don't attempt to add the frame indicator if v.buffer.shiftFrameFocus is called

on a non-HTML document

content/buffers.js

@@ -267,8 +267,9 @@ vimperator.Buffer = function() //{{{
     // TODO: allow callback for filtering out unwanted frames? User defined?
     this.shiftFrameFocus = function(count, forward)
     {
-        try
-        {
+        if (!window.content.document instanceof HTMLDocument)
+            return;
+
         var frames = [];
 
         // find all frames - depth-first search
@@ -284,7 +285,7 @@ vimperator.Buffer = function() //{{{
             return;
 
         // remove all unfocusable frames
-            // TODO: find a better way to do this
+        // TODO: find a better way to do this - walking the tree is too slow
         var start = document.commandDispatcher.focusedWindow;
         frames = frames.filter(function(frame) {
                 frame.focus();
@@ -347,6 +348,8 @@ vimperator.Buffer = function() //{{{
             frames[next].frameElement.scrollIntoView(false);
 
         // add the frame indicator
+        // TODO: make this an XBL element rather than messing with the content
+        // document
         var doc = frames[next].document;
         var indicator = doc.createElement("div");
         indicator.id = "vimperator-frame-indicator";
@@ -359,12 +362,6 @@ vimperator.Buffer = function() //{{{
         // remove the frame indicator
         setTimeout(function() { doc.body.removeChild(indicator); }, 500);
     }
-        catch (e)
-        {
-            // FIXME: fail silently here for now
-            //vimperator.log(e);
-        }
-    }
 
     // updates the buffer preview in place only if list is visible
     this.updateBufferList = function()