This applies to anywhere inside the thread, but I chose this example because it’s very clear to see the issue.
So, the example topic I will use here is this one. By opening the site and having a certain window size that exceeds some threshhold (I changed the window size before reloading), I get to see this:
Notice how I can only see one post but the “current post tracker” (or whatever that is called, it didn’t reveal its name upon hovering) shows I have already read 3 posts of 89. This continues to be offset by roughly 500-700px for the whole page and happens in every topic I have encountered so far.
The offset size made me think that the whole box after the initial post is not considered in the calculation, but collapsing it did not solve the issue. I am still told to be reading the 6th post despite only being able to see the first 4.
As you probably know, I’m on FireFox (currently 29 beta but this bug has existed for a longer time).