Put multiple Word document windows into the same parent window

This addin adds 'document tabs' into Microsoft Word, I wonder how it's done? As we know, according to Word's 'Object Model' API, each Word document is associated with a standalone Word window, how can that addin put multiple Word document windows int开发者_开发技巧o the same parent window and use tabs to switch among them?

Attached this screenshot to illustrate how that addin works:

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No idea how that addin does it specifically, but If I had to guess, i'd say they were putting word into MDI mode, creating the tabs on they're addin ui and then managing the visibility of individual doc windows when you click on the tabs.

You can put Word into MDI mode by unchecking the "Show all windows in the task bar" option.