private boolean isFullscreen() {
			GraphicsConfiguration gc = window.getGraphicsConfiguration();
			if( gc == null )
				return false;
			return gc.getDevice().getFullScreenWindow() == window;
		}
Rounds in the same way as AWT.
See AwtWin32GraphicsDevice::ClipRound()
flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatWindowsNativeWindowBorder.java