[FIX] mail: fix message separator appearing after showing composer

closes odoo/odoo#118875

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/static/src/core_ui/thread.xml

@@ -26,7 +26,7 @@
                         </div>
                         <t t-set="currentDay" t-value="msg.dateDay"/>
                     </t>
-                    <Transition visible="props.thread.seen_message_id === (prevMsg?.id ?? false)" name="'o-fade'" t-slot-scope="transition">
+                    <Transition t-if="props.thread.isChannel"  visible="props.thread.seen_message_id === (prevMsg?.id ?? false)" name="'o-fade'" t-slot-scope="transition">
                         <div class="o-mail-Thread-newMessage d-flex align-items-center fw-bolder" t-att-class="{ 'opacity-0': transition.className.includes('o-fade-leave') }">
                             <hr class="flex-grow-1 border opacity-50"/><span class="px-3">New messages</span>
                         </div>

addons/mail/static/tests/thread/thread_tests.js

@@ -985,3 +985,27 @@ QUnit.test("can be marked as read while loading", async function (assert) {
     await afterNextRender(loadDeferred.resolve);
     assert.containsNone($, ".o-mail-DiscussCategoryItem-counter");
 });
+
+QUnit.test(
+    "New message separator not appearing after showing composer on thread",
+    async (assert) => {
+        const pyEnv = await startServer();
+        pyEnv["mail.message"].create([
+            {
+                model: "res.partner",
+                res_id: pyEnv.currentPartnerId,
+                body: "Message on partner",
+            },
+            {
+                model: "res.partner",
+                res_id: pyEnv.currentPartnerId,
+                body: "Message on partner",
+            },
+        ]);
+        const { openFormView } = await start();
+        await openFormView("res.partner", pyEnv.currentPartnerId);
+        assert.containsNone($, ".o-mail-Thread-newMessage");
+        await click("button:contains(Log note)");
+        assert.containsNone($, ".o-mail-Thread-newMessage");
+    }
+);