This:

    socket.onmessage = function (event) {
      if (typeof event.data == 'string') {
        // control frame
        if (event.data == 'profile is next') {
          socket.binaryType = Blob;
          socket.addEventListener('message', handleGettingProfile, false);
        } else if (event.data == 'game data is next') {
          socket.binaryType = ArrayBuffer;
          socket.addEventListener('message', handleGettingGameData, false);
        }
      }
    };

    function handleGettingProfile(event) {
      handleGettingProfile(event.data); // passed a blob
      event.target.removeEventListener(event.type, handleGettingProfile, false);
    }

    function handleGettingGameData(event) {
      handleGettingGameData(event.data); // passed a blob
      event.target.removeEventListener(event.type, handleGettingGameData, false);
    }

handles cases like:

   "profile is next"
   profile [handled]
   profile [gets ignored]
   "game data is next"
   "profile is next"
   profile [handled]

...without blinking. On the other hand, this:

    socket.onmessage = function (event) {
      if (typeof event.data == 'string') {
        // control frame
        if (event.data == 'profile is next') {
          socket.responseType = Blob;
          socket.addEventListener('message', handleGettingProfile, true);
        } else if (event.data == 'game data is next') {
          socket.responseType = ArrayBuffer;
          socket.addEventListener('message', handleGettingGameData, true);
        }
      } else {
        // happens after the other handlers because they are capture handlers and capture gets called first
        socket.responseType = String;
      }
    };

    function handleGettingProfile(event) {
      handleGettingProfile(event.data); // passed a blob
      event.target.removeEventListener(event.type, handleGettingProfile, true);
    }

    function handleGettingGameData(event) {
      handleGettingGameData(event.data); // passed a blob
      event.target.removeEventListener(event.type, handleGettingGameData, true);
    }

...would result in the following:

   "profile is next"
   profile [handled]
   profile [gets ignored]
   "game data is next"
   "profile is next" [gets treated as game data somehow, though i'm not sure how to convert text to arraybuffer]
   profile [ignored]