private void updateDropShadowWindowBounds() {
			if( dropShadowWindow == null )
				return;

			// calculate size of drop shadow window
			Dimension size = popupWindow.getSize();
			Insets insets = dropShadowPanel2.getInsets();
			int w = size.width + insets.left + insets.right;
			int h = size.height + insets.top + insets.bottom;

			// update drop shadow popup window bounds
			int x = popupWindow.getX() - insets.left;
			int y = popupWindow.getY() - insets.top;
			dropShadowWindow.setBounds( x, y, w, h );
			dropShadowWindow.validate();
		}
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