This specifically applies to the folders in the bookmark bar because, apparently, you can have other bookmark folders showing in the bar that aren’t technically part of the bookmark bar (will explain later)?
Anyway, in the bookmark bar, clicking a folder drops down the contents as expected. However, collapsing the folder exhibits strange behavior. To collapse it, you have to click the mouse on the folder icon very quickly. If you hold for just a split second too long, the folder drops back down.
Now to explain why I draw precise distinctions between the Bookmark bar and Other bookmarks. Collapsing the other bookmarks folder behaves exactly the way I would expect, collapsing regardless whether the mouse click is quick or held for a split second.
I think the expand/collapse behavior should be consistent across all expandable objects, behaving like the hamburger menu icon which registers any click as an instruction to toggle the state of the collapsible regardless of the hold duration of a click.