Fix scroll-up glitch when new message arrives

app/src/main/kotlin/com/simplemobiletools/smsmessenger/activities/ThreadActivity.kt

@@ -36,6 +36,8 @@ import androidx.annotation.StringRes
 import androidx.constraintlayout.widget.ConstraintLayout
 import androidx.core.content.res.ResourcesCompat
 import androidx.core.view.*
+import androidx.recyclerview.widget.LinearLayoutManager
+import androidx.recyclerview.widget.RecyclerView
 import com.google.gson.Gson
 import com.google.gson.reflect.TypeToken
 import com.simplemobiletools.commons.dialogs.ConfirmationDialog
@@ -394,8 +396,10 @@ class ThreadActivity : SimpleActivity() {
         runOnUiThread {
             refreshMenuItems()
             getOrCreateThreadAdapter().apply {
-                val scrollPosition = if (currentList.lastOrNull() != threadItems.lastOrNull()) {
-                    threadItems.lastIndex
+                val lastPosition = itemCount - 1
+                val lastVisiblePosition = (thread_messages_list.layoutManager as LinearLayoutManager).findLastVisibleItemPosition()
+                val scrollPosition = if (currentList.lastOrNull() != threadItems.lastOrNull() && lastPosition - lastVisiblePosition <= 2) {
+                    lastPosition
                 } else {
                     -1
                 }
@@ -1235,7 +1239,7 @@ class ThreadActivity : SimpleActivity() {
 
         val newItems = getThreadItems()
         runOnUiThread {
-            getOrCreateThreadAdapter().updateMessages(newItems)
+            getOrCreateThreadAdapter().updateMessages(newItems, newItems.lastIndex)
             if (!refreshedSinceSent) {
                 refreshMessages()
             }

app/src/main/kotlin/com/simplemobiletools/smsmessenger/adapters/ThreadAdapter.kt

@@ -227,7 +227,7 @@ class ThreadAdapter(
 
     private fun isThreadDateTime(position: Int) = currentList.getOrNull(position) is ThreadDateTime
 
-    fun updateMessages(newMessages: ArrayList<ThreadItem>, scrollPosition: Int = newMessages.lastIndex) {
+    fun updateMessages(newMessages: ArrayList<ThreadItem>, scrollPosition: Int = -1) {
         val latestMessages = newMessages.toMutableList()
         submitList(latestMessages) {
             if (scrollPosition != -1) {

app/src/main/res/drawable/ic_arrow_down_vector.xml

@@ -0,0 +1,3 @@
+<vector xmlns:android="http://schemas.android.com/apk/res/android" android:width="24dp" android:height="24dp" android:viewportWidth="24" android:viewportHeight="24">
+    <path android:fillColor="@android:color/white" android:pathData="M11 5v11.17l-4.88-4.88c-0.39-0.39-1.03-0.39-1.42 0-0.39 0.39-0.39 1.02 0 1.41l6.59 6.59c0.39 0.39 1.02 0.39 1.41 0l6.59-6.59c0.39-0.39 0.39-1.02 0-1.41-0.39-0.39-1.02-0.39-1.41 0L13 16.17V5c0-0.55-0.45-1-1-1s-1 0.45-1 1z"/>
+</vector>