What is the dimming view when a tab-bar sidebar overlaps the selected controller?

When the automatic sidebar is active, a dimming layer covers the selected child. We cannot find a public property to disable it. Is that dimming UITabBarController’s own overlay, and is there a supported way to keep the selected controller fully interactive while the sidebar is visible?

What is the dimming view when a tab-bar sidebar overlaps the selected controller?
 
 
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