When reaching a side, items from the other side are displayed as if the desktop were a sphere. It allows a continuous and infinite scrolling in the desktop. Pinned items are not affected by this option and remain pinned, even items pinned on only one axis.
For the seamless mode to work, the desktop must be large enough for the wrapping effect to occur. An item cannot be displayed twice: it means that if the desktop is too small or the item is too large, it may disappear on one side, and appear on the opposite side. For instance, if the desktop is only one page, the left part of an item crossing the left side will not appear on the right side of the desktop.
Warning: keep in mind this is a visual effect only: it works by displaying items at virtual positions, it doesn't change the real location of items. Scripts in particular are not affected by this option and will still retrieve exact items and scroll positions. Additionally, the seamless mode is automatically disabled in edit mode, otherwise it wouldn't be possible to add items on new pages. When adding new items, make sure to navigate to the right place in edit mode first.