ForceImmersiveActivity.kt

// prefer one
private val immersiveRunnable = Runnable {
    immersiveMode(this@BaseActivity)
}

companion object {
    private const val INITIAL_HIDE_DELAY = 300L
}

override fun onWindowFocusChanged(hasFocus: Boolean) {
    super.onWindowFocusChanged(hasFocus)
    if (hasFocus) {
        handler.postDelayed(immersiveRunnable, BaseActivity.INITIAL_HIDE_DELAY)
    } else {
        handler.removeCallbacksAndMessages(immersiveRunnable)
    }
}

// last resort
override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    
    //   ...

    window.decorView.setOnSystemUiVisibilityChangeListener {
        if (it and View.SYSTEM_UI_FLAG_FULLSCREEN == 0) {
            immersiveMode(this@BaseActivity)
        }
    }
}