transparentKeyboard.java

public static void makeKeyboardTransparent(InputMethodService service) {
    try {
        View decorView = service.getWindow().getWindow().getDecorView();
        final int viewId = fetchInternalRId("fullscreenArea");
        View fullscreenArea = decorView.findViewById(viewId);
        if (fullscreenArea != null) {
            modifyView(fullscreenArea);
            return;
        }
    } catch (Exception e) {
    }

    try {
        Class<?> superClass = service.getClass().getSuperclass();
        Field fullscreenAreaField = superClass.getDeclaredField("mFullscreenArea");
        fullscreenAreaField.setAccessible(true);
        View fullscreenArea = (View) fullscreenAreaField.get(service);
        if (fullscreenArea != null) {
            modifyView(fullscreenArea);
        }
    } catch (Exception e) {
    }
}

private static void modifyView(View fullscreenArea) {
    fullscreenArea.setBackgroundColor(Color.TRANSPARENT);
}

private static int fetchInternalRId(String name) throws Exception {
    Class<?> rIdClass = Class.forName("com.android.internal.R$id");
    return rIdClass.getDeclaredField(name).getInt(rIdClass);
}