(Easiest to see with a graphics debugging tool
like QuartzDebug for the mac - each area that's
repainted flashes yellow).

I had noticed that the text limit line was
sometimes partially hiding characters.  After
watching it in slowed-down debug mode, I know why.
 It goes as follows:
 - Paint one line of text
 - Paint the text limit line from top to bottom
 - Paint another line of text
 - Paint the text limit line from top to bottom
 - Paint another line of text
 - Paint the text limit line from top to bottom

for the whole height of the editor.

Now, drawing lines is a pretty cheap operation,
but editor painting needs to be really fast.  It
seems to me you could probably just draw the once
when the rest of the paint is completed, or before
the text painting starts.