This happens when the mouse is relatively near the edge of a container. If the mouse is released, the dragged container will become a sibling of the target container, placed left/right/up/down according to the position of the indicator. This might or might not create a new v-split or h-split according to the previous layout of the target container. For example, if the target container is in an h-split and you drop the dragged container below it, the new layout will have to be a v-split.