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To continue and expand some things I said in various messages above...
The reason Shell does not and indeed should not have a secondary line prompt is that in Shell, '>>> ' means 'Enter a statement:' rather than just a line. Being able to enter, edit, and recall complete statements rather than just lines is an important feature of Idle's Shell.
For proportional fonts, I suspect the tab stops are every 8 em quads. A em quad (named after M) is square, so it is as wide as the font is high.
In msg151418 I suggested that the solution is to not mix prompts with user input, as is done. I gave the narrow prompt window as fix 3. I suggested as fix 1 to put statement prompts and output indications on separate lines. Here is a mockup example.
>>>:
def f(x):
 if x:
 print('got it')
 return 'something'
>>>:
f(3)
---
got it
>>>:
(The 'blank line' signaling 'end of statement' is intentionally suppressed.) This should be easy to do and, to me, looks pretty decent. The hesitation I had before is the copy for paste problem, but if that is a custom function, it would not be hard to reformat. A minor problem is the output could include '>>>:\n' or '---\n', but today one can do
>>> print(">>> def fake():\n\treturn 'def'\n")
>>> def fake():
	return 'def'
>>>