Fix image viewer & transitions (#1448)

app/src/main/java/com/keylesspalace/tusky/pager/ImagePagerAdapter.kt

@@ -16,6 +16,7 @@ class ImagePagerAdapter(
 ) : FragmentStatePagerAdapter(fragmentManager), SharedElementTransitionListener {
 
     private var primaryItem: ViewMediaFragment? = null
+    private var didTransition = false
 
     override fun setPrimaryItem(container: ViewGroup, position: Int, item: Any) {
         super.setPrimaryItem(container, position, item)
@@ -24,7 +25,14 @@ class ImagePagerAdapter(
 
     override fun getItem(position: Int): Fragment {
         return if (position >= 0 && position < attachments.size) {
-            ViewMediaFragment.newInstance(attachments[position], position == initialPosition)
+            // Fragment should not wait for or start transition if it already happened but we
+            // instantiate the same fragment again, e.g. open the first photo, scroll to the
+            // forth photo and then back to the first. The first fragment will trz to start the
+            // transition and wait until it's over and it will never take place.
+            ViewMediaFragment.newInstance(
+                    attachment = attachments[position],
+                    shouldStartPostponedTransition = !didTransition && position == initialPosition
+            )
         } else {
             throw IllegalStateException()
         }
@@ -39,6 +47,7 @@ class ImagePagerAdapter(
     }
 
     override fun onTransitionEnd() {
+        this.didTransition = true
         primaryItem?.onTransitionEnd()
     }
 }