This feature is quite rarely used and is not very discoverable in dolphin. This would allow us to remove the QWidgets dependecy and move the class to KIO-Gui.
By looking at the available config options it seems that they are not useful to the average user anyways.
If there are legitimate usecases in the future we could consider loading KCMs, similar to how KRunner plugins can have their own config.
+1 referencing a KCM in the metadata for the few that need it is surely better than baking a widgets dependency into the API
I agree that the special API that ThumbCreator uses to provide config widgets is a bit weird. I don't really know anything about KCM (does that add a GUI in ksystemsettings?), but I think it's important to be able to configure the thumbnailer in some way. Even if it's just raw config files without a GUI. The average user doesn't currently need the config options that are present, but I can imagine advanced use cases that do. For example: Over here someone suggested pulling thumbnails from the embedded metadata or even from static image streams of a video file, instead of generating them from the video stream. That would probably a on-by-default option, but I imagine there are cases where users want to disable that, e.g. if the embedded thumbnails are low-quality or very generic. That's one of the options provided by the upstream ffmpegthumbnailer.
Even if it's just raw config files without a GUI.
Then we can keep the config & the kcfg file kinda serves as documentation. Not the best but still better than just reading the source code.
Do you think that in https://invent.kde.org/network/kio-extras/-/merge_requests/114#note_278244 one should keep reading the config?
https://invent.kde.org/frameworks/kio/-/merge_requests/561 Deprecated this in KIO since the consumers in KDE have been taken care of