>  This is not easy to figure out since when a debugger gets opened, the part
> of the stack used to open it is discarded (how?).

Error>>#defaultAction raises a new exception, the UnhandledError.
UnhandledError has an instance-variable to remember the original
exception.

UnhandledError opens the debugger as part of the #defaultAction. The
debugger uses the stack-frame of the original exception to display at
the top level. That's why you don't see the handling code itself.

Lukas

-- 
Lukas Renggli
http://www.lukas-renggli.ch