Get error "Uncaught ReferenceError: Raphael is not defined" when trying 'side-by-side diff'

• Create pull request
• Select branch
• Choose "Diff" view
• Click "side-by-side diff" button

Visual diff shows checkerboard pattern (ie. transparent?), and JS console has the error:
Uncaught ReferenceError: Raphael is not defined e7e31b05ee4d.js:1379