bug#9543: 23.3.50; help-echo property

[Leo]

1. Emacs -Q

2. eval
  (put 'x 'risky-local-variable t)
  (setq x '(:eval (propertize "x" 'help-echo (message "Wow"))))
  (push '("" x) global-mode-string)

3. Move point around and check how many Wow's are printed in *Messages*
   buffer

The form (message "Wow") is eval'd each time mode-line is updated even
though one didn't move mouse over the 'x' string on the mode-line.

If the help-echo property is a function, it is only called when one
moves the mouse over 'x'.

Why this difference?

Leo