为什么 FragmentTwo 从backstack在下列情况下消失：

Why does FragmentTwo disappear from the backstack in the following situation:

在我的应用程序有一个片段名为 FragmentOne 在活动。 FragmentOne 持有按钮。单击时，它会启动 FragmentTwo ，它被添加到片段 backstack。 FragmentTwo 有一个按钮，点击它时，增加了两个选项卡动作条链接到两个片段， FragmentThree 和 FragmentFour 。 FragmentThree 是可见的。 如果我现在单击后退按钮，我期望看到 FragmentTwo 。相反，我看到 FragmentOne 。哪里 FragmentTwo 去了？ My app has a Fragment called FragmentOne in an Activity. FragmentOne holds a Button. When clicked, it launches FragmentTwo, which is added to the Fragment backstack. FragmentTwo has a Button, which when clicked adds two tabs to the ActionBar linked to two Fragments, FragmentThree and FragmentFour. FragmentThree is visible. If I now click the back button, I expected to see FragmentTwo. Instead, I see FragmentOne. Where did FragmentTwo go?

在我重写的onkeydown（），并开始实施自己的backstack的片段我想问问是否有一些明显的我失踪？请注意，没有配置更改测试这个时候发生了。

Before I override onKeyDown() and start implementing my own backstack for Fragments I wanted to ask if there is something obvious I am missing? Note there is no configuration change happening when testing this.

详细信息：

FragmentOne的按钮单击处理程序包含：

FragmentOne's button click handler contains:

    FragmentTransaction ft = getFragmentManager().beginTransaction();
    FragmentTwo fragment = new FragmentTwo();
    ft.addToBackStack(null);
    ft.replace(android.R.id.content, fragment).commit();

FragmentTwo按一下按钮将在活动处理：

FragmentTwo button click is handled in the Activity:

    getActionBar().setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
    FragmentFour fragmentThree = new FragmentThree();
    FragmentFive fragmentFive = new FragmentFive();

    ActionBar.Tab tab = getActionBar().newTab().setText("Frag 3").setTabListener(new CustomTabListener<FragmentThree>(fragmentThree));
    getActionBar().addTab(tab);
    tab = getActionBar().newTab().setText("Frag 4").setTabListener(new CustomTabListener<FragmentFour>(fragmentFour));
    getActionBar().addTab(tab);

，其中选项卡监听器：

where the tab listener is:

public static class CustomTabListener<T extends Fragment> implements TabListener {
    Fragment fragment;

    public CustomTabListener(Fragment fragment) {
        this.fragment = fragment;
    }

    public void onTabSelected(Tab tab, FragmentTransaction ft) {
        ft.replace(android.R.id.content, fragment);
    }

    public void onTabUnselected(Tab tab, FragmentTransaction ft) {
        ft.remove(fragment);
    }

    public void onTabReselected(Tab tab, FragmentTransaction ft) {

    }
}

我如果添加更多的片段在 FragmentThree 来backstack显示，它始终是只有片段立即 FragmentThree 已经消失。

I if add more Fragments to backstack before FragmentThree is shown, it is always and only the Fragment immediately before FragmentThree that has disappeared.

当我离开由$ P $选项卡式用户视图pssing返回键，返回 FragmentOne ，标签仍显示。我明白我需要重新设置动作条到 NAVIGATION_MODE_STANDARD ，但目前还不清楚为什么 FragmentOne 正呈现>而不是 FragmentTwo