title: Xcode 6.3 shows random files instead of counterparts in assistant editor
body: <p>I always have my Xcode assistant editor open with <strong>counterparts</strong> option. It's very handy to have an *.m file opened at standard editor while having <em>corresponding</em> *.h file in assistant editor.</p>
  
  <p>After recent update to version 6.3 Xcode started to show <strong>random</strong> instead of <strong>corresponding</strong> headers quite often. Several clicks to "four squares" icon and mode usually brings it back (but it's very annoying).</p>
  
  <p>Do you experience this too? How to fix it?</p>
  
  <p>Note: deleting DerivedData didn't helped</p>