private void macClearBackgroundForTranslucentWindow( JRootPane c ) {
		if( !SystemInfo.isMacOS )
			return;

		Window window = getParentWindow( c );
		if( window != null ) {
			Color windowBackground = window.getBackground();
			if( windowBackground != null &&
				windowBackground.getAlpha() < 255 &&
				c.getBackground() instanceof UIResource )
			{
				// clear root pane background
				c.setBackground( null );
			}
		}
	}
When setting window background to translucent color (alpha < 255),
Swing paints that window translucent on Windows and Linux, but not on macOS.
The reason for this is that FlatLaf sets the background color of the root pane,
and Swing behaves a bit differently on macOS than on other platforms in that case.
Other L&Fs do not set root pane background, which is {@code null} by default.
<p>
To fix this problem, set the root pane background to {@code null}
if windows uses a translucent background.
flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatRootPaneUI.java