There's a "mark as read" option in the three dots at the upper-right, don't know if that only applies to the current post or all posts. Actually, this version's a little funky now that I'm using it a bit more, but at least it works.
Edit: Just looked at the options accessed via the dots, and they seem to be topic-wide.
So I decided to reset (and not keep files) my SP3 running Windows 10 10240 with the latest updates. After successfully restoring, it's now in a reboot loop of the blue screen saying inaccessible boot device so can't get it to start up. Someone else reported this on this "final" build so warning to all, don't reset your SP3 and choose not to keep files.
All the reviews I've read have echoed me and I hope that doesn't end up being a big problem for Windows 10. Every review says it's great but wait a while before installing because it's still very buggy. Feels like a rushed rollout to me. Don't think it should be out in the hands of regular customers yet.