Hi,

It happens because the MessageBox is shown in the context of DiagramView.OnMouseDown, and once the message box is dismissed, the mouse is at a different position than the coordinates reported in the EventArgs. We have some code that guards against this by handling the OnLostFocus event, but only for situations when a MessageBox is shown from a DiagramView.OnMouseDown handler. As a work-around you could trick the control into doing that check as shown below:

private bool msgbox = false;
private void tbQuery_Leave(object sender, EventArgs e)
{
	if (new Random().Next(2) == 1)
	{
		MessageBox.Show(@"1");
		msgbox = true;
	}
}

private void diagramView_MouseDown(object sender, MouseEventArgs e)
{
	if (msgbox)
		tbQuery.Focus();
	msgbox = false;
	... 

I hope that helps,
Stoyan