> So this issue doesn't occur when linked with a thread-aware Tcl/Tk, 
right? Maybe we should just make sure that's the only configuration we 
ensure?

This would break compatibility, including some usage patterns (see the 
"Reuse the threaded versions" option above).
Can this actually be considered for anything short of the next major 
release?
While official Windows releases use a pocket version, POSIX ones would 
probably link against whatever a distribution provides.