Hierarchical tableselect

The element itself: a table of check boxes over rows that are data rather than form elements. The column and the "select all" cell in the header are added by the element, and the hierarchy is the one thing it knows nothing about -- the indentation is markup the row carries in its first cell, exactly as the overview form of a vocabulary builds it. Two rows are selected to begin with, and one is disabled, which is the state most easily forgotten: a row whose check box cannot be reached still has to line up with the column above it.

The overview form of a menu, which is the same tree in the other element: a #type table whose rows are form elements, so each one carries the state of a link, the weight it is ordered by, the parent it hangs off and its operations, and the whole table can be dragged. The check box column here is built by hand rather than asked of #tableselect, because the two cannot be combined: the check box of a row would be submitted under the name of the row itself, which is the name the weight and the parent of that row are already submitted under. What a theme sees is nearly the same column either way -- a "select all" cell in the header, and a check box in the first cell of every row -- next to the drag handle, the indentation and the hidden fields that make a tree out of a flat table. Nearly, because the cell holding a check box is given a class here and left bare by the element above, which is a difference in core rather than in the example, and one worth knowing before a column like this is styled on one table and checked on the other. The indentation sits in that first cell as well, rather than on the title beside it, because the first cell is the only place tabledrag looks: the handle is put after the last marker it finds there, and prepended to the cell when it finds none, so with the markers left on the title the handle stayed at the left edge of every row however deep the row sat. Moving them costs the title column its indentation and steps the check boxes in instead, which is the trade a table makes by putting a select column in front of the one that carries the tree.

The same element with #multiple turned off, which is what a picker is: the check boxes become radio buttons and the "select all" cell in the header goes away, because selecting every row of a list you may only pick one of means nothing. The tree being picked from is the menu above, so it is a link that is being given a new parent, and the rows the link may not be moved under -- itself, and everything below it -- are disabled. A theme that only ever sees the multiple variation of this element misses both differences.