in the timeline panel you're dragging the scrollbar thumb?
No. I was using the "key" thumb. I'm not sure what it's really called, but it's the red thing that indicates which frame is displayed on the scene. But I need this key thumb more than the scrollbar thumb because I do animation, and it helps to drag that thumb left and right to test out the animation of the frames.
Actually, it looks like the problem fixed itself after I turned off the program and turned it back on again. That's a relief. But just in case the setting saved itself like that, how would I fix it?
i've never seen that problem, so hopefully restarting your flash pro will fix the problem, if it recurs.