iframe navigation affecting window history

[microbit-matt-hillsdon]

After navigating to micro:bit CreateAI and allowing the page to load, pressing the browser back button once appears to do nothing.

We've tracked this as far as navigations that occur in iframes also affect the parent's window.history. On load, there is a navigation inside the MakeCode iframe, presumably when we initialise a project.

The first back press is trying to undo this navigation, but fortunately, this seems to have no impact on MakeCode.

It is not clear how to fix this and it will need some thought. Manually messing with window.history can go badly. Ideally, we'd want MakeCode's initial navigation to be replaceState rather than pushState.