I just tried editing a comment/answer/question on Chrome and while it correctly copies everything, it doesn't highlight the entire copied text.
The first three lines don't get highlighted!. Similar issues with my comments. It works on Safari though...
This was happening on different forums. I also can't successfully highlight multiple lines using my keyboard or mouse
I'm on macOS
10.15.3 and chrome
Screenshot for editing question
Screenshot for editing my comment exactly for this question
I can no longer reproduce this on chrome. Seems that the problem has gone away.