Comment by retzkek

> While many users would at first glance conclude that the "arrow" on the topmost button is an indicator that this is the currently selected button, it is used to indicate that this is a "special" (a.k.a. inconsistent and undesirable) type of button. Clicking on the arrow causes the button to "open" (downward) to reveal a variety of folders. Why the designers chose to use an arrow pointing to the right to indicate downward movement is one of the many mysteries of this program.

And now the expand/collapse UI pattern with right/down arrows or triangles is standard (e.g. https://developer.mozilla.org/en-US/docs/Web/HTML/Element/de...). Don't get me wrong, the example button is terrible UI, but an interesting example of how norms change.

On more than one occasion I have misinterpreted the upvote icon on HN to be "collapse thread".