bun test --update-snapshots rewrites the .snap file in execution order, reordering unchanged entries #42969

What version of Bun is running?

1.4.2

What platform is your computer?

Darwin 24.6.0 arm64 arm

What steps can reproduce the bug?

order.test.ts:

import { describe, expect, test } from "bun:test";
describe("suite", () => {
  test("zeta", () => { expect({ z: 1 }).toMatchSnapshot(); });
  test("alpha", () => { expect({ a: 1 }).toMatchSnapshot(); });
});

1. bun test order.test.ts writes __snapshots__/order.test.ts.snap with suite zeta 1 before suite alpha 1 (execution order).
2. Hand-sort the file (alpha first), run bun test order.test.ts: passes, order kept. Add a third test: the new entry is appended. Good so far.
3. bun test --update-snapshots order.test.ts: the file is rewritten as zeta, mid, alpha, i.e. every unchanged entry moves.

What is the expected behavior?

Jest sorts snapshot keys (natural compare) on every write, so a regenerated file only shows the entries whose values changed. Either sorted keys, or keeping the existing file order and appending new keys (what the plain run already does), would keep --update-snapshots diffs reviewable.

What do you see instead?

--update-snapshots emits entries in the order the tests ran, so a one-value update to a file whose tests are not declared alphabetically reorders the whole file. In a repo with 10 snapshot files and 87 entries the migration diff was about 2900 lines of reorder for one changed value.

Additional information

Related: node:test's t.assert.snapshot is not implemented in the node:test shim, so suites moving from Node's runner also lose the >-joined keys and the <test>.snapshot file next to the test.