I wish the tweet just said what it was. People on mobile have no way of trying it.
It opens Github Codespaces (https://github.com/features/codespaces), which is basically VS Code in the browser, and lets you edit the files of whatever repository you're in.
Also it's Chrome only, it looks like.
It opened for me in firefox on mac, but it broke the back button
Also WFM. And the UI - it is non-GitHub'ey, very much a bolted-on product IMO.
OLE!
Firefox worked fine for me. On linux too.
To be honest I liked the experience of actually doing it. If the tweet had just said it or shown a picture I never would have done it.
I had to be logged in for it to work.
Aha, that was my mistake!
It opens a free, vscode-like editor in the web where you can make quick changes to code