1. Create a new message
2. enter an address in the 'To:' field
3. Realise you don't want to send it to them, but only to CC them.  Decide to copy the address to clipboard, to save it for later.
4. Select the whole address and right-click

Doing so de-selects the address and selecting "copy" from the pop-up menu instead copies a blank string into the clipboard.

Selecting the address and middle-clicking on another field (to past the address) also fails to past the address, instead pasting nothing.

ctrl-C and the menu entry at the top of the message still work, just not the right-click menu or the middle-click option.