Hide close button for controlbox in mobile view.

css/converse.css

@@ -1536,6 +1536,12 @@
       #conversejs #controlbox .controlbox-head {
         border-top-left-radius: 0;
         border-top-right-radius: 0; } }
+    @media screen and (max-height: 450px) {
+      #conversejs #controlbox .controlbox-head .chatbox-btn.close-chatbox-button {
+        display: none; } }
+    @media screen and (max-width: 480px) {
+      #conversejs #controlbox .controlbox-head .chatbox-btn.close-chatbox-button {
+        display: none; } }
   #conversejs #controlbox form.search-xmpp-contact {
     margin: 0;
     padding-left: 5px;

sass/_controlbox.scss

@@ -24,6 +24,16 @@
             height: $chat-head-height;
             margin: 0;
             padding: 6px 6px 6px 0;
+
+            .chatbox-btn.close-chatbox-button {
+                @media screen and (max-height: $mobile-landscape-height) {
+                    display: none;
+                }
+                @media screen and (max-width: $mobile-portrait-length) {
+                    display: none;
+                }
+            }
+
         }
         form.search-xmpp-contact {
             margin: 0;