在toolbar中加入這個屬性即可：
app:layout_scrollFlags=“scroll|enterAlways”
或者enterAlwaysCollapsed
app:layout_scrollFlags=“scroll|enterAlwaysCollapsed”

<?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/ThemeOverlay.AppCompat.Dark.ActionBar">

        <android.support.v7.widget.Toolbar
            android:id="@+id/tb"
            android:layout_width="match_parent"
            android:layout_height="?actionBarSize"
            app:layout_scrollFlags="scroll|enterAlways"
            ></android.support.v7.widget.Toolbar>
    </android.support.design.widget.AppBarLayout>

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