Ilkka,
You can disable/customize the MovingInfoPopup content (information while taskbar drag and drop) like this,
Here is the C# code,
void fxgantt_Loaded(object sender, RoutedEventArgs e)
{
....
this.fxgantt.GanttChart.MovingInfoPopup = null;
}
Similarly, you can disable/customize the ResizeInfoPopup.
Thanks,
Raja.