>> If the crash still occurs please proceed as follows:
 >
 > I checked with master and the crash no longer happens.

Thanks for checking and for reporting this bug in the first place.  It
revealed an awfully silly, costly and completely unnecessary way of
implementing the desired behavior.

Yet, I'd still like to know why and how that crash happened.  So if
you have a few minutes to spend, please shortly undo the fix and try
the two things I asked in my other mail, namely to:

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

(2) Try the attached (again) 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.

Thanks again, martin