How can I debug a problem triggered from jit-lock's background fontification?

[Alan Mackenzie]

Hi, Emacs!

I have a bug in CC Mode which is triggered by jit-lock inside its
background fontifying.  The problem doesn't occur when jit-lock is
disabled.

I have instrumented the CC Mode function for Edebug, yet something seems
to be inhibiting the invocation of Edebug from inside jit.  No doubt
there's a good reason for this, but it has been getting on my nerves for
years.

Would somebody please suggest a way I can debug the actions of
jit-locking from Edebug, or possibly some other way of making progress.

Many thanks in advance!

-- 
Alan Mackenzie (Nuremberg, Germany).