Thank you for re-considering it.
Most video applications i used have this feature built in.
There is a workaround:
Use the old desktop client, if you don't need these "new" features.
And I just need to turn off the app auto update in quest 3.
btw, if you get gray boxes, you need to play the video, then it will generate a screenshot of the content. Me think, it's a lot faster and easier to use the jpg as cover.
