This version is the classic method of creating a repeating pattern and saving it as a reusable swatch. Create a square artboard (here 3"), place design elements freely, allowing elements to cross the border. The trick then is to repeat the "missing" part of the element in the opposite side (left and right, top and bottom).
The precise and simple method to achieve this is by arithmetic based on the size of the container (here 3"). Copy the element, the paste in same position.
Move L-R, add 3"
Move R-L, subtract 3"
Move T-B, add 3"
Move B-T, subtract 3"
Create a background rectangle same size as artboard (3"), position from centre is w/2, h/2 so 1.5" and 1.5". Group the whole caboodle! Use a copy of this square as a clipping mask, bring to the front and right click "create clipping mask" to clip off the extraneous element parts. Then using Pathfinder options, clip it to tell Illustrator that the extraneous elements are not to be considered as part of the swatch size!