With the OS X Cocoa backport of Tcl/Tk 8.5.12 (as seen in ActiveTcl 8.5.12 for OS X), attempting to use clipboard Copy with a text widget causes an immediate crash in the interpreter. This is a serious regression from 8.5.11.
See discussion on tcl-mac list beginning here:
https://sourceforge.net/mailarchive/message.php?msg_id=29646437
Thanks for the report, bug fixed.
Clearly more testers are needed for the branch. Especially around the
time of releases. Please subscribe to tcl-core@lists.sf.net.