ViewPager in a NestedScrollView

I meet this problem ,i solve it by setFillViewport (true)

<android.support.v4.widget.NestedScrollView
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:id="@+id/nest_scrollview"
android:layout_height="match_parent"
app:layout_behavior="@string/appbar_scrolling_view_behavior"
tools:context="mio.kon.yyb.nestedscrollviewbug.ScrollingActivity"
tools:showIn="@layout/activity_scrolling">


    <android.support.v4.view.ViewPager
        android:id="@+id/viewpager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

in activity

  NestedScrollView scrollView = (NestedScrollView) findViewById (R.id.nest_scrollview);
    scrollView.setFillViewport (true);