Gtk.TreeViewColumn.prototype.set_fixed_width

function set_fixed_width(fixed_width:Number):void {
    // Gjs wrapper for gtk_tree_view_column_set_fixed_width()
}

If fixed_width is not -1, sets the fixed width of tree_column; otherwise unsets it. The effective value of fixed_width is clamped between the minumum and maximum width of the column; however, the value stored in the "fixed-width" property is not clamped. If the column sizing is #GTK_TREE_VIEW_COLUMN_GROW_ONLY or #GTK_TREE_VIEW_COLUMN_AUTOSIZE, setting a fixed width overrides the automatically calculated width. Note that fixed_width is only a hint to GTK+; the width actually allocated to the column may be greater or less than requested.

Along with "expand", the "fixed-width" property changes when the column is resized by the user.