Why does my GUI.Window dragging behaviour act erratically on a rotated window?

I am using a GUI.Window, rotated using GUIUtility.RotateAroundPivot(). I am also calling GUI.DragWindow inside the window content function to allow dragging of this window.

The window draws exactly how I want it, and can be dragged around fine most of the time. However, when the line is near vertical, the dragging turns into a massive spiral where the window quickly jumps way out of the screen.

What am I missing here? relevant drawing code is below

 function Draw() {

     var oldMat = GUI.matrix;

     GUIUtility.RotateAroundPivot(rotationAngle, Vector2(rect.x + rect.width/2, rect.y + rect.height/2));

     super.Draw();

     GUI.matrix = oldMat;

 }

super.Draw calls:

 function Draw() {

     rect = GUI.Window (winID, rect, WindowContent, GUIContent.none, GUIStyle.none);

 }

and the relevent WindowContent Function is:

     function WindowContent(windowID : int) {

     if(annotater.currentState == toolState.nothing) {

         if(IsInFocus()) {

              GUI.DragWindow();

         } else { 

             if(GUI.Button(Rect(0,0,rect.width, rect.height), GUIContent.none, GUIStyle.none)) {

                 annotater.SetCurrentFocus(this);

             }

         }

     }

     GUI.DrawTexture(Rect(0,0, rect.width, rect.height), GetTexture());

 }