Trouble with mouse location on pictures

[kamocat]

I'm trying to use the mouse to draw rectangles on a Picture indicator.
However, the mouse coordinates are not corresponding consistently to the Rectangle coordinates, and I can't seem to make sense of it.

Here's a video demonstrating the problem.
http://screencast.com/t/Yzg1YTViYWUt
While you can't see whether I'm clicking or not, every short diagonal movement is a click-and-drag action.
(In one of them, I believe the rectangle it made was actually outside the picture).


Is there anything wrong with my code?

(Note:
I am using the Picture control, not at IMAQ image. This code is not intended to run on the cRIO, and probably won't be usable within the FRC Framework)