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word.tcl in Tcl library contains following lines:
if {$::tcl_platform(platform) eq "windows"} {
 # Windows style - any but a unicode space char
 set ::tcl_wordchars {\S}
 set ::tcl_nonwordchars {\s}
} else {
 # Motif style - any unicode word char (number, letter, or underscore)
 set ::tcl_wordchars {\w}
 set ::tcl_nonwordchars {\W}
}
So by default all works as expected in Motif style, but not in Windows style.
If you want to have same behavior in both styles, defines word chars as:
 tk.call('set', 'tcl_wordchars', r'\w')
 tk.call('set', 'tcl_nonwordchars', r'\W')
GUI tests are not needed, it is enough to test relevant Tcl commands: tcl_wordBreakAfter, tcl_wordBreakBefore, tcl_endOfWord, tcl_startOfNextWord, and tcl_startOfPreviousWord or TextSelectTo. It's interesting, there are no tests for these functions in Tcl test suite.