This does work, but it causes a server postback, and that's not what the original question was asking for. It also doesn't allow for the collapsing of an item after its been expanded.
So I've been scouring the forums for an hour looking for a way to have the expand/collapse client-side functionality apply to the text of the group header item - either instead of, or in addition to, the expand/collapse icon, and haven't been able to find the answer. This thread is the closest I've seen, but it's not quite what we're after. Is this possible? Many thanks,