TypeError: can't access property "getClientRects", t.text.firstChild is null

## Error

```
can't access property "getClientRects", t.text.firstChild is null
```

## Stack trace

```
../node_modules/easymde/node_modules/codemirror/lib/codemirror.js:2352 in ensureLineHeights
../node_modules/easymde/node_modules/codemirror/lib/codemirror.js:2443 in measureCharPrepared
../node_modules/easymde/node_modules/codemirror/lib/codemirror.js:2679 in get
../node_modules/easymde/node_modules/codemirror/lib/codemirror.js:2691 in cursorCoords
../node_modules/easymde/node_modules/codemirror/lib/codemirror.js:3485 in scrollPosIntoView
../node_modules/easymde/node_modules/codemirror/lib/codemirror.js:3943 in endOperation_finish
../node_modules/easymde/node_modules/codemirror/lib/codemirror.js:3862 in endOperations
../node_modules/easymde/node_modules/codemirror/lib/codemirror.js:3845 in endCb
../node_modules/easymde/node_modules/codemirror/lib/codemirror.js:2073 in finishOperation
../node_modules/easymde/node_modules/codemirror/lib/codemirror.js:3842 in endOperation
```

## Tracking Details
- **Project**: exercism-js
- **Sentry**: [TypeError: can't access property "getClientRects", t.text.firstChild is null](https://thalamus-ai.sentry.io/issues/130687053/)
- **Culprit**: ensureLineHeights(easymde/node_modules/codemirror/lib/codemirror)
- **First seen:** Jun 27, 2026

Start with the Sentry report and the CodeMirror stack trace, especially ensureLineHeights and the EasyMDE integration that invokes it. Reproduce the editor state that leaves t.text.firstChild null and trace it back to the website code. Done means the triggering state no longer produces this error and is covered by a regression check.

Written by the indexing model from the issue text.