File name completion glitch

[Chong Yidong]

The new partial completion style completion in Emacs 23 needs to have a
few more glitches worked out before the release.  Here's one problem
that I've observed (similar complaints have been reported before):

When I type `o' in Gnus to save a Mime attachment, I get a minibuffer
prompt like this:

  Save MIME part to: ~/_foo.txt

where _ indicates the position of the cursor.  Suppose I want to save
the file to a directory named `mystuff' in my home directory.  In Emacs
22, I can use completion:

  Save MIME part to: ~/mys_foo.txt
  [TAB]
  Save MIME part to: ~/mystuff/_foo.txt

In Emacs 23, I get this:

  Save MIME part to: ~/mys_foo.txt
  [TAB]
  Save MIME part to: ~/mys_foo.txt [No match]

This happens because the TAB now takes into account the minibuffer
contents after the cursor.

Stefan, can you suggest a way to fix this problem?