Prevent cursor jumping in `textarea`

- On hitting Enter in the `textarea`, the `textarea` jumps to that line (brings it to the top of the box).
- This behaviour is not desired and should be stopped.
- The `textarea` might have to be converted into a [contenteditable](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/contenteditable) `div`, but avoid that if possible.

Start by reproducing the cursor-jumping behavior when pressing Enter in the textarea, then locate the textarea implementation in the preview-markdown application. Done means pressing Enter no longer moves the current line to the top of the box, while the existing Markdown preview behavior remains intact.

Written by the indexing model from the issue text.