To reproduce: run the sample project and follow the illustrates steps from the gif file.

Workaround: close the tooltip programmatically:

        private void radScheduler1_AppointmentMouseDown(object sender, SchedulerAppointmentMouseEventArgs e)
        {
            if (tooltip != null)
            {
                tooltip.Hide(this.radScheduler1);
            }
        }

        ToolTip tooltip = null;

        private void radScheduler1_ToolTipTextNeeded(object sender, Telerik.WinControls.ToolTipTextNeededEventArgs e)
        {
            AppointmentElement app = sender as AppointmentElement;
            if (app != null)
            {
                StringBuilder sb = new StringBuilder();
                for (int i = 0; i < 10; i++)
                {
                    sb.AppendLine(app.Appointment.Summary + i + " " + app.Appointment.Summary + i + i + " " + app.Appointment.Summary);
                }
                e.ToolTipText = sb.ToString();
                tooltip = e.ToolTip;
            }
        }