Expand a Top-level Window with "Knob?"

[Stephen Wynne <stevemw mindspring com>]

I'm interested in giving the user the ability to expand a top-level
window with additional area to offer access to a choice of forms. This
avoids unwanted dialog windows and keeps the GUI small in its normal
usage.

I thought of using a GtkCombo (for a choice of several) or GtkArrow
(for one). What's the best way to do this? Is there an example?

Thanks!