[OnePlus 5T  ROMs, Kernels, Recoveries, & Other Dev] isme4:
Trying to get Developer Option "Kill app back button" working. (Kill the foreground app by long-pressing the back button)

When turned on, then there seems to be two "long-press" periods. And the latter one tries to kill the app. However, at that time the Calculator app switched already into background.

So the result is that I'm back on home screen, but Calculator still running.

Logcat of long-press back while Calculator is in foreground. I also turned off sound feedback to reduce logs.

03-08 12:22:09.847 828 828 E QCOM PowerHAL: Failed to acquire lock.
03-08 12:22:11.859 1287 1337 I ActivityManager: Force stopping com.android.calculator2 appid=10049 user=0: from pid 1287
03-08 12:22:11.860 1287 1337 I ActivityManager: Killing 29540:com.android.calculator2/u0a49 (adj 0): stop com.android.calculator2
03-08 12:22:10.260 828 828 I chatty : uid=1000(system) power@1.1-servi identical 8 lines
03-08 12:22:10.260 828 828 E QCOM PowerHAL: Failed to acquire lock.
03-08 12:22:11.867 1287 1338 W zygote64: kill(-29540, 9) failed: No such process
03-08 12:22:11.870 1287 1337 W ActivityManager: Force removing ActivityRecord{1df23be u0 com.android.calculator2/.Calculator t123}: app died, no saved state
03-08 12:22:11.878 1287 1337 I WindowManager: Failed to capture screenshot of Token{bda4c1f ActivityRecord{1df23be u0 com.android.calculator2/.Calculator t123 f}} appWin=Window{1e795a5 u0 com.android.calculator2/com.android.calculator2.Calculator} drawState=4
03-08 12:22:11.883 1287 1338 I chatty : uid=1000(system) ActivityManager identical 1 line
03-08 12:22:11.908 1287 1338 W zygote64: kill(-29540, 9) failed: No such process
03-08 12:22:11.916 1891 1891 D CarrierSvcBindHelper: No carrier app for: 0
03-08 12:22:11.916 1891 1891 D CarrierSvcBindHelper: No carrier app for: 1
03-08 12:22:11.922 28758 28758 D NlpPackageReceiver: Intent received: Intent { act=android.intent.action.PACKAGE_RESTARTED dat=package:com.android.calculator2 flg=0x10 cmp=com.google.android.gms/org.microg.nlp.PackageReceiver (has extras) }
03-08 12:22:11.922 2890 2890 D RegisteredNfcFServicesCache: Service unchanged, not updating
03-08 12:22:11.928 1287 1349 W ActivityManager: setHasOverlayUi called on unknown pid: 29540
03-08 12:22:11.952 1287 1338 W zygote64: kill(-29540, 9) failed: No such process
03-08 12:22:11.952 1287 1338 I zygote64: Successfully killed process cgroup uid 10049 pid 29540 in 86ms
03-08 12:22:11.958 1287 4268 I OpenGLRenderer: Initialized EGL, version 1.4
03-08 12:22:11.958 1287 4268 D OpenGLRenderer: Swap behavior 2
03-08 12:22:12.486 2979 3091 W OpenGLRenderer: Incorrectly called buildLayer on View: ShortcutAndWidgetContainer, destroying layer...
03-08 12:22:12.486 2979 3091 W OpenGLRenderer: Incorrectly called buildLayer on View: ShortcutAndWidgetContainer, destroying layer...
03-08 12:22:12.489 828 828 E QCOM PowerHAL: Failed to acquire lock.
03-08 12:22:12.505 1287 1337 D AutofillManagerService: onBackKeyPressed()
03-08 12:22:13.953 1287 1287 W WindowManager: removeWindowToken: Attempted to remove non-existing token: android.os.Binder@c9b6df6

When searching for "Back" in settings, I also get some "Back button" search results, but no corresponding settings option exists.

I.e. there may be some other hidden Back button settings.