markdown editor

select any text on the left and a → preview button appears — click it and the matching block on the right scrolls into view and both sides highlight.

the other direction

click any block in the preview. the editor scrolls to it and lays a gray streak over the matching lines.

<Editor value={md} onChange={setMd} renderMarkdown={render} />

• the streak tracks the textarea's own scroll
• editor and preview share one source of line numbers