IVsDropdownBar.RefreshCombo freezes UI

Hi all, 

I have written a plugin for Visual Studio , which among other supports outlining and dropdown bar.

The problem is, that when I have huge files, e.g. 100K lines and I make any change to the file, the file will be reparsed in the backgound and the outlining regions updates. Once this is done, I am doing a forceRepaint of the IVsDropdownBar instance. 

For huge files, this causes the Visual Studio instance to hang. Why can't this be done in the background ?

I tried putting this call to a background task, but it doesn't matter since it has to be called via the dispatcher and it anyway hangs the ui.

Here is some code:

void OnParserReady(object source, Parsing.ParserDatabase.ParserReadyEventEventArgs e)
        {
            //if a which belong to this text view is updated update the combo boxes
            if (e.Filepath == Utilities.RegistryReader.GetTextDocument(_textView.TextBuffer).FilePath && e.IsParsed )
            {
                var dropDownBar = _dropDownBar;
                //get new command list if available
                _body = Parsing.ParserDatabase.Instance.getParsedDocument(e.Filepath, false );
                if (dropDownBar != null)
                {
                    if (mTimer.IsEnabled)
                    {
                        mTimer.Stop();
                    }
                    updateComboBoxCallback = new Action(test);
                    //Action callback = () => { dropDownBar.RefreshCombo(0, 0);};
                    //_dispatcher.BeginInvoke(callback, DispatcherPriority.Loaded);
                    mTimer.Start();
                }
            }
        }

//timer callback actually executes the comborefresh

void test()
        {
            //if (mRefreshTask != null && !mRefreshTask.IsCompleted)
            //{
            //    //task is running so cancel it
            //    mRefreshTokenSrc.Cancel();
            //    mRefreshTask.Wait();
            //}
            //mRefreshTokenSrc = new CancellationTokenSource();
            //mRefreshTask = System.Threading.Tasks.Task.Factory.StartNew(test2, mRefreshTokenSrc.Token, System.Threading.Tasks.TaskCreationOptions.LongRunning, new ThreadPerTaskScheduler());
            if (_dropDownBar != null)
            {
                _dispatcher.BeginInvoke(DispatcherPriority.Input, new ThreadStart(() =>
                {

                     _dropDownBar.RefreshCombo(0, 0); // <---------------- Here UI hangs for a long time if the file is large enough.
                }));
            }
        }