23.0.60; Disappearing cursor over composed char

[Thomas Morgan <tlm@thomasmorgan.net>]

I have a utf-8-unix-encoded file named `norgard' which contains
the following two lines:

Rilke Sings Nørgård
Nørgård

The `å' in `Nørgård' is composed of ASCII `a' and Unicode `̊'
(combining ring above), the second of which appears in my Emacs
(in X Windows) as a hollow box.  The composed char is displayed
as a normal ASCII `a'.  Visited literally, the file looks like this:

Rilke Sings N\303\270rga\314\212rd
N\303\270rga\314\212rd

But visited in utf-8-unix with auto-composition-mode enabled,
the cursor behaves strangely.  When I move the cursor to the `a'
on the first line with `C-f' or `C-b', the cursor disappears.
It reappears when I move the cursor away from the `a' with any
cursor motion command.

Moving the cursor to the same `a' with `M-x goto-char 17'
causes both the cursor and the character to disappear
(it is displayed as an empty space).  `C-l' makes the `a'
reappear, but the cursor remains invisible.

The cursor behaves normally with the `a' in the second line.

I also tried viewing the same file with `emacs -Q -nw norgard'
in an xterm.  Both instances of `Nørgård' appear as `Nørgrd  ',
with a diacritical ring above the `g' and two spaces after the `d'.
With the cursor over `ø', `C-f' moves to the `g', skipping over
the `r'.

I'm using GNU Emacs 23.0.60.3 (i686-pc-linux-gnu, GTK+ Version 2.8.20).