codemirror editing: right click to copy in firefox often doesn't work

[williamstein]

If you try to use the right mouse button to copy some selected text in the codemirror editor in SMC in Firefox, it doesn't work (unless the mouse is over a point with no text).

Observations:

   - only a problem on firefox; nowhere else.
   - not a problem in codemirror itself -- something is messed up by some other library in SMC.
   - what's happening is somehow the div with the text has nothing in it selected, so when user right clicks,
     they don't get a context menu.  If we move the selection div to front with z-index>2 (in codemirror source)
     and make opacity like .4, then it works fine, but of course looks silly with the font color
     changing.
   - not bootstrap
   - not a css problem.
   - not a $(document).click problem.
   - giving up for now.