Think Niri [0], but worse, embedded in a web browser tab, and without keyboard navigation.
Here's a somewhat ancient Stack Overflow screenshot I found: https://i.sstatic.net/yCseI.png
(I think that's from near the transition because it has full "windowing" controls of minimize/maximize/close buttons. I recall a period with only close buttons.)
All that blue space you could keep filling with more "blades" as you clicked on things until the entire page started scrolling horizontally to switch between "blades". Almost everything you could click opened in a new blade rather than in place in the existing blade. (Like having "Open in New Window" as your browser default.)
It was trying to merge the needs of a configurable Dashboard and a "multi-window experience". You could save collections of blades (a bit like Niri workspaces) as named Dashboards. Overall it was somewhere between overkill and underthought.
(Also someone reminded me that many "blades" still somewhat exist in the modern Portal, because, of course, Microsoft backwards compatibility. Some of the pages are just "maximized Blades" and you can accidentally unmaximize them and start horizontally scrolling into new blades.)