I guess I’d ask you that! 
The documentation I’ve read says that it’s the top-left corner. @jane said she would put in a request to change it to the center of the sprite.
But my findings here are really strange. Regardless of whether the x,y position is top-left or center, I’m seeing inconsistencies that prevent me from using the Canvas.
That’s the way I’m heading, too… I just think a drag-and-drop game using the Canvas would be so much more satisfying.