If zooming using the Shift-key and dragging a rectangle, the zoom behaves incorrectly. I've found that the method MapBox.GetImagesAsyncEnd zeroes out the variable _dragEndPoint. This will make the zoom calculations in OnMouseUp to produce the wrong result. When removing the code which change _dragEndPoint the zoom works as expected. I've tried to check everywhere the variable is used (only in OnMouseDown, OnMouseMove and OnMouseUp) and it seems like it would be safe to remove the line in GetImagesAsyncEnd, but I might be missing something.

The error can be reproduced in the WPFSamples application.