gitk's copy pasteboard doesn't persist after it quits

On OS X, if I load gitk, copy a sha1, then quit, the sha1 isn't put  
into the system-wide pasteboard.  It's definitely copied - I can paste  
it back into the sha1 field - but it seems to be some sort of local  
pasteboard that's specific to gitk

If I switch to another app, the sha1 is stored in the pasteboard  
correctly, and I can then quit gitk and still have it available.  I'm  
guessing that gitk (or Tcl/Tk) is syncing with the system-wide  
pasteboard on focus change, but not on quit.

I'm using the version of gitk in 388afe7881b, and Tcl 8.4.7


Any suggestions on fixing / working around this?

Jon