Although, I'm not really sure how to use this thing

Nice this is very cool for emacs folks!

Last time I tried it (forever and a half ago) I had conflicts with modal editing and flickering issues. Works smoothly for you?

No flickering issues. On windows (via WSL) I have issues with certain key combos and I can't easily enter text in input fields. On osx, everything is much better but not perfect. The perf of webkit is noticeably worse than chrome, but after some rendering tweaks, it feels smooth enough.

It should be available via (portal.api/open {:launcher :emacs}) with the latest release of https://github.com/djblue/portal/releases/tag/0.31.0

Any rendering tweaks required by the user?

No, nothing for the user. I had to tweak how the inspector was passing context to prevent re-renders.