1. In rectangle mode, adjust the ( topright, right, bottom right, bottom, bottom left) selection and check mouse and edge position.
2.1 select a rect, make sure the right/bottom edge is beside moniter edge
2.2 drag the topleft, top, top right up quickly or drag top left, left, bottom left to left quickly, check the bottom/ right edge
3.1 select full screen and check size
3.2 save image and check image size