"Preview as Visitor" spinner does not go away with SPA Editor

Reproduce:

• Use headless p13n SPA demo project.
• Open Preview as Visito
• Note that a "spinner" loading indicator is present. (Should not be.)
• Add a trait - like "Age" and set it to "Adult".
• Page updates correctly and shows the correct p13n content, But the spinner is still displayed. (Should not be)

"Preview as Visitor" is a key piece of our p13n support and must work properly even in headless / SPA context. Now it looks like something is broken, and authors will be uncertain, expecting that something else should load.