The new version (attached) is fast enough all the time in every real module
I have <whew!>.  You can make it slow by, e.g., creating an open list with
5,000 90-character identifiers (+ trailing comma) each on its own line, then
adding an item to the end -- but that still consumes less than a second on
my P5-166.  Response time in real code appears instantaneous.

Fixed some bugs.

New feature:  when hitting ENTER and the cursor is beyond the line's leading
indentation, whitespace is removed on both sides of the cursor; before
whitespace was removed only on the left; e.g., assuming the cursor is
between the comma and the space:

def something(arg1, arg2):
                   ^ cursor to the left of here, and hit ENTER
               arg2):   # new line used to end up here
              arg2):    # but now lines up the way you expect

New hack:  AutoIndent has grown a context_use_ps1 Boolean config option,
defaulting to 0 (false) and set to 1 (only) by PyShell.  Reason:  handling
the fancy stuff requires looking backward for a parsing synch point; ps1
lines are the only sensible thing to look for in a shell window, but are a
bad thing to look for in a file window (ps1 lines show up in my module
docstrings often).  PythonWin's shell should set this true too.

Persistent problem:  strings containing def/class can still screw things up
completely.  No improvement.  Simplest workaround is on the user's head, and
consists of inserting e.g.

def _(): pass

(or any other def/class) after the end of the multiline string that's
screwing them up.  This is especially irksome because IDLE's syntax coloring
is *not* confused, so when this happens the colors don't match the
indentation behavior they see.