[HarfBuzz] Display issue with DejaVu Sans Mono font

[Eli Zaretskii]

We have some strange display problem in Emacs with this sequence:

  u+0097 u+0301 u+0302

The problem seems to happen only with DejaVu Sans Mono font.  It looks
like hb-view also displays the sequence with only one of the two
combining accents, see the images in

  https://lists.gnu.org/archive/html/bug-gnu-emacs/2019-08/msg01082.html

Is there something wrong with this font when displaying this sequence,
or is there some kind of bug in Emacs and/or HarfBuzz?

TIA