From the 2015.2 release to the 2015.3 release, the built-in validation on the Start date was removed from the left-hand treelist.  So now I can move the Start date to be after the Finish date, and the Gantt control responds by treating this as a "move".   However, there is still built-in validation on the Finish date so that I cannot move the Finish date to before the start.   I would expect the same behavior for changes in both directions.   If I change the Finish date to before the Start date, then this should also be treated as a "move", where the duration of the task is retained, and the Start is also moved back N days.

This is a trivial change to make.  I think it's removing 1 line of code.  And this would really help with usability for users who want to achieve a "move" action, but who prefer to edit their data using the left-hand treelist instead of the right-hand timeline.