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We have clients in several industries still using old AS/400 5250 Terminals.  The software using the old IBM keyboard layout the control key is enter.

When we had rescue if your mouse was on the remote session all KB strokes were sent to the client.  I believe there was a button to turn that off at the top, but we never did.

We also have a lot of remote IP KVM / IMPI and certain keystroke within SC will not push on those as well.