The current Tk documentation contains this text:
Before Tk 8.5, the widget used the string “chars” to refer to index positions (which included characters, embedded windows and embedded images). As of Tk 8.5 the text widget deals separately and correctly with “chars” and “indices”. For backwards compatibility, however, the index modifiers “+N chars” and “-N chars” continue to refer to indices. One must use any of the full forms “+N any chars” or “-N any chars” etc. to refer to actual character indices. This confusion may be fixed in a future release by making the widget correctly interpret “+N chars” as a synonym for “+N any chars”.
I decided that this is the right place to make the change, in revised implementation "±N chars" is referring to characters, and not to indices.
Of course, this is not a bugfix, but rather – the implementation of an expected change.