1. Disable show-paren-mode if it's enabled.
2. Evalute the attached .el file (which defined a major mode).
3. Create a new bufferand type M-x foo-mode.
4. Type 'def foo do' (without quotes) and press RET.
5. Cursor will hang around on the first line even after the newline is 
inserted.

Key moments: ?o is not in smie-blink-matching-triggers because the
grammar also defines a closer 'dop'. smie-blink-matching-inners is t
(which it is by default).

Here's an old bug report for elixir-mode which, unfortunately, didn't
reach the end of the investigation before the author opted for a
workaround instead: 
https://github.com/elixir-editors/emacs-elixir/issues/363

What would be the better workaround, by the way? I'm thinking of simply 
disabling smie-blink-matching-inners.

In GNU Emacs 27.0.50 (build 52, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
  of 2019-04-28 built on zappa
Repository revision: 80822917736edbab77969c4a18dfb8dd20fe3a88
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12001000
System Description: Ubuntu 18.04.2 LTS