Full stack, not even close. At least if Staff IC is the target. Mobile isn't a wrong move, but it splits your focus at exactly the point where you need to go deeper. Staff roles reward people who can own a problem from end to end, not people who can work across platforms.
What's actually mattered most in my career isn't the stack, it's understanding what happens between a user clicking something and a database changing. Where data lives, how it gets there, what goes wrong when it drifts. That thinking transfers to every codebase I've touched.
The React Server Components + Next.js App Router story has made this concrete in 2026. You can own a full feature slice without leaving the JS ecosystem. That's a much easier sell in a Staff IC conversation than "I do a bit of both."