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Just came back to my desk and Xcode keeps highlighting/underlining white space in files. If I type something it goes away, but if I switch tabs and come back, they reappear.
UPDATE: this isn't caused by the "show invisibles" option. That looks different (try it).
Go to Editor > Hide Invisibles in the toolbar. It's showing all the invisible characters in the document, including spaces.
You seem to have a search active. Look in your search bar at the top and you should find a " " character. Xcode is highlighting all instances of the matching space character.