Odd Rectangular Mask Behavior: Holding Shift + Dragging Snaps Unpredictably.
Clicking two points on a rectangle and attempting to Shift + Drag to keep points locked onto the X or Y axis doesn't work as expected. The axis meant to be locked will shift slightly, preventing you from masking precisely.
Myer Pjani commented
It would be great if it was like AE.
R. Neil Haugen commented
Agreed ... I'm on a PC, and sometimes Shift/drag mostly works, and sometimes it doesn't.