>> ... and what is a "standard event" in this context? And why can a
>> toolbar button not react to such a standard event in the same way?
>
>
> Initialize tab, hide tab, show tab, kill tab, at least.
I don't understand that yet. For me an event is a user action like
hitting a key or moving the mouse. The type of that action and any
additional information are communicated by the OS to Emacs which reacts
to the event by executing some action (or doing nothing). Initializing
or hiding a tab is in my opinion an action performed by Emacs itself,
possibly in reaction to an event or triggered by a timer.
In graphocal contexts , like GTK / motif programming, the concept of 'event has almost nothing to do with the concept of UNIX signal.
Initializing a tab is not an action performed by emacs, but it is what you define it to be.
In gtk for example, if you insert a gtk-notebook widget in emacs, you write the callback functions in C , such that they call elisp function of that signals.