Why does my safari show white line at top?

I'd start by making sure you have the latest updates applied. Go to the Apple Icon on the finder menu and select 'Software Update' when connected to the internet. If you find no updates were needed or you have applied them all and the problem is still present do the following:

Try flushing out your caches and then try resetting everything. WARNING! if you have any saved user accounts/passwords and/or URL's (other than bookmarked) you'll loose them in the reset. If that doesn't clear things do the following:

Go to your system preferences and select 'Accounts' create a new account on your system then log on to your system using this new account. Are you able to see the problem in Safari now? If not your original account files have been corrupted in some way. Depending on what you have on your system you have two different ways to fix this.

1 - Try using a system cleaner app to clear out any junk and also use a Antivirus app that can locate and remove any malware or trojans that may be present. If that doesn't do it go to step 2

2 - If you have setup the new user account as a manager then you could delete the other user and then create a new one. If you do this you must make a backup of your data as you 'll need to delete this users files. Once you create a new user you can copy back your stuff. If you installed any apps that were tied to the user account you'll need to re-install them (It better to install them so all users can access them that way you won't loose them).

A smart thing to do is setup two user accounts one you use for day to day and the other only when you need to install apps. This approach isolates the Admin functions from your day to day access and allows you to delete the day to day user when it becomes compromised without having to rebuild things. The only trick here is making a shared user account that holds your data so you don't need to reinstall all your stuff and don't forget keep your antivirus up to date and check your files often