Why does rotation cause Android fragment replacement to fail?

Question

I have put together a simple program that uses fragment replacement with a single activity and two fragments. One fragment has a button, that when pressed, replaces the frag

Answer 1

The answer is simple. UI state is retained across screen rotations meaning if you do nothing the screen would still show the second fragment once you rotate. Since you add the first fragment in your Activity onCreate method it will be added to what is already shown hence the two fragments overlap.

You could do something like this:

FragmentManager mgr = getFragmentManager();
if (mgr.findFragmentByTag("start") == null &&
    mgr.findFragmentByTag("replacementTest") == null) {

    FragmentTransaction t = mgr.beginTransaction();
    t.add(R.id.fragment_container, startingFragment, "start").commit();             

}