Hey. Well it looks like a bug. Really you can only do the basic troubleshooting here.
1) Quit and reopen XCode
2) Restart machine (could be a very odd graphics glitch)
3) Try opening the file in a different window from XCode
4) Clear your XCode preferences by deleting/renaming ~/Library/Preferences/com.apple.Xcode.plist and reopen XCode (have it closed before you do this)
5) Report it as a bug if it is repeatable and nothing resolves the issue
Sorry, its just one of those things. Not really something you can do about it from your end.