void hideImpl() {
			if( contents instanceof JComponent )
				((JComponent)contents).putClientProperty( KEY_POPUP_USES_NATIVE_BORDER, null );

			if( delegate != null ) {
				delegate.hide();
				delegate = null;
				owner = null;
				contents = null;
			}

			if( popupWindow != null ) {
				// restore background so that it can not affect other LaFs (when switching)
				// because popup windows are cached and reused
				popupWindow.setBackground( oldPopupWindowBackground );
				popupWindow = null;
			}
		}
Fixes popup background flashing effect when using dark theme on light platform theme,
where the light popup background is shown for a fraction of a second before
the dark popup content is shown.
This is fixed by setting popup background to content background.
<p>
Defers hiding of heavy weight popup window for an event cycle,
which allows reusing popup window to avoid flicker when "moving" popup.
flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatPopupFactory.java