Apple Care Support and 2 Finger Swipe in Safari not Working Issue

Today I called Apple Care support to resolve my iMac issue.  The issue is that two finger swipe did not work in the past weeks. When I received this iMac,  I transferred all user accounts from TM to this computer. It was working fine.

BB uses this computer most of time. One day she told me that two-finger-swipe stopped working. I tried to check system settings and could not find anything wrong. I found that other users are OK. Only BB's account has this issue.

I googled web, and found one solution from Apple discussion forum. One person phoned Apple care and got a solution: delete com.apple.systempreferences.plist from user account's Library/preferences folder. I tried but it did not working.

Then I decided to give Apple a call.  I tried twos ago.  The working hours is 8:00am-9:00pm Extern Time.  I missed twice when I remembered to call after work.  I could ask BB to call but I would rather to get experience from Apple Care Support.  Today I made a call once I got home.

A lady answered my call.  It sounds like that she has not much experience with this issue. She told me that  she is going to check with senior manager. I waited 5 minutes. When she was back, she told me that she had a solution. This one is actually as same as the one I found. It did not work. I even tried to reboot iMac after removing the plist file. She went back to her line again asking her manager.

Then the manager was on the phone.  We tried various ways. Including reboot by holding option key, unplugged power cord for 1 minutes, restarted by holding option+command+p+r.  None of those was working.

He admitted it is a really rare issue. I gave all my story about the issue. He said that it must be something screwed up in this account. Finally, after he thought it for a while, he asked me to pull the plist file of Safari out of Library/Preferences.  When it is out, two-fingure -swipe is back to normal after Safari is restarted. It is Safari plist file that is corrupted!

The support call took about 1 hour. I am happy with the result.

After that, I further approve it is the Safari plist issue.  I put the systempreferneces plist back. It is fine. When I put the trouble Safari plist back, two-finger-swip stops working.

Here is the corrupted plist file.  Does any one know what is wrong in the file?