A closing note: Saimadhav Heblikar's initial patch on #17535 was a standalone proof-of-concept file was based on Canvas and Text subclasses.  The latter consisted mostly of two tk.eval(tcl_code) calls.  I rejected the idea of using tk.eval, as review and maintenance would requires knowing tcl.  But the submission won him my GSOC mentorship (and we did quite well working together, even though line-numbering was not finished).

Saimadhav's poc appears to have been inspired by Bryan Oakley's Canvas and Text-based answer a year earlier on https://stackoverflow.com/a/16375233.  But Bryan only used tk.call and tk.createcommand, not tk.eval.  Some of Saimadhav's tcl appears to be a translation of some of Bryan's python.

PR 2183 is a full patch also based on Canvas and Text and explicitly inspired by Oakley's answer.  However, the Text subclass is an edited version of Saimadhav's tk.eval version, not Oakley's.  This is properly closed as a duplicate, but it does provide the Oakley link, which does not appear on #17535.