[ESS] mouse-selecting previous line in *R* buffer

[Stephen Eglen]

 > 'mouse 2: insert after prompt as new input'
 > 
 > I'm a bit anti mice. Nevertheless I was trying to use the thing to
 > copy some text from that line, and the light green message stuff
 > prevented me from selecting the text. I'm not at all claiming that
 > this shouldn't be default behaviour, but how do I turn this
 > functionality off?

That seems to come from comint.el, which is part of Emacs, rather than
ESS:

in comint.el:

	      (unless comint-use-prompt-regexp
		;; Give old user input a field property of `input', to
		;; distinguish it from both process output and unsent
		;; input.  The terminating newline is put into a special
		;; `boundary' field to make cursor movement between input
		;; and output fields smoother.
		(add-text-properties
		 beg end
		 '(mouse-face highlight
		   help-echo "mouse-2: insert after prompt as new input"
		   field input))))

Turning it off might be tricky -- I think we rely on
comint-use-prompt-regexp being t for other reasons.

Stephen