[History Viewer] Comparison view header row has no background and blocks data

[mting314]

Repro steps

1. cd tools/data-viewer
2. bun install
3. bun run dev
4. Navigate to http://localhost:3001/?file=character-info.json
5. Click "history" in top navbar from character-info
6. Choose a commit that has many modifications
7. Scroll and observe header row text blocking