#15141, which I will close as a duplicate, makes the same request (for the edit window). Roger Serwy notes that his extension package
http://idlex.sourceforge.net/extensions.html
has horizontal.py, which does what is requested. He is willing for that (or other) extension(s) to be bundled with idle for default or optional activation. I presume that is done in some .cfg file. I re-opened for that purpose.

(Note: it would seem nice to be able to get a list of extensions available and those enabled from the menu. Or is there a reason not too?)

Some arguments for h. scroll:
1. The shell window is a print output as well as code input window. Output should not necessarily be limited to 80 chars. With variable pitch font, that does not even make much sense.
2. Edit windows are not limited to python code. Else they would *always* syntax hilight. Or Pycode might be embedded in other formats. If #6858 were implemented, h scroll would be more useful.
3. Not everyone agrees with the 80 char limits.
4. People sometimes edit a file written by someone else.

Counter-arguments:
1. A full screen window allows about 200 chars or more, which is enough for any code entry and most print output and most 'files written by someone else'.
2. For exceptions, put cursor on long line and use => arrow key.