"MDL-8642, see tracker, merged from MOODLE_18_STABLE"

mod/chat/gui_header_js/chatinput.php

@@ -53,7 +53,7 @@
           onsubmit="return empty_field_and_submit()" style="margin:0">
         <input type="text" id="input_chat_message" name="chat_message" size="50" value="" />
         <?php helpbutton('chatting', get_string('helpchatting', 'chat'), 'chat', true, false); ?><br />
-        <input type="checkbox" id="auto" size="50" value="" checked /><?php echo get_string('autoscroll', 'chat');?>
+        <input type="checkbox" id="auto" size="50" value="" checked='true' /><?php echo get_string('autoscroll', 'chat');?>
     </form>
 
     <form action="insert.php" method="post" target="empty" id="sendForm">

mod/chat/gui_header_js/jsupdate.php

@@ -141,8 +141,11 @@
             }
         }
         ?>
-        if(parent.msg && parent.input.document.getElementById('auto').checked){
-            parent.msg.scroll(1,5000000);
+        if(parent.input){
+            var autoscroll = parent.input.document.getElementById('auto');
+            if(parent.msg && autoscroll && autoscroll.checked){
+                parent.msg.scroll(1,5000000);
+            }
         }
         //]]>
         </script>