First Checkmark click inverts, the second click resets the list to all checked, third click sets all unchecked, forth click sets list to the original selection.
Watchlist Checkmark that inverts selection and resets.
@Phillip-Weeks You're going to have be more explicit about which version of the Watchlist module you are talking about and which checkmark. Note that the legacy watchlist page is deprecated. There will be no changes to this page.