When toggle_add_mode() has been called, the grab in a GTK_SELECTION_EXTENDED mode list will remain with the anchor item at the start of the selection, regardless of where the focus falls, until another item is actually selected. Effectively it switches the list to something very like GTK_SELECTION_BROWSE mode.

The item that currently has grab has a dashed line around it to signify its status, as well as the usual GTK_STATE_SELECTED style.