将reportpager和tablayout添加到recyclerView时，tablayout标题首先没有显示

Question

RecyclerView的适配器：

public class Adapter extends RecyclerView.Adapter{

private static List<String> mDataSet = new ArrayList<>();

static {
    for (int i = 0 ; i < 20;i ++){
        mDataSet.add(""+i);
    }
}

private AppCompatActivity mContext;

public Adapter(AppCompatActivity mContext) {
    this.mContext = mContext;
}

@Override
public int getItemViewType(int position) {

    if (position == getItemCount() -1) return -1;

    return super.getItemViewType(position);
}

@Override
public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {

    if (i == -1) return new PagerHolder(LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.view_pager_tab_layout,viewGroup,false),mContext);

    return new ViewHolder(LayoutInflater.from(viewGroup.getContext()).inflate(android.R.layout.simple_list_item_1,viewGroup,false));
}

@Override
public void onBindViewHolder(RecyclerView.ViewHolder viewHolder, int i) {

    Log.d("position",""+i);
    if (viewHolder instanceof ViewHolder) {
        Log.d("ViewHolder", "" + i);
        ((ViewHolder) viewHolder).textView.setText(mDataSet.get(i));
    }
    else if (viewHolder instanceof PagerHolder){
        Log.d("PagerHolder", "" + i);
    }

}


@Override
public int getItemCount() {
    return mDataSet.size()+1;
}

public static class ViewHolder extends RecyclerView.ViewHolder {
    public final TextView textView;
    public ViewHolder(View itemView) {
        super(itemView);
        textView = (TextView) itemView;
    }
}

public static class PagerHolder extends RecyclerView.ViewHolder {

    public final ViewPager viewPager;
    public final TabLayout tab;

    public PagerHolder(View itemView ,AppCompatActivity mContext) {
        super(itemView);

        itemView.setLayoutParams(new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, getHeight(mContext)));
        tab = (TabLayout) itemView.findViewById(R.id.tab);
        viewPager = (ViewPager) itemView.findViewById(R.id.view_pager);
        viewPager.setAdapter(new PagerAdapter(mContext.getSupportFragmentManager()));
        tab.setupWithViewPager(viewPager);
    }


    public int getHeight(AppCompatActivity mContext){
        final float scale      mContext.getResources().getDisplayMetrics().density;
        return (int)(440 * scale +0.5f);
    }
}
}

的xml：

<LinearLayout 
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">

<android.support.design.widget.TabLayout
    android:id="@+id/tab"
    android:textColor="#000000"
    android:layout_width="match_parent"
    android:layout_height="56dp"/>
<android.support.v4.view.ViewPager
    android:id="@+id/view_pager"
    android:layout_height="match_parent"
    android:layout_width="match_parent"/>
</LinearLayout>

当我第一次向下滚动到viewpager时，tablayout中的标题不会出现，

然后我向上和向下滚动回到viewpager，显示tablayout中的标题，

看起来像布局问题，

我的代码中有什么问题吗？我对自己可怜的英语感到抱歉......