private void removeOldPopupReferences() {
		for (Iterator<WeakReference<PopupWindow>> iterator = VISIBLE_POPUPS.iterator(); iterator
				.hasNext();) {
			WeakReference<PopupWindow> reference = iterator.next();
			PopupWindow window = reference.get();
			if (window == this) {
				reference.clear();
				iterator.remove();
				return;
			}
		}
	}
Area where user can mouse without hiding the window (in screen coordinates).  A.K.A., the
mouse neutral zone.
Ghidra/Framework/Docking/src/main/java/docking/widgets/PopupWindow.java