Android Studio - XML Editor autocomplete not working with support libraries

Issue

I just started using the new android.support.design library. When using any of the widgets inside the XML editor I stop getting the XML autocomplete suggestions!

For example,

<android.support.design.widget.CoordinatorLayout
    android:id="@+id/header_root"
    android:layout_width="match_parent"
    android:layout_height="200dp">

    <ImageView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@color/primary_dark" />

    <android.support.design.widget.FloatingActionButton
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|right"
        android:src="@drawable/ic_action_add"
        android:layout_marginLeft="16dp"
        android:layout_marginRight="16dp"
        android:layout_marginTop="56dp"
        app:fabSize="normal"
        app:layout_anchor="@id/header_root"
        app:layout_anchorGravity="bottom|right|end" />

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

None of the tags will show the autocomplete popup, like when I start typing "android:i" no popup appears, the only suggestion I get is shown in the following picture.

I have tried cleaning my project, restarting the pc, restarting Android Studio.. nothing is working!

Solution

I have tried a lot of things (restart Android Studio, PC, Invalidate Caches, Power Saver mode,...).

Finally, deleting the .idea folder and all .iml files from the project, restarting Android Studio, and rebuilding Gradle did the trick. Autocomplete in XML support library is working again.

Checking out files from Version Control or copying all the source files in a new project would do the trick as well.

Answered By - Marko