bug#11226: format-mode-line, %z, mode-line-mule-info, and unibyte

[Glenn Morris]

Package: emacs
Version: 24.0.95

mode-line-mule-info contains this:

(propertize
 "%z" 'help-echo [...]
 (format "Buffer coding system %s\nmouse-1: describe coding system"
  (if enable-multibyte-characters
       (concat "(multi-byte): "
                    (symbol-name buffer-file-coding-system))
                       (concat "(unibyte): "
                                  (symbol-name buffer-file-coding-system))))

The intent is obviously that you see (multibyte) in the mode-line
tooltip in a multibyte buffer, and (unibyte) otherwise.

However, you will never see the unibyte tooltip, because:

(set-buffer-multibyte nil)
(format-mode-line "%z")
    -> ""

ie, the empty string, so there is nowhere for the help-echo to display.
Compare with "-" in multibyte buffers.

If it returned " " instead of "", you would see the tooltip.