Last modified: 2010-05-15 15:40:39 UTC
The field wl_notificationtimestamp in the watchlist table is set back to NULL
when visiting the page but set back to 0 when the 'Mark all pages visited'
button on the Special:Watchlist page is clicked.
The UserMailer.php checks for entries that have this timestamp set to NULL
rather than NULL or 0, so clicking the button doesn't re-enable emails.
*** This bug has been marked as a duplicate of 6023 ***