This is a problem that has been going on since we moved to the new OpenTK version (1.0) and I haven't been able to fix it since. When I try to resize my game window from the top, the Window all of a sudden resizes down. It feels like the system cursor is automatically gets moved back inside the game window, resulting in the title bar being dragged down and hence the window resized down. Because I can't really explain it, I made a little video showing: http://youtu.be/uvFzta6COYw
I personally can not reproduce it very reliably, but following the procedure seen in the video (compile, open another window and then resizing the top side of the game) seems to cause it.
We don't really do anything special with the mouse, except for subscribing to mouse movement, so we can hide the cursor after a short while of inactivity. Removing this event subscription does not resolve the bug.
Current version: 1.1.1
Operating system: Windows 8 Ultimate 64-bit