis

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

@@ -82,7 +82,7 @@ public void ancestorAdded( AncestorEvent event ) {
 	@Override
 	public void ancestorMoved( AncestorEvent event ) {
 		JViewport viewport = (JViewport)event.getComponent();
-		if(viewport.isDisplayable()) {
+		if(viewport.isShowing()) {
 			SmoothScrollingHelper.registerViewport( viewport );
 		} else {
 			SmoothScrollingHelper.unregisterViewport( viewport );