bug#1453: 23.0.60; Dead keys do not work in dired and similar modes in Cocoa port

[Alan Third]

Wolfgang Lux <wolfgang.lux@gmail.com> writes:

> Dired mode binds ~ to dired-flag-backup-files. When one attempts
> entering
> ~ via the dead-key combination Alt-n (assuming the Alt key is not used
> as
> a modifier key and Command is interpreted as Meta key), Emacs first
> reports
> an error message "Buffer is read-only: #<buffer ~>" followed by an error
> message "Wrong type argument: overlayp, nil" upon the next event.

This is still sort of an issue in Emacs 25. As far as I can tell Emacs
still gives the "buffer is read-only" message, but as soon as another
key is hit actually executes dired-flag-backup-files.

I also suspect this won't be limited to the NS port, but will be an
issue anywhere you can use dead-keys in a read-only buffer.

I can't think of any reason to try and display the partially complete
input when the buffer's read-only, but I can't see where this dead-key
entry code is located to see if it's possible to do a check.
-- 
Alan Third