diff --git a/addons/mail/static/src/js/thread.js b/addons/mail/static/src/js/thread.js
index 0c57ea8ad88..aee712ea905 100644
--- a/addons/mail/static/src/js/thread.js
+++ b/addons/mail/static/src/js/thread.js
@@ -38,9 +38,9 @@ var Thread = Widget.extend({
         "click .oe_mail_expand": function (event) {
             event.preventDefault();
             event.stopPropagation();
-            var $source = $(event.currentTarget);
-            $source.parents('.o_thread_message_core').find('.o_mail_body_short').toggle();
-            $source.parents('.o_thread_message_core').find('.o_mail_body_long').toggle();
+            var $message = $(event.currentTarget).parents('.o_thread_message');
+            $message.addClass('o_message_expanded');
+            this.expanded_msg_ids.push($message.data('message-id'));
         },
     },
 
@@ -55,6 +55,7 @@ var Thread = Widget.extend({
             shorten_messages: true,
             squash_close_messages: true,
         });
+        this.expanded_msg_ids = [];
     },
 
     render: function (messages, options) {
@@ -132,6 +133,10 @@ var Thread = Widget.extend({
             msg.hour = msg.date.format('LT');
         }
 
+        if (_.contains(this.expanded_msg_ids, message.id)) {
+            msg.expanded = true;
+        }
+
         msg.display_subject = message.subject && !message.is_system_notification;
         return msg;
     },
diff --git a/addons/mail/static/src/less/thread.less b/addons/mail/static/src/less/thread.less
index be86e577b08..7f0f144ffd8 100644
--- a/addons/mail/static/src/less/thread.less
+++ b/addons/mail/static/src/less/thread.less
@@ -145,6 +145,15 @@
                 padding: 4px;
             }
         }
+
+        &.o_message_expanded .o_thread_message_core {
+            .o_mail_body_short {
+                display: none;
+            }
+            .o_mail_body_long {
+                display: block;
+            }
+        }
     }
     .o_thread_title {
         margin-top: 20px;
diff --git a/addons/mail/static/src/xml/thread.xml b/addons/mail/static/src/xml/thread.xml
index 83436f37495..ca863d9ad8f 100644
--- a/addons/mail/static/src/xml/thread.xml
+++ b/addons/mail/static/src/xml/thread.xml
@@ -46,7 +46,7 @@
     </t>
 
     <t t-name="mail.ChatThread.Message">
-        <div class="o_thread_message" t-att-data-message-id="message.id">
+        <div t-att-class="'o_thread_message' + (message.expanded ? ' o_message_expanded' : '')" t-att-data-message-id="message.id">
             <div t-if="options.display_avatar" class="o_thread_message_sidebar">
                 <t t-if="!message.mailto &amp;&amp; message.author_id[0]">
                     <img t-if="message.avatar_src &amp;&amp; message.display_author"