diff --git a/packages/layout_editor/src/lib/isMessageSequential.test.js b/packages/layout_editor/src/lib/isMessageSequential.test.js
new file mode 100644
index 000000000..d951bce31
--- /dev/null
+++ b/packages/layout_editor/src/lib/isMessageSequential.test.js
@@ -0,0 +1,99 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import isMessageSequential from './isMessageSequential.js';
+
+test('does not group when there is no previous message', () => {
+  assert.equal(
+    isMessageSequential(
+      { u: { _id: 'user-1' }, ts: '2026-09-23T10:00:00' },
+      undefined,
+      300
+    ),
+    false
+  );
+});
+
+test('groups consecutive messages from the same user inside the grouping range', () => {
+  assert.equal(
+    isMessageSequential(
+      { u: { _id: 'user-1' }, ts: '2026-09-23T10:02:00' },
+      { u: { _id: 'user-1' }, ts: '2026-09-23T10:00:00' },
+      300
+    ),
+    true
+  );
+});
+
+test('does not group messages written by different users', () => {
+  assert.equal(
+    isMessageSequential(
+      { u: { _id: 'user-2' }, ts: '2026-09-23T10:02:00' },
+      { u: { _id: 'user-1' }, ts: '2026-09-23T10:00:00' },
+      300
+    ),
+    false
+  );
+});
+
+test('does not group once the gap is wider than the grouping range', () => {
+  assert.equal(
+    isMessageSequential(
+      { u: { _id: 'user-1' }, ts: '2026-09-23T10:10:00' },
+      { u: { _id: 'user-1' }, ts: '2026-09-23T10:00:00' },
+      300
+    ),
+    false
+  );
+});
+
+test('does not group system messages', () => {
+  assert.equal(
+    isMessageSequential(
+      { u: { _id: 'user-1' }, ts: '2026-09-23T10:02:00', t: 'uj' },
+      { u: { _id: 'user-1' }, ts: '2026-09-23T10:00:00' },
+      300
+    ),
+    false
+  );
+  assert.equal(
+    isMessageSequential(
+      { u: { _id: 'user-1' }, ts: '2026-09-23T10:02:00' },
+      { u: { _id: 'user-1' }, ts: '2026-09-23T10:00:00', t: 'uj' },
+      300
+    ),
+    false
+  );
+});
+
+test('does not group a message that is explicitly marked as not groupable', () => {
+  assert.equal(
+    isMessageSequential(
+      { u: { _id: 'user-1' }, ts: '2026-09-23T10:02:00', groupable: false },
+      { u: { _id: 'user-1' }, ts: '2026-09-23T10:00:00' },
+      300
+    ),
+    false
+  );
+});
+
+test('does not group messages sent under different aliases', () => {
+  assert.equal(
+    isMessageSequential(
+      { u: { _id: 'user-1' }, ts: '2026-09-23T10:02:00', alias: 'bot' },
+      { u: { _id: 'user-1' }, ts: '2026-09-23T10:00:00' },
+      300
+    ),
+    false
+  );
+});
+
+test('does not group across a day boundary even inside the grouping range', () => {
+  assert.equal(
+    isMessageSequential(
+      { u: { _id: 'user-1' }, ts: '2026-09-24T00:01:00' },
+      { u: { _id: 'user-1' }, ts: '2026-09-23T23:59:00' },
+      300
+    ),
+    false
+  );
+});