in Premiere CC, how do you create a TALL rectangular mask?
Finally figured it out (with some help). You can drag a whole side at a time! Here's how:


Click one of the 2 corners to (subtly) select it.
Shift-click-and-drag the other corner.


That will select the whole side between the two corners. It will also limit your "dragging path" to paths that are at 45 degrees or 90 degrees relative to the corner's original position. That enables you to keep your rectangle rectangular.