Docked Web Inspector doesn't track properly once the mouse has left the window.
Click to drag the resizer of the web inspector. Shrink it by moving towards the bottom of the window.
Once you move the mouse pointer off the bottom of the window, then start moving back up, the resizer starts growing again, even before the mouse pointer reaches the window.
Created attachment 49245 [details]
Screencast showing the problem
*** This bug has been marked as a duplicate of bug 22263 ***