When clicking on a row, the selection wasn't set properly to a non-empty area, and this was required to trigger the "copy" event of the browser. Fixed.
The user needs to wait for 2 seconds before copying a row. The reason is unknown. It could be a Confluence script which interacts with the event and delays it. If the user copies before waiting, nothing will be copied into the clipboard.
It sometimes happens that copying a row (notably in IE) and pasting it in Excel doesn't paste anything.