Spinoff from #38673, about standard REPL, msg356271 (me) and msg356348 (Guido).  In the following interactions, no blank lines were entered.

3.9 behavior
>>> #a
>>> # a
>>>  #a
>>>  # a
 
>>> 
Mystery 1: why the blank continuation line?

I previously wrote
"ast.dump(ast.parse(' # a\n', '', 'single')) gives the same result, 'Module(body=[], type_ignores=[])', as without [space after #]".
Today, 3.8.10, 3.9.5, 3.10, and 3.11 say "unexpected EOF while parsing".

3.10 behavior
>>> #a
...  
>>> # a
>>>  #a
>>>  # a
...  
>>>
Mystery 2: why the new continuation line after '#a'?

3.11 behavior
>>> #a
>>> # a
>>> #a
>>>  #a
>>>  # a
...  
>>>
Mystery 3: why does the 3.10 regression for '#a' disappear?

Perhaps IDLE should handle initial blank lines itself, but I will investigate what codeop._maybe_compile is getting and doing in the different cases first.