We had a discussion in a Mozilla newsgroup if the disclosure triangle should also be used in Windows based installations.

In my opinion this triangle is unclear to what it does.
It might be discoverable when used in a tree, like More does, but it isn't when used at the bottom of a panel, to let it expand that panel.

If I am correct the triangle originated from More.
Who designed it, and what are it's design ideas?

Actually I think the "triangle" which we refer to as a "wedge" originated in Mac System 7. I never really liked it, but we adapted it and made it work better, imho, in Frontier's outliner.

It has two states, gray and black. If black, there's something to expand. If gray, there's no point 2clicking on the line, because there either is no subordinate text, or the subordinate text is already expanded. That way your eye can scan down the left edge and quickly see where there's hidden content.

Apple had an animated triangle that was pointing down in our gray state, and to the right in our black state.