Don't try to scroll to show composebox when no messages are showing.

(imported from commit 3835c7de0f3cad777b1fd5b609e9dc71635422ac)

zephyr/static/js/compose.js

@@ -21,6 +21,11 @@ function show(tabname, focus_area) {
     $('.message_comp').slideDown(100, function () {
         // If the compose box is obscuring the currently selected message,
         // scroll up until the message is no longer occluded.
+        if (selected_message_id === -1) {
+            // If there's no selected message, there's no need to
+            // scroll the compose box to avoid it.
+            return;
+        }
         var cover = selected_message.offset().top + selected_message.height()
             - $("#compose").offset().top;
         if (cover > 0) {