Remove the #controlbox-panes element. Not needed.

converse.css

@@ -904,24 +904,6 @@ dd.available-chatroom:hover a.room-info {
     background-color: #DCEAC5;
 }
 
-#conversejs div.controlbox-panes {
-    background: -moz-linear-gradient(top, rgba(255,255,255,1) 0%, rgba(240,240,240,1) 100%); /* FF3.6+ */
-    background: -ms-linear-gradient(top, rgba(255,255,255,1) 0%,rgba(240,240,240,1) 100%); /* IE10+ */
-    background: -o-linear-gradient(top, rgba(255,255,255,1) 0%,rgba(240,240,240,1) 100%); /* Opera 11.10+ */
-    background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,rgba(255,255,255,1)), color-stop(100%,rgba(240,240,240,1))); /* Chrome,Safari4+ */
-    background: -webkit-linear-gradient(top, rgba(255,255,255,1) 0%,rgba(240,240,240,1) 100%); /* Chrome10+,Safari5.1+ */
-    background: linear-gradient(top, rgba(255,255,255,1) 0%,rgba(240,240,240,1) 100%); /* W3C */
-    background-color: white;
-    border-bottom-left-radius: 4px;
-    border-bottom-right-radius: 4px;
-    width: 200px;
-    height: -moz-calc(100% - 38px);
-    height: -o-calc(100% - 38px);
-    height: calc(100% - 38px);
-    overflow-y: auto;
-    position: relative;
-}
-
 form#converse-login {
     background: white;
     padding: 2em 0 0.3em 0.5em;
@@ -1023,7 +1005,11 @@ div#settings {
     border-bottom-right-radius: 4px;
     border-bottom-left-radius: 4px;
     width: 100%;
-    height: 100%;
+    height: -moz-calc(100% - 38px);
+    height: -o-calc(100% - 38px);
+    height: calc(100% - 38px);
+    overflow-y: auto;
+    position: relative;
 }
 
 div#chatrooms {