The breakpoint currently being edited in responsive controls (e.g. the 'Container size' toggle). Persisted here so it survives element re-selection and can be driven by the viewport selector. 'mobile' maps to 'Small', 'desktop' to 'Large'.
Optional
The breakpoint currently being edited in responsive controls (e.g. the 'Container size' toggle). Persisted here so it survives element re-selection and can be driven by the viewport selector. 'mobile' maps to 'Small', 'desktop' to 'Large'.