Maybe I am late but thought it might be helpful for someone who faced the same issue as me.
final SearchView searchView = (SearchView) myActionMenuItem.getActionView();
searchView.setMaxWidth(Integer.MAX_VALUE);//set search menu as full width
in onCreateOptionsMenu
of your class may solve this problem but one thing we need to understand.
If you just want to show search menu (on clicking of search button) and not any other menu items in actionbar then you need to keep the search item as a first item in menu file. Here is an example
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto">
<!--keeping searach as first item-->
<item
android:id="@+id/action_search"
android:icon="@drawable/search_ab"
android:title="@string/search"
app:actionViewClass="android.support.v7.widget.SearchView"
app:showAsAction="ifRoom" />
<item
android:id="@+id/itemFilter"
android:icon="@drawable/filter"
android:title="@string/filter"
app:showAsAction="ifRoom" />
</menu>
And it will be displayed as
on clicking search it will be shown as full width
And if you keep search as second or last item in menu then it will be show like this.
and on clicking search