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When I attempted to navigate within the 3D viewport by using the CTRL-numpad options, it didn't seem to work, and I noticed a message in the communication part of the main header that gave the usual warning sign (an exclamation point within a red edged yellow triangle), followed by the text "View offset is locked". The warning appeared and closed so quickly I was not able to obtain a screenshot.
I don't know what I did to cause this, and cannot find any information in the manual or other Blender documentation about this warning message. How do I unlock the "viewport offset"?