Fragment from View Pager hiding behind Tab Bar

Adding :

app:layout_behavior="@string/appbar_scrolling_view_behavior"

in ViewPager resolved issue in my case.

if we will wrap app bar layout with linear layout than toolbar will not hide when you scroll so accepted answer might not help you if you want to hide toolbar when you scroll.

Smeet did it right way but not explained! here is full example with explanation.

add app namspace to CoordinatorLayout

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout
xmlns:app="http://schemas.android.com/apk/res-auto"
......
>

and just add below line in your ViewPager

app:layout_behavior="@string/appbar_scrolling_view_behavior"

complete xml will be as below

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout 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:layout_height="match_parent"
   >


    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:layout_scrollFlags="scroll|enterAlways"
            app:popupTheme="@style/AppTheme.PopupOverlay" />

        <android.support.design.widget.TabLayout
            android:id="@+id/tabLayout"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"

            />

    </android.support.design.widget.AppBarLayout>

    <android.support.v4.view.ViewPager
        android:id="@+id/viewPager"
        android:layout_width="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior"
        android:layout_height="match_parent" />


</android.support.design.widget.CoordinatorLayout>

Edit: As suggested below by @smeet and @hardik, adding the scroll behavior app:layout_behavior="@string/appbar_scrolling_view_behavior" should fix the problem while preserving the scroll behavior. Scroll behaviors only work if the view is a direct child of the coordinator layout.

---

Old Answer

Just Wrap your appbar layout and viewpager in a vertical LinearLayout

<android.support.design.widget.CoordinatorLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    tools:context="com.BoardiesITSolution.CritiMonApp.AppsActivity">

    <LinearLayout
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        //appbar layout

        //viewpager    


    </LinearLayout>

</android.support.design.widget.CoordinatorLayout>

From the docs, CoordinatorLayout is a super-powered FrameLayout. So you can expect the typical "lay views on top of other views" FrameLayout behavior.