Another difference is that vscode's extensions are very limited, as they run in their own process and can only access what has been explicitly made available. Today that excludes any means to build more advanced views. You could for instance, not build something like the git panel as an extension.

Vscode's architecture is not fit for that, as it runs additional node processes (e.g. the extension host process) that are chatting with the renderer process on a very fine-grained level.

Interesting. This is possibly too late to get a response, but can you give me any more detail? What constitutes chatty? What IPC mechanism? Is it just that the API for extensions is so wide open?

And still, do those things prevent something else from synchronizing UI state? Completely let VS Code run normally, even with a fake X server if I need... and then another extension that just syncs the current state of each UI panel to an emulated one in browser?

Anyway, sounds like you've thought this through or see obvious holes... I'd really like to understand more of why it's unworkable, or how it could be doable with other compromises. Thanks in advance.

Jupyterlab is really nice, too! A significant difference is that it is deeply integrated with jupyter notebooks. So it really targets data scientists working with jupyter. Theia is more general, by being a platform for all kinds of IDEs and tools. They could have built Jupyterlab on top of Theia but not the other way around.