Yep, there was some cheating done. Correct, predictable navigation probably needs to be generalized as a parallel hierarchical finite state machine (e.g. xstate). Things also get complex when authoring a deck directly in deck view (adding new cells).

Just for the record: if I make all subslides into slides, then Space / Shift-Space navigation
is smooth. So a quick fix might be to have a dedicated Space/Shift-Space navigation
that just ignore the difference between slides and subslides.

@nthiery @bollwyvl I can take a look at it.
Just wondering what should be the correct workflow.
Let's say we have the following schema:

Slide 1
   Sub slide 1.1
   Sub slide 1.2
Slide 2
   Sub slide 2.1

Pressing right arrow from Sub slide 1.1 should redirect to Slide 2.
But then, should pressing left arrow (or shift space) redirect back to Sub slide 1.1 or to Sub slide 1.2 ?

The first case would require keeping the state of each slide when moving to another slide.

Oops, I missed your question. I am not sure myself what the semantic should be
for left/right arrow (I rarely use them). reveal.js being widely used, following the
same convention is presumably a safe starting point:

https://revealjs.com/vertical-slides/

The documentation there is a bit ambiguous. However my understanding of the
default behavior is that, when moving back and forth from one stack to the other,
one always lands on the first slide of the stack.