A code block which shows how to show Android keyboard after user press back button when current Activity has focus.

KeyboardShowing.kt

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val button: Button = findViewById(R.id.button)
        button.setOnClickListener {
            startActivity(Intent(this, DetailsActivity::class.java))
        }
    }

    override fun onStart() {
        super.onStart()

        // show the keyboard if has focus
        currentFocus?.let {
            showSoftKeyboard(it)
        }
    }

    private fun showSoftKeyboard(view: View) {
        if (view.requestFocus()) {
            val imm = view.context.getSystemService(Context.INPUT_METHOD_SERVICE) as InputMethodManager?

            // here is one more tricky issue
            // imm.showSoftInputMethod doesn't work well
            // and imm.toggleSoftInput(InputMethodManager.SHOW_IMPLICIT, 0) doesn't work well for all cases too
            imm?.toggleSoftInput(InputMethodManager.SHOW_FORCED, 0)
        }
    }
}