fix: make the sphinx-gallery scraper capture every shown figure

[larsoner]

Link to issue

Closes #4722
Closes #4959
Closes sphinx-gallery/sphinx-gallery#1238
Closes sphinx-gallery/sphinx-gallery#1632

Description of change

Fix sphinx-gallery thumbnail generation!

Demo

Testing strategy

Added some unit tests to make sure it works. It uses SG (added to dev_optional so CIs install it) to make sure the contract holds.

Additional information (optional)

I am a SG maintainer. I used Claude Opus 5 to draft the changes here but I iterated with it a lot and have reviewed the final diff.

Guidelines

• I have reviewed the pull request guidelines and the Code of Conduct and confirm that this PR follows them.
• I have added an entry to the changelog if needed (not required for documentation PRs).

[sr-murthy]

Tried it (replaced plotly dep in project TOML with a ref to your fork branch of Plotly), cleared out the docs build folder, and ran Sphinx build again (make -C docs html), but still get the same placeholder thumbs (in the docs auto_examples/images/thumb folder - the full size figures are correctly drawn in the HTMLs.

I can see that the Plotly version in my env. is your branch because the UV log has the following line:

...
DEBUG Requirement already installed: plotly==6.9.0 (from git+https://github.com/larsoner/plotly.py@5ef36f39d12315835d211c39ae5ff2357c06a70b)
...

Conf changes for Plotly PNG rendering are as described here. Folder structure for the examples folder containing the Python files, is also as recommended, with auto_examples adjacent to examples. Here is the gallery conf. in docs/conf.py:

# Sphinx gallery conf.
sphinx_gallery_conf = {
    "examples_dirs": "sources/plot-gallery/examples",  # `docs`-relative path to Python scripts
    "gallery_dirs": "sources/plot-gallery/auto_examples",  # `docs`-relative path to gallery outputs
    "image_scrapers": ("matplotlib", "plotly.io._sg_scraper.plotly_sg_scraper",),
}

Wouldn't it be simpler for Sphinx gallery to support a post-build hook (a custom function maybe) that created the thumbnails by resizing and exporting the full sized figures into PNGs in the appropriate location?

[larsoner]

Wouldn't it be simpler for Sphinx gallery to support a post-build hook (a custom function maybe) that created the thumbnails by resizing the full sized PNGs into the appropriate location?

I don't think we should rework / discuss the SG API contracts here -- it'll be a much bigger discussion if needed, and require an understanding of design decisions and code evolution over the years where we settled on the existing design.

Instead I'd like to see if we can get things working properly for you using this branch. Can you make clean and try again with the latest version of the PR? I pushed some fixe -- the previous code required some SG changes and fig.show() to be used etc. but this version should work, even with the simpler:

    "image_scrapers": ("matplotlib", "plotly"),

I confirmed this works in sphinx-gallery/sphinx-gallery#1635 (see the updated example render) so hopefully it works for you now!

[sr-murthy]

Thx, I'll try it out, and let you know.