The <fo:leader> object can be used used to:
If the leader's length is too long to fit in the area, the leader will begin a new line.
The children of the <fo:leader> object are ignored unless the leader-pattern property is "use-content". If the <fo:leader> has no children and the leader-pattern property is "use-content", the leader is filled with blank space.
If the leader should fill all space on a line, the max length of the leader should be at least as large as the width of the column.