[FIX] mail: prevent chat window buttons to wrap on multiple lines

addons/mail/static/src/less/chat_window.less

@@ -47,14 +47,18 @@
             .o-text-overflow();
         }
 
-        .o_chat_window_close, .o_chat_window_expand {
-            color: white;
-            padding: 3px;
-            margin-left: 5px;
-            &:hover {
-                background-color: darken(@odoo-brand-primary, 10%);
+        .o_chat_window_buttons {
+            .o-flex(0, 0, auto);
+            .o_chat_window_close, .o_chat_window_expand {
+                color: white;
+                padding: 3px;
+                margin-left: 5px;
+                &:hover {
+                    background-color: darken(@odoo-brand-primary, 10%);
+                }
             }
         }
+
     }
 
     .o_mail_thread {

addons/mail/static/src/xml/chat_window.xml

@@ -46,7 +46,7 @@
             <t t-esc="title"/>
             <span t-if="unread_counter"> (<t t-esc="unread_counter"/>)</span>
         </span>
-        <span>
+        <span class="o_chat_window_buttons">
             <a href="#" class="o_chat_window_close fa fa-close"/>
         </span>
     </t>