Message Thread Sorting/Display Order?

Okay - So we're now on the new messaging board beastie...

I've managed to locate all the items I've been tracking and update their shortcuts - but so far I can't seem to figure out how to set the message threads listing/sorting/display order to list by "most recent reply date".  Selecting "Sort / Date" seems to toggle between ascending/descending order - but only based on the original message/thread creation date - not the last (most recent) reply within the thread.

Obviously I'm missing setting it somewhere...

So can someone kindly point me to the appropriate setting?

Thanks,

John