The control uses its first child as an examplar. It uses that child's width as the preferred width for all children. This width is used to calculate the number of columns which can fit into the control. The first child's right and bottom margin are also used, respectively, to determine the preferred spacing between columns and the preferred vertical spacing between children within a column.
The layout heuristic is simple and fast: the control examines each of its children in turn, and adds that child to whichever column is currently shortest.
This control was discussed in a user interface blog post.