In user research we quite often saw people accidentally scroll the
branding preview `<iframe>` when trying to scroll the page.
This is suboptimal because they:
- were confused why the page wasn’t scrolling
- lost visibility of the branding they were trying to preview because
it scrolled outside the bounds of the `<iframe>`
The previewPane JS used selectors that targeted
the old form of radios HTML.
The JS tests also contained selectors like this
and fragments of HTML, used for fixtures, modelled
on the old radios HTML.
