When using instrument tracks I always use the key command “Edit VST Instrument” to open and close the instrument interface. When used on a sampler track, it does open the sampler interface as expected, but when used to close the interface the Lower Zone remains open and prompts the user to “Drop Audio Sample, or MIDI Part Here!”
I think it would make a lot more sense for it to close the lower window all together rather then having to use a separate key command. That or have a preference to keep sampler as separate windows, like the key editor, which then opens and closes as stated above.