ThumbSequenceCreator in KIO not working properly

[David Lerch]

Hi,

Is there someone here in the know about KIO and specifically its
handling of thumbnails?

I'm trying to implement a custom thumbnail generator based on
kffmpegthumbnailer, but using ThumbSequenceCreator instead of
ThumbCreator, to display an "animated" thumbnail for video files
(mplayerhumbs did that, but was apparently dropped a while ago).
However, when viewing a video file icon in Dolphin,
ThumbSequenceCreator::sequenceIndex() always just returns 0.0. It
looks like whoever actually requests the thumbnail generation does not
call setSequenceIndex() properly. I _am_ using HandleSequences=true in
the .desktop file.

I have tried sifting through KDE code and it seems like the
sequenceIndex is passed through gazillions of layers, but it seems
like the one responsible for updating it is KIO's
DelegateAnimationHandler. Today is my first day developing something
to do with KDE, and debugging this has been a nightmare. I have not
been able to find out if DelegateAnimationHandler is even being used
during thumbnail generation. I'm trying to avoid having to compile
basically the entire KDE core system from source just to debug this.

Is ThumbSequenceCreator supposed to work in current KDE versions?
Might this be something specific to Dolphin? Is there anything else I
need to do to get a sequenceIndex != 0? Another mailing list I should
write to instead? I'm currently using KDE Frameworks 5.54.0.