TypeError: can't access property "up", focus is undefined

## Error

```
can't access property "up", focus is undefined
```

## Stack trace

```
/bd97baa3-35bc-4de7-ae28-1b5b4cf6c23c:10 in Zipper@blob:
/bd97baa3-35bc-4de7-ae28-1b5b4cf6c23c:44 in fromTree@blob:
/61d59459-397c-477b-bc49-e3abcdb13ed3:236 in @blob:
/61d59459-397c-477b-bc49-e3abcdb13ed3:175 in queueTest@blob:
/61d59459-397c-477b-bc49-e3abcdb13ed3:114 in test/run.current<@blob:
```

## Tracking Details
- **Project**: exercism-js
- **Sentry**: [TypeError: can't access property "up", focus is undefined](https://thalamus-ai.sentry.io/issues/141162990/)
- **Culprit**: Zipper@blob:(bd97baa3-35bc-4de7-ae28)
- **First seen:** Aug 17, 2026

Start with the linked Sentry report and the stack-trace entries for Zipper@blob, fromTree@blob, and test/run.current. Reproduce the exercism-js failure and trace why focus is undefined; done means the error no longer occurs and the relevant test coverage verifies the fix.

Written by the indexing model from the issue text.