When you are in white type field - a simple right-click, to copy, doesn't work: I have never known it to work?
The workaround that I have been using for ages is the usual keyboard [control] X, C, or V [for windows] or my browser's 'EDIT' commands (as har_3100 said).
Weird isn't, LOL. Outside the white text field, I can copy/paste until my fingers are full of UHU.
Maybe the script assumes you don't need to copy anything - since you
are given a 'reply' and 'quote' option
and afterall thats all you are supposed to be doing (sic).
.