Using develop.element.io for Element Web causes frames to nest the harness

While polishing up Patience yesterday, I encountered one of the stranger web behaviours I've seen in a while...

The clients are loaded into `iframe`s inside the Patience test harness page. When Element Web tries to adjust the URL for going to a room (`location.assign("#/room/!1234:matrix.org")`), somehow the browser decides it should use the parent document (the test harness page) as the base URL, and nests a second copy of the test harness inside itself... 🤯

This was not happening a few months ago, so... that's going to be a strange one to sort out. 😵‍💫

It seems to reproduce consistently in Chrome, but only sometimes in Firefox.

It only seems to happen when using https://develop.element.io as the Element Web deployment. If I instead run Element Web locally, it works fine. So far, I've tried comparing HTTP headers and Element Web configs, but nothing seems to cause my local copy to behave like develop in this way.

Start by reproducing the issue in Chrome with https://develop.element.io loaded in an iframe inside the Patience test harness, then compare it with a local Element Web deployment and check Firefox behavior. Done means navigating to a room with location.assign("#/room/!1234:matrix.org") no longer nests a second copy of the harness.

Written by the indexing model from the issue text.