Avoid chat position jumps during message loading

Sometimes, the chat list would jump without the user scrolling:
- During intial loading of a room content, i.e. when it is expected
  that the list stays scrolled to bottom
- During loading of messages after jumping to a linked message

With this commit, the target event is repeatedly scrolled to upon list
changes until the users scroll themselves, to avoid above scenarios.

Change-Id: Iabbe76832e7e68686431b0baed9356c88eb50901

matrix-sdk-android/src/main/java/org/matrix/android/sdk/api/session/room/timeline/Timeline.kt

@@ -56,6 +56,11 @@ interface Timeline {
      */
     fun restartWithEventId(eventId: String?)
 
+    /**
+     * Event that should be displayed first, before the user scrolls.
+     */
+    fun getInitialEventId(): String?
+
     /**
      * Check if the timeline can be enriched by paginating.
      * @param direction the direction to check in

matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/session/room/timeline/DefaultTimeline.kt

@@ -221,6 +221,10 @@ internal class DefaultTimeline(
         postSnapshot()
     }
 
+    override fun getInitialEventId(): String? {
+        return initialEventId
+    }
+
     override fun getTimelineEventAtIndex(index: Int): TimelineEvent? {
         return builtEvents.getOrNull(index)
     }

vector/src/main/java/im/vector/app/features/home/room/detail/RoomDetailFragment.kt

@@ -740,6 +740,8 @@ class RoomDetailFragment @Inject constructor(
     }
 
     private fun navigateToEvent(action: RoomDetailViewEvents.NavigateToEvent) {
+        scrollOnNewMessageCallback.initialForceScroll = true
+        scrollOnNewMessageCallback.initialForceScrollEventId = action.eventId
         val scrollPosition = timelineEventController.searchPositionOfEvent(action.eventId)
         if (scrollPosition == null) {
             scrollOnHighlightedEventCallback.scheduleScrollTo(action.eventId)
@@ -1067,6 +1069,8 @@ class RoomDetailFragment @Inject constructor(
         layoutManager = LinearLayoutManager(context, RecyclerView.VERTICAL, true)
         val stateRestorer = LayoutManagerStateRestorer(layoutManager).register()
         scrollOnNewMessageCallback = ScrollOnNewMessageCallback(layoutManager, timelineEventController)
+        // Force scroll until the user has scrolled to address the bug where the list would jump during initial loading
+        scrollOnNewMessageCallback.initialForceScroll = true
         scrollOnHighlightedEventCallback = ScrollOnHighlightedEventCallback(views.timelineRecyclerView, layoutManager, timelineEventController)
         views.timelineRecyclerView.layoutManager = layoutManager
         views.timelineRecyclerView.itemAnimator = null
@@ -1080,6 +1084,15 @@ class RoomDetailFragment @Inject constructor(
         }
         timelineEventController.addModelBuildListener(modelBuildListener)
         views.timelineRecyclerView.adapter = timelineEventController.adapter
+        views.timelineRecyclerView.addOnScrollListener(object: RecyclerView.OnScrollListener() {
+            override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {
+                if (dy != 0) {
+                    // User has scrolled, stop force scrolling
+                    scrollOnNewMessageCallback.initialForceScroll = false
+                }
+                super.onScrolled(recyclerView, dx, dy)
+            }
+        })
 
         if (vectorPreferences.swipeToReplyIsEnabled()) {
             val quickReplyHandler = object : RoomMessageTouchHelperCallback.QuickReplayHandler {

vector/src/main/java/im/vector/app/features/home/room/detail/ScrollOnNewMessageCallback.kt

@@ -28,8 +28,23 @@ class ScrollOnNewMessageCallback(private val layoutManager: LinearLayoutManager,
 
     private val newTimelineEventIds = CopyOnWriteArrayList<String>()
     private var forceScroll = false
+    var initialForceScroll = false
+    var initialForceScrollEventId: String? = null
+        get() = field ?: timelineEventController.timeline?.getInitialEventId()
 
     fun addNewTimelineEventIds(eventIds: List<String>) {
+        // Disable initial force scroll
+        initialForceScroll = false
+        // Update force scroll id when sticking to the bottom - TODO try this if staying at bottom is not reliable as well
+        /*
+        if (eventIds.isNotEmpty()) {
+            initialForceScrollEventId.let {
+                if (it != null && it == timelineEventController.timeline?.getInitialEventId()) {
+                    initialForceScrollEventId = eventIds[0]
+                }
+            }
+        }
+         */
         newTimelineEventIds.addAll(0, eventIds)
     }
 
@@ -38,6 +53,12 @@ class ScrollOnNewMessageCallback(private val layoutManager: LinearLayoutManager,
     }
 
     override fun onInserted(position: Int, count: Int) {
+        if (initialForceScroll) {
+            timelineEventController.searchPositionOfEvent(initialForceScrollEventId)?.let {
+                layoutManager.scrollToPosition(it)
+            }
+            return
+        }
         if (position != 0) {
             return
         }