Incomplete diff of `toMatchSnapshot(properties)` mismatch error

Describe the bug

(not sure how to describe, so 🤖's best shot. repro is mine though.)

When toMatchSnapshot(properties) fails, the diff misleadingly shows fields like x as additions, as if they shouldn't be there. But x is a valid part of the object — the only actual problem is that y changed from 5 to 10. The diff should focus on the mismatched values, not imply that extra fields are wrong.

• repro.test.ts

import { expect, test } from 'vitest';

test('example', () => {
  // initially snapshot created with
  // { x: 'hello', y: 5 }
  expect({ x: 'world', y: 10 }).toMatchSnapshot({ y: 5 });
});

Test run:

 FAIL  test/repro.test.ts > example
Error: Snapshot properties mismatched

- Expected
+ Received

  {
-   "y": 5,
+   "x": "world",
+   "y": 10,
  }

 ❯ test/repro.test.ts:6:33
      4|   // initially snapshot created with
      5|   // { x: 'hello', y: 5 }
      6|   expect({ x: 'world', y: 10 }).toMatchSnapshot({ y: 5 });
       |                                 ^
      7| });
      8|
 ❯ new Promise ../../../blitz.4c73681d.js:31:30606

if (!propertiesPass) {
  return {
    pass: false,
    message: () => 'Snapshot properties mismatched',
    actual: received,      // full object
    expected: properties,  // subset — missing fields show as additions
  }
}

Reproduction

https://stackblitz.com/edit/vitest-dev-vitest-xnjrwzha?file=test%2Frepro.test.ts

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 22.22.0 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    @vitest/ui: latest => 4.1.2 
    vite: latest => 8.0.3 
    vitest: latest => 4.1.2

Used Package Manager

npm

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

Reproduce the failure with test/repro.test.ts, then inspect the properties mismatch path in packages/snapshot/src/client.ts and how packages/utils/src/error.ts renders actual and expected. Determine the intended subset-property diff behavior and add coverage so a valid x field is not shown as an addition while the changed y value remains highlighted.