Right - so, I still can't reliably get the other behaviors happening, but
they still happen.
I had a couple variables concatenated together, and the autocomplete
overwrote the dot before the next variable. It highlights in yellow/red
what it's going to replace, and it was highlighting the incorrect thing,
then replaced exactly what it had highlighted.
This happens more often when the autocomplete suggestions box is visible,
but still not all the time.
I wonder if this is related to mark occurrences being a little whacky?
Mine highlights all sorts of garbage sometimes.