In the new version I no longer can expand and see the code, if it is not my notebook. Is this on purpose? Do I have to fork it first? That would be unfortunate.

On any notebook, you can click the pencil button in the bottom-left corner to enter tinker mode which allows you to see (and edit) the code without leaving the page.

Perfect! Would be nice if I could toggle it. For now I can only enable but not disable again. :)