I'm not sure I'd say it injects code in to 'their site', as slacks site never sees the code.

It injects code into the dom model of the users browser.

> the extension injects code into their site.

But it doesn't inject code into their site, right? The HTTP request is made to the server all the same, nothing modified via the browser, it's all code additive/correction in the extension?

So, even more nefarious, and in violation of their terms?

That’s what we’re arguing. Slack should not be able to restrict me from modifying the experience on my hardware.

Those terms bind Slack users. This problem goes away if the maintainer doesn't use Slack and has never used Slack. It wouldn't be efficient, but it'd be possible.

If you are using IE to browsing slack, and IE couldn't correctly render it. It is illegal?

JavaScript runs on the client genius, not the server. You're not changing their product, your modifying how it is run on your personal machine.