@Override
			public Dimension getPreferredSize() {
				Dimension size = super.getPreferredSize();
				int titleBarHeight = clientPropertyInt( rootPane, TITLE_BAR_HEIGHT, -1 );
				if( titleBarHeight >= 0 )
					return new Dimension( size.width, UIScale.scale( titleBarHeight ) );

				if( buttonMaximizedHeight > 0 && isWindowMaximized() && !hasVisibleEmbeddedMenuBar( rootPane.getJMenuBar() ) ) {
					// make title pane height smaller when frame is maximized
					size = new Dimension( size.width, Math.min( size.height, UIScale.scale( buttonMaximizedHeight + buttonsMargins.top + buttonsMargins.bottom ) ) );
				}
				return size;
			}
This panel paint a border at the top of the window in fullWindowContent mode,
if FlatLaf window decorations are enabled.
Only used on Windows 10.
<p>
This panel is not a child of the title pane.
Instead it is added by FlatRootPaneUI to the layered pane at a layer over all other layers.
flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatTitlePane.java