add drop shadow to controller view when going back

adds some depth, as shadows do

app/src/main/java/eu/kanade/tachiyomi/ui/main/MainActivity.kt

@@ -2,6 +2,7 @@ package eu.kanade.tachiyomi.ui.main
 
 import android.Manifest
 import android.animation.AnimatorSet
+import android.animation.ObjectAnimator
 import android.animation.ValueAnimator
 import android.annotation.SuppressLint
 import android.app.Dialog
@@ -526,6 +527,21 @@ open class MainActivity : BaseActivity<MainActivityBinding>() {
                     syncActivityViewWithController(to, from, isPush)
                     binding.appBar.isVisible = true
                     binding.appBar.alpha = 1f
+                    if (binding.backShadow.isVisible && !isPush) {
+                        val alpha = binding.backShadow.alpha
+                        val bA = ObjectAnimator.ofFloat(binding.backShadow, View.ALPHA, alpha, 0f)
+                        from?.view?.let { view ->
+                            bA.addUpdateListener {
+                                binding.backShadow.x = view.x - binding.backShadow.width
+                            }
+                        }
+                        bA.doOnEnd {
+                            binding.backShadow.alpha = 0.25f
+                            binding.backShadow.isVisible = false
+                        }
+                        bA.duration = 150
+                        bA.start()
+                    }
                     if (!isPush || router.backstackSize == 1) {
                         nav.translationY = 0f
                     }

app/src/main/java/eu/kanade/tachiyomi/util/view/ControllerExtensions.kt

@@ -887,6 +887,21 @@ interface BackHandlerControllerInterface {
             view?.let { view ->
                 view.alpha = 1f - progress
                 view.x = progress * view.width * 0.15f
+
+                activityBinding?.let { activityBinding ->
+                    val container = activityBinding.controllerContainer
+                    val backShadow = activityBinding.backShadow
+                    if (container.indexOfChild(backShadow) != container.indexOfChild(view) - 1) {
+                        container.removeView(backShadow)
+                        container.addView(backShadow, container.indexOfChild(view))
+                    }
+                    if (!backShadow.isVisible) {
+                        backShadow.isVisible = true
+                    }
+                    backShadow.x = view.x - backShadow.width
+                    backShadow.alpha = 0.33f * view.alpha
+                }
+
                 router.backstack[router.backstackSize - 2].controller.view?.let { view2 ->
                     view2.alpha = progress
                     view2.x = view.x - view.width * 0.2f
@@ -906,5 +921,7 @@ interface BackHandlerControllerInterface {
                 view.x = 0f
             }
         }
+        activityBinding?.backShadow?.isVisible = false
+        activityBinding?.backShadow?.alpha = 0.25f
     }
 }

app/src/main/res/layout-w720dp/main_activity.xml

@@ -17,7 +17,14 @@
         app:layout_constraintStart_toEndOf="@id/side_nav"
         app:layout_constraintTop_toTopOf="parent"
         app:layout_constraintVertical_bias="1.0">
-
+        <ImageView
+            android:id="@+id/back_shadow"
+            android:layout_width="6dp"
+            android:alpha="0.25"
+            android:visibility="gone"
+            android:layout_height="match_parent"
+            android:src="@drawable/shape_gradient_start_shadow"
+            android:importantForAccessibility="no" />
     </com.bluelinelabs.conductor.ChangeHandlerFrameLayout>
 
     <eu.kanade.tachiyomi.ui.base.ExpandedAppBarLayout

app/src/main/res/layout/extension_detail_controller.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <androidx.coordinatorlayout.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:id="@+id/coordinator"
+    android:background="?background"
     android:layout_width="match_parent"
     android:layout_height="match_parent" >
     <androidx.recyclerview.widget.RecyclerView

app/src/main/res/layout/main_activity.xml

@@ -18,6 +18,14 @@
         app:layout_constraintTop_toTopOf="parent"
         app:layout_constraintVertical_bias="0.0">
 
+        <ImageView
+            android:id="@+id/back_shadow"
+            android:layout_width="6dp"
+            android:alpha="0.25"
+            android:visibility="gone"
+            android:layout_height="match_parent"
+            android:src="@drawable/shape_gradient_start_shadow"
+            android:importantForAccessibility="no" />
     </com.bluelinelabs.conductor.ChangeHandlerFrameLayout>
 
     <eu.kanade.tachiyomi.ui.base.ExpandedAppBarLayout

app/src/main/res/layout/source_global_search_controller.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:tools="http://schemas.android.com/tools"
+    android:background="?background"
     android:layout_width="match_parent"
     android:layout_height="wrap_content">