The "Mark thread as read" button has an undo button. Like it, we should have
an undo button for "Mark all as read" as well. It is pretty easy to accidentally
click the wrong buttons since "Mark thread as read" for the topmost thread and
screen contents around for quite a while without signalling when done, so one
may assume that this phase was over before it really is, and thus involuntarily
click the place which will later be occupied by the wrong button.