I once had to debug a sporadic problem in a prod system I wasn't familar with, which seemed to be caused by two requests made simultaneously by different users. I could have assembled curl commands but the easiest thing to reproduce and verify the issue was to run a setTimeout(clickSubmit, 5s) in both browser session's consoles. Weirdly niche use case, but something like this extension would've been useful.
I don't know of a reliable way of calling two separate curl commands at once though. Maybe abusing the shell job system, or learning the 'at' command. It has been a while since I've needed to do something like this.
Oh I don't know, I could see it being useful it it's possible to hook up with rtk-query or react-query, to prevent duplicate requests from multiple instances of an app that might be running.
Though I guess this might just trigger all the requests on all that tabs anyway. I suppose you'd have to add architecture to start one instance of that app as the 'controller' and then a way to pass off controller responsibilities to other tabs... which sounds like basically "step 2: draw the rest of the owl"
Since I just noticed this is an extension rather than a library, I think it's probably more useful for automating interactions with a number of tabs of the same page (but puppeteer or something similar would probably be a better fit for this also)
For user-driven automations, something like Tampermonkey would be way better (and safer, since they can be domain-limited).
I can think of no reason why a regular user would want to allow tabs to execute each other's code... that just seems like a setup for self-XSS attacks by people who don't know any better :(
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edit: Sorry, re: the second part of your comment, I misunderstood what you meant. Like if you wanted to purposely script multiple identical tabs at the same time.