Tab contents: Structure, etc. visible in another screen?

Sometimes, yes. You can show/hide many views within the main window layout (Structure, Snippets, Editor, Tracks, etc). Library and Palette offer optional standalone windows. 

DAWs, graphics editors and other media authoring apps have abandoned multi-window UIs though. It really gets complicated when you open multiple arrangements.

I often make visible the structure view to get an idea of what I'm doing on the Tracks page. Whether a few clips should be grouped into a container, etc. On a large enough monitor this works fine.

You can show/hide the structure view on other pages, too (palette, harmonizer, progression, snippets). It is convenient to navigate containers wherever you are.

Andre, we are glad that works for you but you cant assume that your workflow fits everyone else's workflow. As the software and the community grows, many people will have more different types of requests that we may or may not be used to.

Bitwig has 3 options of multiscreen UI among other softwares that I use. Synfire is the most expensive piece of software I own at the moment and the option to have the pages in separate windows should definitely be there, its up to the user if we actually use it or not. Personally, and other users have mentioned it here before, we would love to still to have the output in a separate window we can pin at the very least.

I agree it could be useful. But it adds a complexity that's extremely hard to handle and maintain. What about instrument, container, parameter selection? If they are shared (same state), that's relatively easy but somewhat limiting. It's essentially the same view as now, only distributed over more screen real estate. But it's doable.

Please share your ideas what those windows should be able to do. 

I should mention however, that with separate selection states in every window it gets hairy. That's essentially a second/third arrange window operating on a shared document model. With all the complexities of the full Synfire UI. Example: If you open a secondary window on a progression, that window must be closed before the progression is deleted. Same for containers, instruments, etc. Maintaining this referential integrity is already complex enough within a single window. I just want to illustrate that rewriting the entire UI framework is a major project that consumes a lot of resources.

By the way, Synfire v1 had secondary windows for phrases/clips (including progressions).

I would assume that it would just send mirror whatever is being done on the main screen.

We believe it's good because it prevents having to switch back and forth in the same ui. We can share the same data across different uis. 

 Example, output page. That would exactly mirror whatever is being done on the main screen. No functions needed on this screen it's just displaying whatever we do on the track and Arrangement page.