Android: Back button does not work

When attempting to exit the app with the back button (capacitive right button on OnePlus), the app fails to exit. I am currently using the minimize key (center key, fingerprint reader) to close.

App 4.0.0. Android 9.

Start by reproducing the failure on Android 9 with the Keybase 4.0.0 app, ideally using the reported OnePlus capacitive back button. Trace the Android back-button handling and compare it with the minimize key behavior. Done means the back button exits the app as expected and the behavior is covered by an appropriate regression check.

Written by the indexing model from the issue text.