Do I understand you correctly that this is the expected result with
that font?  Because what Emacs displays, even without HarfBuzz as its
shaping engine, looks exactly like that: the leftmost part of the
letter is off-screen.

> Also, if you don't pass neighboring context text to HarfBuzz, again,
> nothing we can do.

I believe this is about the other part: displaying text which is
partially selected, when selection is shown as a different background
color.  You are saying that to do its job, a shaping engine needs to
see the entire text, not just the part which has the same colors.
Right?

Thanks again for your comments.