Avoid taking window focus by Android Spinner's Dropdown to keep setSystemUiVisibility flags (such as Immersive Mode).

AvoidSpinnerDropdownFocus.java

import android.widget.ListPopupWindow;
import android.widget.PopupWindow;
import android.widget.Spinner;

public static void avoidSpinnerDropdownFocus(Spinner spinner) {
    try {
        Field listPopupField = Spinner.class.getDeclaredField("mPopup");
        listPopupField.setAccessible(true);
        Object listPopup = listPopupField.get(spinner);
        if (listPopup instanceof ListPopupWindow) {
            Field popupField = ListPopupWindow.class.getDeclaredField("mPopup");
            popupField.setAccessible(true);
            Object popup = popupField.get((ListPopupWindow) listPopup);
            if (popup instanceof PopupWindow) {
                ((PopupWindow) popup).setFocusable(false);
            }
        }
    } catch (NoSuchFieldException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    }
}

AvoidSpinnerDropdownFocus.kt

import android.widget.ListPopupWindow
import android.widget.PopupWindow
import android.widget.Spinner

fun Spinner.avoidDropdownFocus() {
    try {
        val listPopup = Spinner::class.java
                .getDeclaredField("mPopup")
                .apply { isAccessible = true }
                .get(this)
        if (listPopup is ListPopupWindow) {
            val popup = ListPopupWindow::class.java
                    .getDeclaredField("mPopup")
                    .apply { isAccessible = true }
                    .get(listPopup)
            if (popup is PopupWindow) {
                popup.isFocusable = false
            }
        }
    } catch (e: Exception) {
        e.printStackTrace()
    }
}