When a contact is not select, I can resize the Zoiper window which has the list of contacts.
If I select a contact, then the window expands to show the contact details / chat panel on the right.
At this point, resizing the window only resizes the chat panel, I can't resize the main part or shrink it, and the splitter bar doesn't work. Once a contact is selected there is no easy way of unselecting it, apart from deleting the conversation history, so I'm just stuck with a big window.