diff --git a/addons/mail/static/src/core/common/chat_window.scss b/addons/mail/static/src/core/common/chat_window.scss
index cccb2eb74ab..d1fcdec3d74 100644
--- a/addons/mail/static/src/core/common/chat_window.scss
+++ b/addons/mail/static/src/core/common/chat_window.scss
@@ -2,6 +2,9 @@
     height: 480px;
     width: $o-mail-ChatWindow-width;
     z-index: 999; // messaging menu is dropdown (1000)
+    &.o-mobile {
+        z-index: 1001; // above messaging menu (chat window takes whole screen)
+    }
     box-shadow: -5px -5px 10px rgba(#000000, 0.09);
     outline: none;
 
diff --git a/addons/mail/static/src/core/common/chat_window.xml b/addons/mail/static/src/core/common/chat_window.xml
index 24859b7a496..792da3b6f90 100644
--- a/addons/mail/static/src/core/common/chat_window.xml
+++ b/addons/mail/static/src/core/common/chat_window.xml
@@ -5,7 +5,7 @@
     <div class="o-mail-ChatWindow"
         t-att-style="style"
         t-att-class="{
-                'w-100 h-100': ui.isSmall,
+                'w-100 h-100 o-mobile': ui.isSmall,
                 'o-folded': props.chatWindow.folded,
                 'position-absolute bottom-0 overflow-hidden d-flex flex-column': !props.chatWindow.hidden,
                 'rounded-top-3': !props.chatWindow.hidden and !ui.isSmall,
@@ -54,7 +54,7 @@
                 <t t-set="itemClass" t-value="'me-1'"/>
             </t>
         </div>
-        <div t-if="!props.chatWindow.folded" class="o-mail-ChatWindow-content bg-view d-flex flex-column h-100 overflow-auto position-relative" t-ref="content">
+        <div t-if="!props.chatWindow.folded or ui.isSmall" class="o-mail-ChatWindow-content bg-view d-flex flex-column h-100 overflow-auto position-relative" t-ref="content">
             <button t-if="threadActions.activeAction" class="o-mail-ChatWindow-closePanel btn position-absolute top-0 end-0 btn opacity-75 opacity-100-hover" title="Close panel" t-on-click.stop="() => threadActions.activeAction.close()"><i class="fa fa-close"/></button>
             <t t-if="thread" name="thread content">
                 <t t-if="threadActions.activeAction?.componentCondition" t-component="threadActions.activeAction.component" thread="thread" t-props="threadActions.activeAction.componentProps"/>
diff --git a/addons/mail/static/src/core/web/messaging_menu.xml b/addons/mail/static/src/core/web/messaging_menu.xml
index 22d59c16b82..15f02d4605d 100644
--- a/addons/mail/static/src/core/web/messaging_menu.xml
+++ b/addons/mail/static/src/core/web/messaging_menu.xml
@@ -2,7 +2,7 @@
 <templates xml:space="preserve">
 
 <t t-name="mail.MessagingMenu" owl="1">
-    <Dropdown t-if="!env.inDiscussApp" autoOpen="false" beforeOpen.bind="beforeOpen" onStateChanged="state => this.state.isOpen = state.open" position="'bottom-end'" menuClass="`p-0 ${ ui.isSmall ? 'start-0 w-100 mh-100 bottom-0 d-flex flex-column' : ''}`">
+    <Dropdown t-if="!env.inDiscussApp" autoOpen="false" beforeOpen.bind="beforeOpen" onStateChanged="state => this.state.isOpen = state.open" position="'bottom-end'" menuClass="`p-0 ${ ui.isSmall ? 'start-0 w-100 mh-100 bottom-0 d-flex flex-column m-0' : ''}`">
         <t t-set-slot="toggler">
             <i class="fa fa-lg fa-comments" role="img" aria-label="Messages" t-on-click="() => store.discuss.activeTab = ui.isSmall and store.discuss.activeTab === 'mailbox' ? 'all' : store.discuss.activeTab"></i>
             <span t-if="counter" class="o-mail-MessagingMenu-counter badge rounded-pill"><t t-esc="counter"/></span>
diff --git a/addons/mail/static/src/core/web/messaging_service_patch.js b/addons/mail/static/src/core/web/messaging_service_patch.js
index 79abc7c57fc..9025f0d1b66 100644
--- a/addons/mail/static/src/core/web/messaging_service_patch.js
+++ b/addons/mail/static/src/core/web/messaging_service_patch.js
@@ -23,7 +23,7 @@ patch(Messaging.prototype, "mail/core/web", {
             );
             this.notificationService.add(notification, { type: "info" });
             const chat = await this.threadService.getChat({ partnerId });
-            if (chat) {
+            if (chat && !this.ui.isSmall) {
                 this.chatWindowService.insert({ thread: chat });
             }
         });
@@ -32,7 +32,7 @@ patch(Messaging.prototype, "mail/core/web", {
         this.loadFailures();
         for (const channelData of data.channels) {
             const thread = this.threadService.createChannelThread(channelData);
-            if (channelData.is_minimized && channelData.state !== "closed") {
+            if (channelData.is_minimized && channelData.state !== "closed" && !this.ui.isSmall) {
                 this.chatWindowService.insert({
                     autofocus: 0,
                     folded: channelData.state === "folded",
diff --git a/addons/mail/static/tests/chat_window/chat_window_tests.js b/addons/mail/static/tests/chat_window/chat_window_tests.js
index 75e85c41fd8..67bbac9bd13 100644
--- a/addons/mail/static/tests/chat_window/chat_window_tests.js
+++ b/addons/mail/static/tests/chat_window/chat_window_tests.js
@@ -1007,6 +1007,35 @@ QUnit.test("Chat window in mobile are not foldable", async (assert) => {
     assert.containsOnce($, ".o-mail-ChatWindow-content"); // content => non-folded
 });
 
+QUnit.test("Server-synced chat windows should not open at page load on mobile", async (assert) => {
+    const pyEnv = await startServer();
+    pyEnv["discuss.channel"].create({
+        channel_member_ids: [
+            Command.create({
+                fold_state: "open",
+                is_minimized: true,
+                partner_id: pyEnv.currentPartnerId,
+            }),
+        ],
+    });
+    patchUiSize({ size: SIZES.SM });
+    await start();
+    assert.containsNone($, ".o-mail-ChatWindow");
+});
+
+QUnit.test("chat window of channels should not have 'Open in Discuss' (mobile)", async (assert) => {
+    const pyEnv = await startServer();
+    pyEnv["discuss.channel"].create({ name: "General" });
+    patchUiSize({ size: SIZES.SM });
+    await start();
+    await click(".o_menu_systray i[aria-label='Messages']");
+    await click(".o-mail-NotificationItem");
+    assert.containsOnce($, ".o-mail-ChatWindow");
+    assert.containsOnce($, "[title='Open Actions Menu']");
+    await click("[title='Open Actions Menu']");
+    assert.containsNone($, "[title='Open in Discuss']");
+});
+
 QUnit.test("Open chat window of new inviter", async (assert) => {
     const pyEnv = await startServer();
     await start();