> 1) trailing whitespace (' ' and '\t' at least) is removed before this function is called.  I presume in IDLE rather than code.II, but cannot find where.  It is not with .rstrip.

You're probably looking for this code in EditorWindow.newline_and_indent_event():

# Strip whitespace after insert point.
while text.get("insert") in " \t":
    text.delete("insert")

See: https://github.com/python/cpython/blob/89e50ab36fac6a0e7f1998501f36fcd2872a6604/Lib/idlelib/editor.py#L1390