private Window getParentWindow( JRootPane c ) {
		// not using SwingUtilities.windowForComponent() or SwingUtilities.getWindowAncestor()
		// here because root panes may be nested and used anywhere (e.g. in JInternalFrame)
		// but we're only interested in the "root" root pane, which is a direct child of the window
		Container parent = c.getParent();
		return (parent instanceof Window) ? (Window) parent : 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