Stack Exchange seems to be using the new CSS property
display:contents in a way that interferes with copying the text of comments. (Using Firefox 58.0.2 or Chromium 64.0.3282.167 on Mint Linux)
If you highlight multiple comments, then copying them to clipboard doesn't work anymore. If I Ctrl+C on this selection of text, then my clipboard has only a single space character in it.
If I toggle-off the
display:contents property, then copying works normally but it explodes visually.
display:contents is an experimental new CSS feature
I don't really understand what it is meant to be used for, but I don't think this text-copying-prevention is the intended purpose (either from W3C or hopefully from Stack Exchange!).