Overlays, nobreak-char-display and NO-BREAK SPACE

[Romain Francoise]

A Debian user reported that inserting the entity corresponding to the
Unicode character "NO-BREAK SPACE" in nxml-mode reliably freezes Emacs.
Indeed I can reproduce this here: when `nobreak-char-display' is non-nil
(which is the default), then inserting this entity in a nXML buffer has
strange effects.  I took screenshots to better demonstrate the problem:

nxml_nbsp.png
Description: PNG image

nxml_nbsp_nok.png
Description: PNG image

Trying to move point in the overlay area makes Emacs go in an infinite
loop, C-g sometimes interrupt it, and sometimes not.  Unfortunately,
trying to catch it in gdb is impossible, the bug appears to trigger a
problem in the debugger:

| (gdb) bt
| Cannot fetch general-purpose registers for thread -1486281024: generic error
| (gdb) The program is running.  Exit anyway? (y or n) y
| Quitting: thread_db_get_info: cannot get thread info: generic error

I guess the new feature to display non-breaking spaces doesn't work well
with overlays.  Can someone who knows help me debug this problem?

Thanks,

-- 
Romain Francoise <address@hidden> | The sea! the sea! the open
it's a miracle -- http://orebokech.com/ | sea! The blue, the fresh, the
                                        | ever free! --Bryan W. Procter