Fix navigation if previous is subslide

[brichet]

Fixes a a navigation problem with sub-slide.

A sub-slide (1) without fragment and (2) followed by another sub-slide is ignored in the navigation.
This PR solve it by checking if a sub-slide is preceded by another sub-slide.

Checklist

• ran doit lint locally

References

Expressed in #49

Expected: the following subslide "Subslide1" is displayed
Got: the subslide "subslide2" is displayed

Code changes

When creating the extent (navigation) for a sub-slide, check if there is a previous sub-slide.

User-facing changes

None except for the navigation.

Backwards-incompatible changes

None

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (d24d22c) 93.66% compared to head (059b6f1) 93.45%.

❗ Current head 059b6f1 differs from pull request most recent head 38b3623. Consider uploading reports for the commit 38b3623 to get more accurate results

Files	Patch %	Lines
js/jupyterlab-deck/src/notebook/presenter.ts	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
- Coverage   93.66%   93.45%   -0.22%     
==========================================
  Files          16       16              
  Lines        1327     1330       +3     
  Branches      274      275       +1     
==========================================
  Hits         1243     1243              
- Misses         42       44       +2     
- Partials       42       43       +1     

Flag	Coverage Δ
front-end	93.40% <0.00%> (-0.22%)	⬇️

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[bollwyvl]

Yep, we'll want a fixture that concretely demonstrates this pattern.

This could either be through adding a new fixture notebook in examples, or updating an existing one to have this specific pattern so it can be observed, and therefore covered, and stay fixed.

[brichet]

@bollwyvl I added a test on this specific pattern.

[nthiery]

Thanks for the work here! I'll try it soon :-)