MouseState.Button out of date

MonoGame stops processing mouse events when the game is not active, leading to the mouse buttons being 'out of date' when the game window becomes active again. 

Fix is to reset to current actual state on window activate.
According to MSDN, Control.MousePosition is defined in screen coordinates.
You need to convert it to client coordinates:

var position = Control.PointToClient(Control.MousePosition);


http://msdn.microsoft.com/en-us/library/system.windows.forms.control.mouseposition%28v=vs.110%29.aspx
http://msdn.microsoft.com/en-us/library/system.windows.forms.control.pointtoclient%28v=vs.110%29.aspx