fixed broken rendering after resizing window to minimum size and then increasing size again (issue #767)

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

@@ -499,7 +499,7 @@ public class FlatRootPaneUI
 		@Override
 		public void invalidateLayout( Container parent ) {
 			if( titlePane != null )
-				titlePane.menuBarChanged();
+				titlePane.menuBarInvalidate();
 		}
 
 		@Override

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

@@ -608,6 +608,10 @@ public class FlatTitlePane
 		doLayout();
 	}
 
+	void menuBarInvalidate() {
+		menuBarPlaceholder.invalidate();
+	}
+
 	@Override
 	public void paint( Graphics g ) {
 		super.paint( g );