Entering 'pass' or a completely blank line results in a new primary prompt, at least on Windows. The Windows REPL otherwise prints ... even for effectively blank lines.  IDLE usually prints a new prompt for effectively blank lines.

>>> 
>>> #a
>>> # a
>>>  #a
>>>

I agree that these look better.  This behavior comes from code.InteractiveInterpreter and ultimately codeop.

def _maybe_compile(compiler, source, filename, symbol):
    # Check for source consisting of only blank lines and comments
    for line in source.split("\n"):
        line = line.strip()
        if line and line[0] != '#':
            break               # Leave it alone
    else:
        if symbol != "eval":
            source = "pass"     # Replace it with a 'pass' statement

As noted above, 'pass\n' is treated the same as '\n'

The first line above originally had a space, but IDLE appears to strip trailing whitespace also, even outside of comments.  (For an ending '\ ', this prevents SyntaxError, but maybe this is a bad lesson for beginners.)  However, I did find a case with an unnecessary continuation line.

>>>  # a
 
>>> 

This puzzles me, as it should be treated exactly the same as without the space after '#'.  ast.dump(ast.parse(' # a\n', '', 'single')) gives the same result, 'Module(body=[], type_ignores=[])', as without.