In the message above, I asked "why this code runs in 3.x but eventually fails in 2.x?".  The answer is almost certainly that I used 3.5 with tk 8.6 compiled *with* thread support and 2.7 with tk 8.5 compiled without thread support.

Serhiy, if you use a system supplied 8.6 with thread support for all Python versions, that would explain why these thread examples work for you with all Python versions.  If non-thread tk works for you, then the code in _tkinter.c works better on Linux (where it must have been developed) than Windows.

The original failure report duplicates other similar reports.  My previous responses, including proposed doc change were incomplete, as I only learned about the tcl compile option a month ago.  I opened #33479, with more extensive proposed tkinter doc changes, in favor of this.