The change is also in 3.6.6, released the same day as 3.7.0.  It was done intentionally, without, as far as I know, consulting Python doc users.  Users were previously free to select the max line width they wanted.

On https://github.com/python/python-docs-theme/issues/4, on April 26, Julien Palard noted that the change was made in sphinx-doc and was automatically incorporated into python-docs when the sphinx-doc version was updated.  So a new python-docs-theme issue would have to be opened to avoid or modify the change.  Issue 4 ended there, though I have added a note.

I see about what roelschroeven does, 80 char lines, but the 50 char line are pretty bad.

Anyway, this tracker is for patching the cpython repository.  This should only be left open if there is a possibility of patching the code that generates the chm file to, for instance, remove max-lines.

Side-notes: 'Python x.y Manuals' open full-screen.  I think part-screen would be better.  'Python 3.7 Module Docs' is duplicated under Python 3.7.

I believe a chm replacement could be written with HTMLParser and tkinter.  IDLE parses a subset of the html used in our docs to display a local (html-encoded) copy of https://docs.python.org/3/library/idle.html in a tk Text (without the sidebar).  The result is nearly identical to what I see in Firefox.  (At present, astral chars would have to be replaces by their escapes.)