23.3.4 Properties in the Mode Line

Starting in Emacs 21, certain text properties are meaningful in the mode line. The face property affects the appearance of text; the help-echo property associate help strings with the text, and local-map can make the text mouse-sensitive.

There are three ways to specify text properties for text in the mode line:

1. Put a string with the local-map property directly into the mode-line data structure.
2. Put a local-map property on a mode-line %-construct such as ‘%12b’; then the expansion of the %-construct will have that same text property.
3. Use a list containing :eval form in the mode-line data structure, and make form evaluate to a string that has a local-map property.

You use the local-map property to specify a keymap. Like any keymap, it can bind character keys and function keys; but that has no effect, since it is impossible to move point into the mode line. This keymap can only take real effect for mouse clicks.