If you like to mark shown post in the NewsBar, there are three ways you can use.
WAY 1 (semi-automatic):
Recall that when you keep your mouse cursor over the title of a post for a second, a small window pops up which enables you to read the content of that post. If you keep your mouse a little bit longer that post is automatically marked as read since you have enough time to read its content thru that popped up window.
WAY 2 (thru context menu):
WAY 3 (cumulative):
Recall that posts from one source are grouped together inside NewsBar and caption of that group includes two buttons shown in the screenshot below. By using those buttons you can mark all those grouped as read or delete them at once.
Keep in mind that posts loop inside NewsBar and based on your preferences it may be shown once or more. When posts are marked as read, those posts are removed from NewsBar and not shown again at the next loop, if there is.
There is also automated way of marking posts as read, as described in related help topic listed below.