How can I predict which regions xdisp will present me for font-locking?

[Alan Mackenzie]

Hi, all.

I hope this isn't one of these "send complete details" questions, but:

In a file.hpp, inside a 3355 line macro (I kid you not), I type a single
character.  xdisp responds by presenting jit-lock, successively, with
four areas to fontify.  (This was determined by
trace-function-background on `font-lock-fontify-region').

None of these areas starts/stops at the top/bottom of the window.  It is the
third area which encloses the changed line.  The fourth area finishes
below the window's last line.

I can't see how this wierd sequence of fontification areas can be
being calculated by CC Mode.

This seemingly superfluous fontification seems to be causing a
performance degradation.

How can this series of fontification regions be explained?

-- 
Alan Mackenzie (Nuremberg, Germany).