Can get the base size and the display aspect ratio from ffprobe by extending the call to something like this:

$ ffprobe -v error -show_streams -select_streams v:0 -show_entries 'format=duration:stream=width,height,display_aspect_ratio:stream_tags=:stream_disposition=' -of default=noprint_wrappers=1:nokey=0 Xylocopa_violacea_male_-_2012-10-23.webm 
width=1440
height=1080
display_aspect_ratio=16:9
duration=82.440000

Will take a gander at patching the thumbor plugin later.

It's extracting a frame with ffmpeg, ideally that can somehow be made correct, without having to parse WebM-specific display_aspect_ratio, crop_* etc?

https://github.com/wikimedia/operations-debs-python-thumbor-wikimedia/blob/47fea15502b5248a76201adebe067cfbaf16f35e/wikimedia_thumbor/loader/video/__init__.py#L170-L200

Seems like https://linux.goeszen.com/how-to-take-screen-caps-from-do-thumbnails-of-anamorphic-video-in-correct-display-aspect-ratio.html has the answer, adding

-vf 'scale=iw*sar:ih'

to the ffmpeg -ss command.

Seems to work when testing it on https://commons.wikimedia.org/wiki/File:Xylocopa_violacea_male_-_2012-10-23.webm yielding this:

instead of (currently) this:

Change 427612 had a related patch set uploaded (by Gilles; owner: Gilles):
[operations/debs/python-thumbor-wikimedia@master] Upgrade to 2.0

https://gerrit.wikimedia.org/r/427612

Change 427612 merged by Filippo Giunchedi:
[operations/debs/python-thumbor-wikimedia@master] Upgrade to 2.0

https://gerrit.wikimedia.org/r/427612

*chef's kiss emoji* that's a perfect fix, and much simpler than extracting the info and adjusting size manually. Many thanks Gilles!