TreeViewColumn.setFixedWidth

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 minimum and maximum width of the column; however, the value stored in the “fixed-width” property is not clamped. If the column sizing is [gtk.types.TreeViewColumnSizing.GrowOnly] or [gtk.types.TreeViewColumnSizing.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.