I've clearly hit some hotkey in Excel, and have odd copy/paste behaviour now.
The green dotted line that appears around cells when you ctrl-C them is now permanent. Normally it disappears once you paste somewhere, or if you move to another cell and edit it, or hit delete in an empty cell. Now the only way I can get rid of it is to hit escape.
What have I done, and how can I revert to the "normal" behaviour?