Windows 10 activate window by mouse hover delay?
Rename your registry value from ActiveWndTrackTimeout to ActiveWndTrkTimeout. Then sign out from Windows and sign in again.
It seems that this name changed in Windows 8.1.
I had this problem when I upgraded from Windows 8 to Windows 10, and after searching for a while, I eventually found the answer on Super User, so do vote up over there if this helps you!
Also see the Microsoft TechNet reference.