PC> For example, how does a color syntax-highlighting editor know to print
 PC> a word in blue in the middle of the page, and print everything before
 PC> and after it in different colors (and then be able to scroll it off and
 PC> back onto the screen.

I recently had to code something like this (it was for printing,  not for
displaying -- the the issue was not colours but different fonts (which is
even worse than colours because the fonts can have a different highest, 
yet still be in the same line).

What I had to do was loose the concept of a "line",  and work
with a linked-list of "text runs".  A line becomes a linked-list
of runs (the line ends when the list ends).

Each run has a style (part of which can be colour), typeface, x coordinate,
and pel-width.  The pel-height has to be calculated by scanning the whole
run to find the tallest font.


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