In a topic's gray "bar" on a Flow board, click in the area between Comment (n) and the timestamp. Although it shows the hyperlink hand pointer and the topic acts like a button when you mouse-down, nothing happens when you click. I would expect it to collapse/expand the topic's posts, just like clicking other background gray areas.
The problem might be that the li .flow-topic-comments is masked from clicks on the button background, but only its child .flow-reply-link acts as a hyperlink. I think replacing the former with the latter in the ignore array in "Set up folding" in modules/discussion/ui.js might fix the bug.