An option to activate a biometric lock before you can access tabs in private mode. The biometric lock should kick in after a short delay when switching to another app, another tab or when locking the device.
Rationale: It's paranoia-friendly. When showing your device to someone (ticket verification, identity check or just something informational), you don't want them to see the private mode tabs whether it happens intentional or accidentally. It's more convenient and safer than having to use app pinning or closing the tabs. Also app pinning doesn't help if what you're showing is actually browser content.