diff --git a/features/messages/impl/src/test/kotlin/io/element/android/features/messages/impl/timeline/TimelineItemIndexerTest.kt b/features/messages/impl/src/test/kotlin/io/element/android/features/messages/impl/timeline/TimelineItemIndexerTest.kt
new file mode 100644
index 0000000000..642f07a3f9
--- /dev/null
+++ b/features/messages/impl/src/test/kotlin/io/element/android/features/messages/impl/timeline/TimelineItemIndexerTest.kt
@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2024 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 io.element.android.features.messages.impl.timeline
+
+import com.google.common.truth.Truth.assertThat
+import io.element.android.features.messages.impl.timeline.model.TimelineItem
+import io.element.android.features.messages.impl.timeline.model.virtual.TimelineItemReadMarkerModel
+import io.element.android.libraries.matrix.api.core.EventId
+import io.element.android.libraries.matrix.test.AN_EVENT_ID
+import org.junit.Test
+
+class TimelineItemIndexerTest {
+    @Test
+    fun `test TimelineItemIndexer`() {
+        val eventIds = mutableListOf<EventId>()
+        val data = listOf(
+            aTimelineItemEvent().also { eventIds.add(it.eventId!!) },
+            aTimelineItemEvent().also { eventIds.add(it.eventId!!) },
+            aGroupedEvents().also { groupedEvents ->
+                groupedEvents.events.forEach { eventIds.add(it.eventId!!) }
+            },
+            TimelineItem.Virtual(
+                id = "dummy",
+                model = TimelineItemReadMarkerModel
+            ),
+        )
+        assertThat(eventIds.size).isEqualTo(4)
+        val sut = TimelineItemIndexer()
+        sut.process(data)
+        eventIds.forEach {
+            assertThat(sut.isKnown(it)).isTrue()
+        }
+        assertThat(sut.indexOf(eventIds[0])).isEqualTo(0)
+        assertThat(sut.indexOf(eventIds[1])).isEqualTo(1)
+        assertThat(sut.indexOf(eventIds[2])).isEqualTo(2)
+        assertThat(sut.indexOf(eventIds[3])).isEqualTo(2)
+
+        // Unknown event
+        assertThat(sut.isKnown(AN_EVENT_ID)).isFalse()
+        assertThat(sut.indexOf(AN_EVENT_ID)).isEqualTo(-1)
+    }
+}