There is also a "lazy" loading ad, which pops up between threads as you scroll down further on the page.
A lot of these changes are driven by our advertisers. They've become more sophisticated with their metrics, and require a certain amount of "viewability" for our ads. We're playing around with different placements, and open to changing things around. It isn't set in stone; we'll probably continue to make changes over the next few weeks.
Link: Forum layout
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