Xcode odd behaiviour and bad rendering code
Turn off the Show live issues checkbox in Xcode > Preferences > General (found near the top of the panel).

I was experiencing the same issue. I found this answer through Editor glitch when editing near end of long file in Apple's Developer Forums. 

It worked for me running Xcode 7.3 with a 2706 line file. The glitch happened around line 2250.