For the box to be positioned correctly, we need to know the width of the borders.
Shift-click on the map. A red box should be visible around the mouse cursor position, with 20 pixels to the top and right, 10 pixels to the left and 30 pixels to the bottom edge of the box.
Drag the box both to the top-left and the bottom-right. The cursor should always be at the top-left or bottom-right inner corner of the box.