Nice project. I would appreciate putting some common content in the demo, such as videos, dropdowns, large amount of text and large size text to show how they can be handled.
Also is there a version without window chrome, just panes?
After tinking with some similar toy projects, I feel like iframes with a well-defined APi to use with postMessage() (and maybe a small library to provide frame-internal matching toolbars/controls) are definitely the way to go here, since they remove the need to tightly couple your "OS" with your "apps".
Instead of content, put there webview/iframe – https://github.com/mlajtos/mosaic