p. 99, end of first paragraph about backgrounds. "If the script of a button is empty, the group won't receive a mouseUp message"
Not exactly true: Buttons are created with an empty mouseUp handler:
which actually blocks the message. But if you remove this script and click "apply", or create a new script without a mouseUp handler, the mouseUp message will be passed along the message path normally.
But be careful! if you re-open the script of a button with an empty script, the empty mouseUp handler will be recreated ! To be sure that your button remains functionally "scriptless" (what can be useful in some circumstances) you have either to correct the default script with a "pass mouseUp" statement or to create a dummy scipt like