diff --git a/vector/src/main/java/im/vector/app/core/utils/DataSource.kt b/vector/src/main/java/im/vector/app/core/utils/DataSource.kt
index 32c1cf9424..60ad91272b 100644
--- a/vector/src/main/java/im/vector/app/core/utils/DataSource.kt
+++ b/vector/src/main/java/im/vector/app/core/utils/DataSource.kt
@@ -26,8 +26,6 @@ interface DataSource<T> {
 
 interface MutableDataSource<T> : DataSource<T> {
 
-    suspend fun set(value: T)
-
     fun post(value: T)
 }
 
@@ -45,10 +43,6 @@ open class BehaviorDataSource<T>(private val defaultValue: T? = null) : MutableD
         return mutableFlow
     }
 
-    override suspend fun set(value: T) {
-        mutableFlow.emit(value)
-    }
-
     override fun post(value: T) {
         mutableFlow.tryEmit(value)
     }
@@ -56,19 +50,17 @@ open class BehaviorDataSource<T>(private val defaultValue: T? = null) : MutableD
 
 /**
  * This datasource only emits all subsequent observed values to each subscriber.
+ *
+ * bufferSize - number of buffered items before it starts dropping oldest. Should be at least 1
  */
-open class PublishDataSource<T> : MutableDataSource<T> {
+open class PublishDataSource<T>(bufferSize: Int = 10) : MutableDataSource<T> {
 
-    private val mutableFlow = MutableSharedFlow<T>(replay = 0, extraBufferCapacity = 1, onBufferOverflow = BufferOverflow.DROP_OLDEST)
+    private val mutableFlow = MutableSharedFlow<T>(replay = 0, extraBufferCapacity = bufferSize, onBufferOverflow = BufferOverflow.DROP_OLDEST)
 
     override fun stream(): Flow<T> {
         return mutableFlow
     }
 
-    override suspend fun set(value: T) {
-        mutableFlow.emit(value)
-    }
-
     override fun post(value: T) {
         mutableFlow.tryEmit(value)
     }
diff --git a/vector/src/main/java/im/vector/app/features/home/room/detail/composer/MessageComposerViewModel.kt b/vector/src/main/java/im/vector/app/features/home/room/detail/composer/MessageComposerViewModel.kt
index d0247553aa..eca5c846ca 100644
--- a/vector/src/main/java/im/vector/app/features/home/room/detail/composer/MessageComposerViewModel.kt
+++ b/vector/src/main/java/im/vector/app/features/home/room/detail/composer/MessageComposerViewModel.kt
@@ -472,16 +472,14 @@ class MessageComposerViewModel @AssistedInject constructor(
                             Unit
                         }
                         is ParsedCommand.UpgradeRoom                       -> {
-                            viewModelScope.launch {
-                                _viewEvents.set(
-                                        MessageComposerViewEvents.ShowRoomUpgradeDialog(
-                                                parsedCommand.newVersion,
-                                                room.roomSummary()?.isPublic ?: false
-                                        )
-                                )
-                                _viewEvents.set(MessageComposerViewEvents.SlashCommandResultOk(parsedCommand))
-                                popDraft()
-                            }
+                            _viewEvents.post(
+                                    MessageComposerViewEvents.ShowRoomUpgradeDialog(
+                                            parsedCommand.newVersion,
+                                            room.roomSummary()?.isPublic ?: false
+                                    )
+                            )
+                            _viewEvents.post(MessageComposerViewEvents.SlashCommandResultOk(parsedCommand))
+                            popDraft()
                         }
                     }
                 }
diff --git a/vector/src/test/java/im/vector/app/core/utils/DataSourceTest.kt b/vector/src/test/java/im/vector/app/core/utils/DataSourceTest.kt
new file mode 100644
index 0000000000..f7b6d4a2c8
--- /dev/null
+++ b/vector/src/test/java/im/vector/app/core/utils/DataSourceTest.kt
@@ -0,0 +1,81 @@
+/*
+ * Copyright (c) 2022 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.utils
+
+import im.vector.app.test.test
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.flow.launchIn
+import kotlinx.coroutines.flow.onEach
+import kotlinx.coroutines.test.TestCoroutineScheduler
+import kotlinx.coroutines.test.UnconfinedTestDispatcher
+import kotlinx.coroutines.test.runTest
+import kotlinx.coroutines.withContext
+import org.amshove.kluent.shouldContainSame
+import org.junit.Test
+
+class DataSourceTest {
+
+    @Test
+    fun `given PublishDataSource, when posting values before observing, then no value is observed`() = runTest {
+        val publishDataSource = PublishDataSource<Int>()
+        publishDataSource.post(0)
+        publishDataSource.post(1)
+
+        publishDataSource.stream()
+                .test(this)
+                .assertNoValues()
+                .finish()
+    }
+
+    @Test
+    fun `given PublishDataSource with a large enough buffer size, when posting values after observing, then only the latest values are observed`() = runTest {
+        val valuesToPost = listOf(2, 3, 4, 5, 6, 7, 8, 9)
+        val publishDataSource = PublishDataSource<Int>(bufferSize = valuesToPost.size)
+        publishDataSource.test(testScheduler, valuesToPost, valuesToPost)
+    }
+
+    @Test
+    fun `given PublishDataSource with a too small buffer size, when posting values after observing, then we are missing some values`() = runTest {
+        val valuesToPost = listOf(2, 3, 4, 5, 6, 7, 8, 9)
+        val expectedValues = listOf(2, 9)
+        val publishDataSource = PublishDataSource<Int>(bufferSize = 1)
+        publishDataSource.test(testScheduler, valuesToPost, expectedValues)
+    }
+
+    private suspend fun PublishDataSource<Int>.test(testScheduler: TestCoroutineScheduler, valuesToPost: List<Int>, expectedValues: List<Int>) {
+        val values = ArrayList<Int>()
+        val job = stream()
+                .onEach {
+                    // Artificial delay to make consumption longer than production
+                    delay(10)
+                    values.add(it)
+                }
+                .launchIn(CoroutineScope(UnconfinedTestDispatcher(testScheduler)))
+
+        valuesToPost.forEach {
+            post(it)
+        }
+        withContext(Dispatchers.Default) {
+            delay(11L * valuesToPost.size)
+        }
+        job.cancel()
+
+        values shouldContainSame expectedValues
+    }
+}