Ho ho ho -- that's trickier than it sounded!  The colorizer is working with
"line.col" strings instead of Text marks, and the absolute coordinates of
the point of interest can change across the self.update call (voice of
baffled experience, when two quick backspaces no longer fooled it, but a
backspace followed by a quick ENTER did <wink>).

Anyway, the attached appears to do the trick.  CPU usage goes way up when
typing quickly into a long triple-quoted string, but the latency is fine for
me (a relatively fast typist on a relatively slow machine).  Most of the
changes here are left over from reducing the # of vrbl names to help me
reason about the logic better; I hope the code is a *little* easier to