I think this was where the height calculation was added originally, FIX: Scroll jumping in some dimensions of browser · discourse/discourse@a77f5a7 · GitHub which confirms that it’s meant to help control scroll jumpiness.
For fixing your specific issue, it seems that the height of .topic-post is what’s being measured… in your theme is that div taller than the content within for some reason? I wonder why it seems to be miscalculated…