diff --git a/vector/src/main/java/im/vector/app/core/flow/ChunkOperator.kt b/vector/src/main/java/im/vector/app/core/flow/ChunkOperator.kt
new file mode 100644
index 0000000000..533a060883
--- /dev/null
+++ b/vector/src/main/java/im/vector/app/core/flow/ChunkOperator.kt
@@ -0,0 +1,79 @@
+/*
+ * Copyright (c) 2021 New Vector Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package im.vector.app.core.flow
+
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.channels.Channel
+import kotlinx.coroutines.channels.ClosedReceiveChannelException
+import kotlinx.coroutines.channels.ReceiveChannel
+import kotlinx.coroutines.channels.produce
+import kotlinx.coroutines.coroutineScope
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.collect
+import kotlinx.coroutines.flow.flow
+import kotlinx.coroutines.isActive
+import kotlinx.coroutines.selects.select
+
+@ExperimentalCoroutinesApi
+fun <T> Flow<T>.chunk(durationInMillis: Long): Flow<List<T>> {
+    require(durationInMillis> 0) { "Duration should be greater than 0" }
+    return flow {
+        coroutineScope {
+            val events = ArrayList<T>()
+            val ticker = fixedPeriodTicker(durationInMillis)
+            try {
+                val upstreamValues = produce(capacity = Channel.CONFLATED) {
+                    collect { value -> send(value) }
+                }
+                while (isActive) {
+                    var hasTimedOut = false
+                    select<Unit> {
+                        upstreamValues.onReceive {
+                            events.add(it)
+                        }
+                        ticker.onReceive {
+                            hasTimedOut = true
+                        }
+                    }
+                    if (hasTimedOut && events.isNotEmpty()) {
+                        emit(events.toList())
+                        events.clear()
+                    }
+                }
+            } catch (e: ClosedReceiveChannelException) {
+                // drain remaining events
+                if (events.isNotEmpty()) emit(events.toList())
+            } finally {
+                ticker.cancel()
+            }
+        }
+    }
+}
+
+private fun CoroutineScope.fixedPeriodTicker(delayMillis: Long, initialDelayMillis: Long = delayMillis): ReceiveChannel<Unit> {
+    require(delayMillis >= 0) { "Expected non-negative delay, but has $delayMillis ms" }
+    require(initialDelayMillis >= 0) { "Expected non-negative initial delay, but has $initialDelayMillis ms" }
+    return produce(capacity = 0) {
+        delay(initialDelayMillis)
+        while (true) {
+            channel.send(Unit)
+            delay(delayMillis)
+        }
+    }
+}
diff --git a/vector/src/main/java/im/vector/app/features/home/room/detail/RoomDetailViewModel.kt b/vector/src/main/java/im/vector/app/features/home/room/detail/RoomDetailViewModel.kt
index ee929243b5..1b765f81b9 100644
--- a/vector/src/main/java/im/vector/app/features/home/room/detail/RoomDetailViewModel.kt
+++ b/vector/src/main/java/im/vector/app/features/home/room/detail/RoomDetailViewModel.kt
@@ -33,6 +33,7 @@ import dagger.assisted.AssistedInject
 import im.vector.app.BuildConfig
 import im.vector.app.R
 import im.vector.app.core.extensions.exhaustive
+import im.vector.app.core.flow.chunk
 import im.vector.app.core.mvrx.runCatchingToAsync
 import im.vector.app.core.platform.VectorViewModel
 import im.vector.app.core.resources.StringProvider
@@ -58,13 +59,17 @@ import im.vector.app.features.settings.VectorPreferences
 import im.vector.app.features.voice.VoicePlayerHelper
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.flow.MutableSharedFlow
+import kotlinx.coroutines.flow.buffer
 import kotlinx.coroutines.flow.collect
 import kotlinx.coroutines.flow.combine
 import kotlinx.coroutines.flow.distinctUntilChanged
+import kotlinx.coroutines.flow.filter
 import kotlinx.coroutines.flow.filterIsInstance
+import kotlinx.coroutines.flow.flowOn
 import kotlinx.coroutines.flow.launchIn
 import kotlinx.coroutines.flow.map
 import kotlinx.coroutines.flow.onEach
+import kotlinx.coroutines.flow.sample
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
 import org.matrix.android.sdk.api.MatrixPatterns
@@ -863,13 +868,13 @@ class RoomDetailViewModel @AssistedInject constructor(
     private fun observeEventDisplayedActions() {
         // We are buffering scroll events for one second
         // and keep the most recent one to set the read receipt on.
-        /*
+
         visibleEventsSource
                 .stream()
-                .buffer(1, TimeUnit.SECONDS)
+                .chunk(1000)
                 .filter { it.isNotEmpty() }
-                .subscribeBy(onNext = { actions ->
-                    val bufferedMostRecentDisplayedEvent = actions.maxByOrNull { it.event.displayIndex }?.event ?: return@subscribeBy
+                .onEach { actions ->
+                    val bufferedMostRecentDisplayedEvent = actions.maxByOrNull { it.event.displayIndex }?.event ?: return@onEach
                     val globalMostRecentDisplayedEvent = mostRecentDisplayedEvent
                     if (trackUnreadMessages.get()) {
                         if (globalMostRecentDisplayedEvent == null) {
@@ -883,10 +888,9 @@ class RoomDetailViewModel @AssistedInject constructor(
                             tryOrNull { room.setReadReceipt(eventId) }
                         }
                     }
-                })
-                .disposeOnClear()
-
-         */
+                }
+                .flowOn(Dispatchers.Default)
+                .launchIn(viewModelScope)
     }
 
     private fun handleMarkAllAsRead() {