Document and workspace have uncoupled

While working on a document I've done something I don't understand and cannot undo! I'm on an iMac running InDesign 18.1

Normally when I open my document, a 56 page magazine layout, the document and workspace are "coupled" together. I can scale them, move them monitor to monitor etc and everything moves together. I can open and close them with a single click on the green or red dots. Suddenly that no longer happens. Now the document is a separate window that opens/closes, moves, and scales on it's own. The "workspace" - with tools, properties, pages, etc, are also separate and I can no longer move or scale them?

What's happened and how do I undo this? (FYI I tried User Interface Scaling and no matter what I do it is greyed out. I'm using a 3 monitor set up with the iMac as main monitor and two externals as extended monitors).