mmm, i’m on OS X and it looks like there is an offset with mouse capturing.
It can be see in ted2 with the dockers/separators and buttons where:
hovering with the mouse (on the uptown docker) the docker activates when the mouse is not over the left side and deactivates before the mouse has left on the right – try it and see
Or the horizontal one the behaviour is the same E.G.
Activates when the mouse is above (not in) a control button/docker etc and deactivates before the mouse has left the control
 
Has anyone else seen this? it is about 2 pixels off in all directions (OS X)