This seems to be a problem only when the image displayed is much bigger than the window, so it may only be a minor problem. For example, if an image displayed at 100% in a namedWindow has width 3000 pixels and is much wider than the screen size, then OpenCV will open a window much smaller than this width. And the full width of the image will not be displayed in the namedWindow. However, setting up a callback, the coordinates from CV_EVENT_MOUSEMOVE seem to be incorrect. Maybe the coordinates from the mouse event gets scaled even though the namedWindow's width is not the same as the image width.
Perhaps this HighGUI portion of OpenCV is already considered deprecated? I'm using version 2.4.3 on 32 bit Windows with Visual C++. Thanks.