You can go to this Dojo to play around with this.
But, this happens because you seem to have 'selectable' set to true, so when you click an event, it focuses that event and jumps to its start.
Hold down the 'ctrl' button, then try clicking on the resize indicator of your long event where the top is out of view, then drag to resize while continuing to hold the 'ctrl' button. Release mouse, then 'ctrl' when you are finished. You should find that it did not jump when you used 'ctrl'.
This may have to be a 'gotcha' you put in documentation and or somehow inform your users about.