Add shadow under compose view on scroll

Fixes #5098
Closes #5796
// FREEBIE

res/drawable/compose_divider_background.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android">
+    <gradient
+        android:angle="90"
+        android:dither="true"
+        android:endColor="@android:color/transparent"
+        android:startColor="@color/conversation_compose_divider" />
+</shape>

res/layout/conversation_fragment.xml

@@ -1,16 +1,24 @@
 <?xml version="1.0" encoding="utf-8"?>
 
-<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
-              android:layout_width="fill_parent"
-              android:layout_height="match_parent"
-              android:orientation="vertical">
+<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
+             android:layout_width="fill_parent"
+             android:layout_height="match_parent">
 
     <android.support.v7.widget.RecyclerView
             android:id="@android:id/list"
-            android:layout_width="fill_parent"
-            android:layout_height="0dp"
-            android:layout_weight="1.0"
+            android:layout_width="match_parent"
+            android:layout_height="match_parent"
             android:scrollbars="vertical"
             android:cacheColorHint="?conversation_background" />
 
-</LinearLayout>
+
+    <!--suppress AndroidLintUnusedAttribute-->
+    <View android:id="@+id/compose_divider"
+          android:layout_width="match_parent"
+          android:layout_height="2dp"
+          android:layout_gravity="bottom"
+          android:background="@drawable/compose_divider_background"
+          android:alpha="0"
+          android:visibility="invisible" />
+
+</FrameLayout>

res/values/colors.xml

@@ -25,6 +25,8 @@
 
     <color name="gray95_transparent50">#7F111111</color>
 
+    <color name="conversation_compose_divider">#32000000</color>
+
     <color name="conversation_list_item_background_read_light">@color/gray5</color>
     <color name="conversation_list_item_background_unread_light">#ffffffff</color>
     <color name="conversation_list_item_background_read_dark">#ff000000</color>

src/org/thoughtcrime/securesms/ConversationFragment.java

@@ -33,6 +33,7 @@ import android.support.v7.view.ActionMode;
 import android.support.v7.widget.LinearLayoutManager;
 import android.support.v7.widget.RecyclerView;
 import android.support.v7.widget.RecyclerView.ItemAnimator.ItemAnimatorFinishedListener;
+import android.support.v7.widget.RecyclerView.OnScrollListener;
 import android.text.ClipboardManager;
 import android.text.TextUtils;
 import android.util.Log;
@@ -78,6 +79,7 @@ public class ConversationFragment extends Fragment
 
   private final ActionModeCallback actionModeCallback     = new ActionModeCallback();
   private final ItemClickListener  selectionClickListener = new ConversationFragmentItemClickListener();
+  private final OnScrollListener   scrollListener         = new ConversationScrollListener();
 
   private ConversationFragmentListener listener;
 
@@ -88,6 +90,7 @@ public class ConversationFragment extends Fragment
   private Locale       locale;
   private RecyclerView list;
   private View         loadMoreView;
+  private View         composeDivider;
 
   @Override
   public void onCreate(Bundle icicle) {
@@ -99,10 +102,13 @@ public class ConversationFragment extends Fragment
   @Override
   public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle bundle) {
     final View view = inflater.inflate(R.layout.conversation_fragment, container, false);
-    list = ViewUtil.findById(view, android.R.id.list);
+    list           = ViewUtil.findById(view, android.R.id.list);
+    composeDivider = ViewUtil.findById(view, R.id.compose_divider);
+
     final LinearLayoutManager layoutManager = new LinearLayoutManager(getActivity(), LinearLayoutManager.VERTICAL, true);
     list.setHasFixedSize(false);
     list.setLayoutManager(layoutManager);
+    list.addOnScrollListener(scrollListener);
 
     loadMoreView = inflater.inflate(R.layout.load_more_header, container, false);
     loadMoreView.setOnClickListener(new OnClickListener() {
@@ -395,6 +401,37 @@ public class ConversationFragment extends Fragment
     void setThreadId(long threadId);
   }
 
+  private class ConversationScrollListener extends OnScrollListener {
+    private boolean wasAtBottom = true;
+
+    @Override
+    public void onScrolled(final RecyclerView rv, final int dx, final int dy) {
+      boolean currentlyAtBottom = isAtBottom();
+
+      if (wasAtBottom != currentlyAtBottom) {
+        composeDivider.setVisibility(currentlyAtBottom ? View.INVISIBLE : View.VISIBLE);
+
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB_MR1) {
+          composeDivider.animate().alpha(currentlyAtBottom ? 0 : 1);
+        } else if (Build.VERSION.SDK_INT > Build.VERSION_CODES.HONEYCOMB) {
+          composeDivider.setAlpha(currentlyAtBottom ? 0 : 1);
+        }
+
+        wasAtBottom = currentlyAtBottom;
+      }
+    }
+
+    private boolean isAtBottom() {
+      if (list.getChildCount() == 0) return true;
+
+      View    bottomView       = list.getChildAt(0);
+      int     firstVisibleItem = ((LinearLayoutManager) list.getLayoutManager()).findFirstVisibleItemPosition();
+      boolean isAtBottom       = (firstVisibleItem == 0);
+
+      return isAtBottom && bottomView.getBottom() <= list.getHeight();
+    }
+  }
+
   private class ConversationFragmentItemClickListener implements ItemClickListener {
 
     @Override