Window decorations: support moving/resizing JInternalFrame that is …

…child of `JLayeredPane` and overlaps FlatLaf title bar (issue #658)

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

@@ -57,6 +57,7 @@
 import javax.swing.JButton;
 import javax.swing.JComponent;
 import javax.swing.JDialog;
+import javax.swing.JInternalFrame;
 import javax.swing.JLabel;
 import javax.swing.JMenuBar;
 import javax.swing.JPanel;
@@ -952,6 +953,13 @@ protected void updateNativeTitleBarHeightAndHitTestSpots() {
 			}
 		}
 
+		// allow internal frames in layered pane to be moved/resized when placed over title bar
+		for( Component c : rootPane.getLayeredPane().getComponents() ) {
+			r = (c instanceof JInternalFrame) ? getNativeHitTestSpot( (JInternalFrame) c ) : null;
+			if( r != null )
+				hitTestSpots.add( r );
+		}
+
 		Rectangle minimizeButtonBounds = boundsInWindow( iconifyButton );
 		Rectangle maximizeButtonBounds = boundsInWindow( maximizeButton.isVisible() ? maximizeButton : restoreButton );
 		Rectangle closeButtonBounds = boundsInWindow( closeButton );