I checked in a fix to the basic routine deciding which text to show in
the tooltip.  Please retry with latest master.

If the crash still occurs please proceed as follows:

(1) Try with `x-gtk-use-system-tooltips' nil.  This should tell
     whether GTK tooltip handling interferes with our routines.

(2) Try the attached diff.  When the crash occurs please do

     p old_len_0
     p new_len_0

     in the debugger and post the values it prints.  This should tell
     whether a timer is added while we construct the list for the copy.
     Undo the change after you're done.

Thank you, martin