Fix freeze of first story first post.

app/src/main/java/org/thoughtcrime/securesms/stories/viewer/page/StoryViewerPageFragment.kt

@@ -257,6 +257,7 @@ class StoryViewerPageFragment :
           progressBar.setPosition(viewModel.getRestartIndex())
           videoControlsDelegate.restart()
         }
+        viewModel.setIsFirstPage(parentState.page == 0)
         viewModel.setIsSelectedPage(true)
         when (parentState.crossfadeSource) {
           is StoryViewerState.CrossfadeSource.TextModel -> storyCrossfader.setSourceView(parentState.crossfadeSource.storyTextPostModel)

app/src/main/java/org/thoughtcrime/securesms/stories/viewer/page/StoryViewerPageState.kt

@@ -3,7 +3,8 @@ package org.thoughtcrime.securesms.stories.viewer.page
 data class StoryViewerPageState(
   val posts: List<StoryPost> = emptyList(),
   val selectedPostIndex: Int = 0,
-  val replyState: ReplyState = ReplyState.NONE
+  val replyState: ReplyState = ReplyState.NONE,
+  val isFirstPage: Boolean = false
 ) {
   /**
    * Indicates which Reply method is available when the user swipes on the dialog

app/src/main/java/org/thoughtcrime/securesms/stories/viewer/page/StoryViewerPageViewModel.kt

@@ -116,7 +116,9 @@ class StoryViewerPageViewModel(
     }
 
     val postIndex = store.state.selectedPostIndex
-    setSelectedPostIndex(max(-1, postIndex - 1))
+    val minIndex = if (store.state.isFirstPage) 0 else -1
+
+    setSelectedPostIndex(max(minIndex, postIndex - 1))
   }
 
   fun getRestartIndex(): Int {
@@ -155,6 +157,10 @@ class StoryViewerPageViewModel(
     storyViewerPlaybackStore.update { it.copy(isDisplayingSlate = isDisplayingSlate) }
   }
 
+  fun setIsFirstPage(isFirstPage: Boolean) {
+    store.update { it.copy(isFirstPage = isFirstPage) }
+  }
+
   fun setIsSelectedPage(isSelectedPage: Boolean) {
     storyViewerPlaybackStore.update { it.copy(isSelectedPage = isSelectedPage) }
   }

app/src/test/java/org/thoughtcrime/securesms/stories/viewer/page/StoryViewerPageViewModelTest.kt

@@ -38,6 +38,46 @@ class StoryViewerPageViewModelTest {
     RxJavaPlugins.reset()
   }
 
+  @Test
+  fun `Given first page and first post, when I goToPreviousPost, then I expect storyIndex to be 0`() {
+    // GIVEN
+    val storyPosts = createStoryPosts(3) { true }
+    whenever(repository.getStoryPostsFor(any())).thenReturn(Observable.just(storyPosts))
+    val testSubject = createTestSubject()
+    testSubject.setIsFirstPage(true)
+    testScheduler.triggerActions()
+
+    // WHEN
+    testSubject.goToPreviousPost()
+    testScheduler.triggerActions()
+
+    // THEN
+    val testSubscriber = testSubject.state.test()
+
+    testSubscriber.assertValueAt(0) { it.selectedPostIndex == 0 }
+  }
+
+  @Test
+  fun `Given first page and second post, when I goToPreviousPost, then I expect storyIndex to be 0`() {
+    // GIVEN
+    val storyPosts = createStoryPosts(3) { true }
+    whenever(repository.getStoryPostsFor(any())).thenReturn(Observable.just(storyPosts))
+    val testSubject = createTestSubject()
+    testSubject.setIsFirstPage(true)
+    testScheduler.triggerActions()
+    testSubject.goToNextPost()
+    testScheduler.triggerActions()
+
+    // WHEN
+    testSubject.goToPreviousPost()
+    testScheduler.triggerActions()
+
+    // THEN
+    val testSubscriber = testSubject.state.test()
+
+    testSubscriber.assertValueAt(0) { it.selectedPostIndex == 0 }
+  }
+
   @Test
   fun `Given no initial story and 3 records all viewed, when I initialize, then I expect storyIndex to be 0`() {
     // GIVEN