thick horizontal red line has appeared across the calc page

I cannot get rid of a thick red line that suddenly appeared where before it was the usual thin horizontal black cell wall,
It runs ad infinitum.

Perhaps you accidentally pressed the keyboard shortcut Ctrl+Shift+C. This turned on the change recording mode.
After that you copied and pasted the whole row - now Calc shows where this action was. Just choose Edit-Track Changes-Record and confirm that you do not want to continue recording.

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