The GApplication::handle-local-options signal has its own accumulator,
g_application_handle_local_options_accumulator(), with a strange behaviour.
As soon as a signal handler returns a negative value (e.g. -1), the accumulator
returns FALSE, thus aborting signal emission. The default signal handler
returns -1.

If I connect my signal handler after the default handler, my handler is not
called.

If I connect it before the default handler, it is called. If it returns >= 0,
the accumulator returns TRUE, and the default handler is called.
The return value of the signal emission is the return value of the last called
signal handler, which will always be -1 as long as the default handler is not
overridden. Is this really the intended behaviour?

It would be more reasonable to abort the signal emission when a signal handler
returns >= 0. Let the accumulator finish with
  return value < 0;
instead of
  return value >= 0;

Alternatively, use g_signal_accumulator_first_wins(), like ::command-line.