A worktree is basically equivalent to a cp -R + git branch, which allows this new workflow to occur.

I loved this particular historical insight as to why git worktree was added in 2015 -

Before worktrees, kernel developers faced a major inconvenience when switching contexts (e.g., stopping feature work to fix an urgent bug on a release branch).

Running git stash and switching branches alters timestamps on thousands of files. This forces the build system (make) to perform a full re-compile, which can take up to an hour on large kernels.

Target market for copilot includes people with actual purchasing power and also many new users where this is an actual make or break feature. So this is worth the investment into design while stacked PRs is questionable. I actually question why they bothered with anything more than a blog post at all for stacked PRs (looking at the post it doesn't seem like too too much more than a blog post though).

There's support in VS Code and Jetbrains IDEs. You can access your agent sessions on the web.

(I work at GitHub, but not on Copilot)