Chatting to Ondrej, this behaviour is apparently correct. The mystery purple box is rendered correctly, and is not a channel picker.
The install ubuntu button is grey because my theme (stock CM) chooses that for active buttons. When I pressed the button, the install worked.
So I think the bug (if any) is that the button is poorly named. 'Choose channel' would be clearer than 'Install Ubuntu' at this point in the workflow, IMHO.
Chatting to Ondrej, this behaviour is apparently correct. The mystery purple box is rendered correctly, and is not a channel picker.
The install ubuntu button is grey because my theme (stock CM) chooses that for active buttons. When I pressed the button, the install worked.
So I think the bug (if any) is that the button is poorly named. 'Choose channel' would be clearer than 'Install Ubuntu' at this point in the workflow, IMHO.