How does "mark as read" work.

... which should be be indicated by a yellow 'paper' icon versus a white one. Any posts made after the time saved when you 'Marked' the forum/group as read should be yellow. The time stamps are stored on the DU servers, it shouldn't be affected by cookies (as far as I can tell).

There is a setting that toggles the yellow paper indicators to simply highlight the '10 newest replies' in a thread instead of using the time stamped 'Mark as read' feature. You can find the links to change the setting underneath any opening post on the right hand side.

https://www.democraticunderground.com/125617353#toc

Unless you're viewing the mobile version of the site - then I believe those links are hidden. Here is a URL to change the settings if you cannot see them:

https://www.democraticunderground.com/?com=thread&address=125617268&togglemark=1#toc

You'll have to copy and paste that to toggle the setting (I didn't want to put an active link where people could mess up their settings by inadvertently hitting it). I've seen google search results come back with that togglemark parameter included in the URL - which means people may unwittingly change their settings by just clicking the google search result link.

The Forums & Groups page has a 'Mark all as read' button near the top of the page:

https://www.democraticunderground.com/?com=forums#tab-home