Hi Ed:
I think I’ve found the solution.
Apparently, in timeline setting, there is a checkbox labeled “position bar”
I used to have it checked.
What it does is showing a whole timeline at the very bottom (looked like ruler measurement) to allow you to park your cursor anywhere on that bar to be able to jump where you want to be in timeline.
and apparently, make your magic mouse horizontal movement doing scaling as well. (for some reason I wouldn’t understand.)
After I removed the box, my scrolling on timeline function restored.
I feel like I can breathe again. lol