pre-patch "Mark All Read" marks *all* unread notifications as read. This is a race condition in that the user is looking at some set of notiifcations and that set may update such that the newest notifications aren't shown. An example might be if sitting on the notifications page or having the menu open while a new notification comes in... Note re-opening the menu would show the latest notifications.
This patch makes it so "Mark All Read" links only marks the notifications currently loaded (and older.) Fixes T5764.
Additionally, if there is nothing to "mark read" the button / link "Mark All Read" will have a disabled style and yield a dialog saying "nothing to mark as read".