Scrolling ListBox via mouse clicks pauses timeSetEvent timer/callback

Found this in our C app but was able to replicate it in a simple C# WinForms app for demo purposes. We set up a timer using the multi-media timer timeSetEvent in order to have the UI drawing continuously (it monitors patient vital signs). The user launches a configuration dialog box that has a ListBox control in it. When the user scrolls the ListBox via the mouse (doing either clicking line up/down or page up/down) the timer thread pauses. However when the user scrolls the ListBox via the keyboard (up/down arrow or page up/down) the thread doesn't pause. And when the user drags the scrollbar via the mouse the thread doesn't pause. Only when the user clicks in the scrollbar's line up/down or pages up/down. Huh? The affect is the patient's vital signs are either not drawing correctly or completely stop during this scrolling via mouse clicks and clinicians seeing this get understandably nervous.

My test app is pretty simple. Has a ListBox, 2 labels, and 2 buttons. A button starts the timer, which every 10ms updates the labels (simulating patient vital waveform drawing). Another button loads up the listbox with items. Then when you click line down repeatedly, the labels stop displaying (showing the timer thread is not being called during that time) then when you stop clicking they pick up again, many steps ahead - I am assumming all the bufferred up callbacks occur very rapidly. I also included a simple vertical scrollbar in the form. It does NOT have the same affect on the time thread - can click line down all day and the timer thread isn't interupted.

The code is simple (cannot yet post an image of the form):

        private void button1_Click(object sender, EventArgs e)
        {
            for (int i = 1; i < 101; i++)
                listBox1.Items.Add(String.Format("Item " + i.ToString ()));
        }

        private void button2_Click(object sender, EventArgs e)
        {
            showTickHandler = new ShowTickHandler(ShowTick);
            timeBeginPeriod(1);
            timerHandler = new TimerEventHandler(TimerCallback);
            timerId = timeSetEvent(100, 1, timerHandler, IntPtr.Zero, TIME_PERIODIC);
            mTestStart = DateTime.Now;
            mTestTick = 0;
        }

        private void TimerCallback(int id, int msg, IntPtr user, int dw1, int dw2)
        {
            mTestTick += 1;
            if (timerId != 0)
                this.BeginInvoke(showTickHandler, mTestTick, DateTime.Now - mTestStart);
        }

        private void ShowTick(int msec, TimeSpan span)
        {
            label1.Text = msec.ToString();
            label2.Text = span.TotalMilliseconds.ToString();
        }

Is is a bug or expected behavior from mouse scrolling a ListBox?

Thanks,

Rich Sienkiewicz
Philips Healthcare