Thanks Ilian. I would never have guessed that one! It takes an expert... ;-)

In case anyone makes this change and then wants to go back, you can recover the original behavior by executing:

SetOptions[$FrontEnd,  PrivateCellOptions -> {"EvaluationUnmatchedStyle" ->  "EvaluationUnmatched"}]

It's also worth noting that the "EvaluationUnmatched" style is defined in the Core.nb StyleSheet.

I am not particularly annoyed by the feature. For some perspective, this is associated with the new sophisticated multiple undo/redo facility... read the following thread to learn about what went into this:

http://community.wolfram.com/groups/-/m/t/290130

The functional reason for the greying out of the output when one edits the input is to designate visually that the output is no longer associated with the revised input. You will notice that it actually is quite clever--if you make edits to the input that has no effect on a possible output (e.g., adding some spaces) the output does not grey out.

But it would perhaps be nice in a future update to have the option of changing this behavior -- perhaps choosing the color of the change (I cannot find an option in the Option Inspector that controls this though there may be one...) or perhaps to modify the color of the output cell's cell bracket,...

I do that too often anyway. This is how my files grow large and become unmanagable.

Is there no one else who is annoyed by this new "feature"?