I've been learning about this topic.
It really seems like a very hard feature to implement, and it could cause performance and stability issues. Only File Explorer seems to be able to have this feature, because of its deeper integration with the Windows OS.
But I wouldn't mind another trade-off, to make this work in XnView.
You could show the video thumbnails in a folder, only after we entered that folder and we have the video thumbnails in the database. This way XnView doesn't have to preemptively scan all the folders, before we open them.
Once the video thumbnails are generated, they are just tiny JPEG or WebP images, no different from the regular image thumbnails. They should be easy to integrate in the folder's icon, right ?
Optionally, if you can easily detect that a folder has only video files inside, and you don't have the thumbnails yet, you could display a folder with a generic movie icon inside.
Add folders thumbnails from video files
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