Fix Z-ordering of preview media.

app/src/main/java/org/thoughtcrime/securesms/animation/DepthPageTransformer.java

@@ -9,7 +9,7 @@ import androidx.viewpager2.widget.ViewPager2;
 /**
  * Based on https://developer.android.com/training/animation/screen-slide#depth-page
  */
-public final class DepthPageTransformer implements ViewPager.PageTransformer, ViewPager2.PageTransformer {
+public final class DepthPageTransformer implements ViewPager.PageTransformer {
   private static final float MIN_SCALE = 0.75f;
 
   public void transformPage(@NonNull View view, float position) {

app/src/main/java/org/thoughtcrime/securesms/animation/DepthPageTransformer2.kt

@@ -0,0 +1,41 @@
+package org.thoughtcrime.securesms.animation
+
+import android.view.View
+import androidx.annotation.RequiresApi
+import androidx.viewpager2.widget.ViewPager2
+
+private const val MIN_SCALE = 0.75f
+
+/**
+ * Lifted from https://developer.android.com/develop/ui/views/animations/screen-slide-2#depth-page
+ */
+@RequiresApi(21)
+class DepthPageTransformer2 : ViewPager2.PageTransformer {
+
+  override fun transformPage(view: View, position: Float) {
+    view.apply {
+      val pageWidth = width
+      when {
+        position < -1 -> alpha = 0f
+        position <= 0 -> {
+          alpha = 1f
+          translationX = 0f
+          translationZ = 0f
+          scaleX = 1f
+          scaleY = 1f
+        }
+        position <= 1 -> {
+          alpha = 1 - position
+
+          translationX = pageWidth * -position
+          translationZ = -1f
+
+          val scaleFactor = (MIN_SCALE + (1 - MIN_SCALE) * (1 - Math.abs(position)))
+          scaleX = scaleFactor
+          scaleY = scaleFactor
+        }
+        else -> alpha = 0f
+      }
+    }
+  }
+}

app/src/main/java/org/thoughtcrime/securesms/mediapreview/MediaPreviewV2Fragment.kt

@@ -18,6 +18,7 @@ import androidx.core.view.WindowInsetsCompat
 import androidx.fragment.app.Fragment
 import androidx.fragment.app.viewModels
 import androidx.recyclerview.widget.LinearLayoutManager
+import androidx.viewpager2.widget.ViewPager2.OFFSCREEN_PAGE_LIMIT_DEFAULT
 import androidx.viewpager2.widget.ViewPager2.OnPageChangeCallback
 import com.google.android.material.appbar.MaterialToolbar
 import com.google.android.material.dialog.MaterialAlertDialogBuilder
@@ -26,7 +27,7 @@ import io.reactivex.rxjava3.android.schedulers.AndroidSchedulers
 import org.signal.core.util.concurrent.SignalExecutors
 import org.signal.core.util.logging.Log
 import org.thoughtcrime.securesms.R
-import org.thoughtcrime.securesms.animation.DepthPageTransformer
+import org.thoughtcrime.securesms.animation.DepthPageTransformer2
 import org.thoughtcrime.securesms.attachments.DatabaseAttachment
 import org.thoughtcrime.securesms.components.ViewBinderDelegate
 import org.thoughtcrime.securesms.conversation.mutiselect.forward.MultiselectForwardFragment
@@ -69,16 +70,7 @@ class MediaPreviewV2Fragment : Fragment(R.layout.fragment_media_preview_v2), Med
 
     initializeViewModel(args)
     initializeToolbar(binding.toolbar, args)
-    binding.mediaPager.offscreenPageLimit = 1
-    binding.mediaPager.setPageTransformer(DepthPageTransformer())
-    val adapter = MediaPreviewV2Adapter(this)
-    binding.mediaPager.adapter = adapter
-    binding.mediaPager.registerOnPageChangeCallback(object : OnPageChangeCallback() {
-      override fun onPageSelected(position: Int) {
-        super.onPageSelected(position)
-        viewModel.setCurrentPage(position)
-      }
-    })
+    initializeViewPager()
     initializeFullScreenUi()
     initializeAlbumRail()
     anchorMarginsToBottomInsets(binding.mediaPreviewDetailsContainer)
@@ -102,6 +94,21 @@ class MediaPreviewV2Fragment : Fragment(R.layout.fragment_media_preview_v2), Med
     }
   }
 
+  private fun initializeViewPager() {
+    binding.mediaPager.offscreenPageLimit = OFFSCREEN_PAGE_LIMIT_DEFAULT
+    if (Build.VERSION.SDK_INT >= 21) {
+      binding.mediaPager.setPageTransformer(DepthPageTransformer2())
+    }
+    val adapter = MediaPreviewV2Adapter(this)
+    binding.mediaPager.adapter = adapter
+    binding.mediaPager.registerOnPageChangeCallback(object : OnPageChangeCallback() {
+      override fun onPageSelected(position: Int) {
+        super.onPageSelected(position)
+        viewModel.setCurrentPage(position)
+      }
+    })
+  }
+
   private fun initializeAlbumRail() {
     binding.mediaPreviewAlbumRail.itemAnimator = null // Or can crash when set to INVISIBLE while animating by FullscreenHelper https://issuetracker.google.com/issues/148720682
     binding.mediaPreviewAlbumRail.layoutManager = LinearLayoutManager(requireContext(), LinearLayoutManager.HORIZONTAL, false)