Has something recently changed with Answer and Comment fields? Pasting text into them has suddenly become very unreliable. Sometimes, after selecting text, I need to mouse-2 several times before the text is pasted into the edit field.
I don't know if it's related, but spelling fixes also seem to suffer the same problem - selecting a replacement spelling has no effect most of the time.
I've noticed a recent style change - a pale blue border when the field is focused. The times when the input is ignored seem to be when the border is not shown.
Browser: Iceweasel 52.9.0 (i.e. Firefox 52.9.0esr-1) on Debian "testing".