Steps to reproduce:
1. In System Settings -> Window Behavior -> Window Behavior -> Focus ->
set Policy to "Focus Follows Mouse"
2. Open Dolphin and go to any video file
3. Open that video in SMPlayer
4. Press 'f' key to turn on fullscreen mode
5. Enjoy focus stealed from SMPlayer by Dolphin window. You have to use
alt+Tab or click your mouse to control SMPlayer

This bug seems to be quite unpredictible. Sometimes if you switch
workspaces between opening Dolphin and playing video, SMPlayer does not
loses focus. SMPlayer seems to be working just fine when run from
terminal as well. But above steps gives me that bug each time I have
tried.

I'm not really sure if it is bug in KDE or SMPlayer. On Fluxbox
(with session.focusModel set to "MouseFocus", which is quite the
same as KDE's "Focus Follows Mouse") I was not able to reproduce that
bug. Hence, I file it against KWin.

I have found similar problem on SMPlayer's bugs tracking system, but it
slipped without any attention:
<http://sourceforge.net/tracker/?func=detail&aid=2997281&group_id=185512&atid=913573>

Best regards
Mirosław Zalewski