It's easy to find a tutorial on changing the delay time of taskbar preview like this one:
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while it's a little hard to find the answer to my question.
In fact, when your mouse pointer moves out of the video display region, the seek bar(or navigation bar? forgive my poor English) disappears in one second or two, which is kind of reasonable.
But when your mouse pointer is inside of it or you're viewing in full screen mode, it takes maybe 10 seconds to disappear.
Also the trigger region for showing the seek bar is not only where the seek bar resides, but the entire video display region, which is kind of annoying.
Why WMP12 does have a seek bar trigger mode smart enough just like what MPC's seek bar does, and why MPC just never have all the fancy looking of WMP12? orz