For added security, turn on Screen Lock by going to Settings > Privacy.
When Screen Lock is enabled, you’ll be required to open Signal in the same way you unlock your phone–by entering your PIN, passphrase, Touch ID, or Face ID. You can choose the length of time for the timeout from instant to 1 hour.
@signalapp Can you make a function to do the same on desktop (and why not allowing a "secure mode" that ask the mobile to allow the unlock on device.