[FIX] web_responsive: mentions overlapping

When we have the chatter in the right side, mentions tend to overlap
with other screen elements, so the dialog is not fully visible. With
this change, we move the mentions dialog to the bottom of the composer
window to avoid such problem.

TT38108

web_responsive/static/src/scss/web_responsive.scss

@@ -317,6 +317,18 @@ $chatter_zone_width: 35%;
     }
 }
 
+// Put mentions dialog below the chatter to avoid overlapping it with other elements
+.o_thread_composer {
+    .o_composer_mention_dropdown {
+        position: absolute;
+        top: 100%;
+        .dropdown-menu {
+            top: 100%;
+            bottom: auto;
+        }
+    }
+}
+
 // Scroll all but top bar
 html .o_web_client .o_action_manager .o_action {
     @include media-breakpoint-down(sm) {