title: Android Navigation: Keep previous fragment behind dialog. Works only with BottomSheetDialogFragment
body: <p>I have a video application - a video list and tapping on an item goes to video details. I want video details to be a full screen dialog fragment (to be able to do the motion layout animation from here <a href="https://medium.com/vrt-digital-studio/picture-in-picture-video-overlay-with-motionlayout-a9404663b9e7" rel="nofollow noreferrer">https://medium.com/vrt-digital-studio/picture-in-picture-video-overlay-with-motionlayout-a9404663b9e7</a>). But the previous dialog is not preserved behind (a blank screen is displayed). Weird enough BottomSheetDialogFragment works but DialogFragment does not. </p>
  
  <p>So the question is - Should a dialog fragment have the previous fragment displayed under it when using  jetpack navigation? Why bottom sheet works and dialog not?</p>
  
  <p>Navigation is done using android jetpack navigation:</p>
  
  <pre><code>&lt;action
              android:id="@+id/action_videosFragment_to_video_details_nav_graph"
              app:destination="@id/video_details_nav_graph"/&gt;
  </code></pre>
  
  <p>and details nav graph:</p>
  
  <pre><code>&lt;navigation 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:id="@+id/video_details_nav_graph"
      app:startDestination="@id/videoDetailsFragment"&gt;
  
      &lt;dialog
          android:id="@+id/videoDetailsFragment"
          android:name="com.myapp.mobile.ui.video.VideoDetailsFragment"
          android:label="VideoDetailsFragment"
          tools:layout="@layout/fragment_video_details"&gt;
      &lt;/dialog&gt;
  &lt;/navigation&gt;
  </code></pre>
  
  <p>Am I missing something? Thanks</p>