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I committed a patch that works well enough for release as it. I attached the diff for changed files in case anyone wants to view in Rietveld. 
Still to do for this issue:
* Automate getting the 2.7 and earliest 3.x idle.html copied to idlelib and whitespace-normalized. (Commit/push should stay manual.) Several lines have 1 to many blanks at the end, while the file lacks a final \n. See e66fbfa282c6. (These anomalies come from Sphinx; idle.rst has been normalized to be checked in.)
I have a .bat file that builds any and copies as needed. I will to add a pass through Idle's trailing-whitespace deleter, which did just what was needed to commit. (Patchcheck.normalize_docs_whitespace does a here unneeded comparision (always unequal), creates an unwanted and nuisance .bak, and does not appear to work when the last list has no '\n'.)
When I have automation working for me, I would like to add make targets for others to use (Zach's patch 4).
* Edit idle.rst
I will push NEWS and idlelib.NEWS entries separately, along with others for Idle.
I opened new issue #25198 for further viewer improvements (and bug fixes, if any).