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.