What causes the additional panic!() newline? I’m not seeing it here with Rust 1.84.1 on macOS, and the tests are failing as a result. Is it a 1.85 change?

I don't exactly know what you are referring to but am assuming it's a failing journey test that wants to see specific line numbers in a traceback?
If so, then yes, there is a test that is prone to failure either if the underlying code changes, or if something in the panic handler changes.

Talking about cf98a15; I'm not getting that extra newline and so the journey tests are broken for me. Doesn't seem to be a line number thing, I'm just not seeing the leading newline.

Right - I remember that with some new Rust release, the leading newline appeared which was taken as expected value then.
CI still seems to agree which I suppose makes it a requirement. Ideally, this would be no dependency on something this 'flaky', so it's worth rewriting the test so the output of the program is filtered down to what matters. The test-suite has support for filters already, and doing so might be relevant for portability as different versions of Rustc produce difference outputs.