Text Editor Graphical Glitch on Xcode 6.4
Go to Preferences->General tab and uncheck "Show live issues" under Issues. Notice that this is a work around to be able to continue working with Xcode, at least until they issue an official fix. This glitch seems to be noticeable when you have lots of lines of code (1000+).

After this, the glitch will still be happening but not while coding but during building phase. Not perfect but at least you will be able to continue working.